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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/19395-8.txt b/19395-8.txt new file mode 100644 index 0000000..bb5ee0d --- /dev/null +++ b/19395-8.txt @@ -0,0 +1,2310 @@ +The Project Gutenberg EBook of The New Heavens, by George Ellery Hale + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The New Heavens + +Author: George Ellery Hale + +Release Date: September 28, 2006 [EBook #19395] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + + + + +Produced by Robert J. Hall + + + + +[Illustration: Fig. 1. The Constellation of Orion (Hubble). + +Photographed with a small camera lens of 1 inch aperture and 5 +inches focal length. The three bright stars in the centre of the +picture form the belt of Orion. Just below, in the sword handle, is +an irregular white patch about one-eighth of an inch in diameter. +This is a small-scale image of the great nebula in Orion, shown +on a larger scale in Fig. 2.] + + + + +THE NEW HEAVENS + + +BY + +GEORGE ELLERY HALE + +DIRECTOR OF THE MOUNT WILSON OBSERVATORY OF THE CARNEGIE INSTITUTION +OF WASHINGTON + +WITH NUMEROUS ILLUSTRATIONS + + +NEW YORK + +CHARLES SCRIBNER'S SONS + +1922 + + + + +TO MY WIFE + + + + +PREFACE + +Fourteen years ago, in a book entitled "The Study of Stellar Evolution" +(University of Chicago Press, 1908), I attempted to give in untechnical +language an account of some modern methods of astrophysical research. +This book is now out of print, and the rapid progress of science has +left it completely out of date. As I have found no opportunity to +prepare a new edition, or to write another book of similar purpose, +I have adopted the simpler expedient of contributing occasional +articles on recent developments to _Scribner's Magazine_, three +of which are included in the present volume. + +I am chiefly indebted, for the illustrations, to the Mount Wilson +Observatory and the present and former members of its staff whose +names appear in the captions. Special thanks are due to Mr. Ferdinand +Ellerman, who made all of the photographs of the observatory buildings +and instruments, and prepared all material for reproduction. The +cut of the original Cavendish apparatus is copied from the +_Philosophical Transactions for 1798_ with the kind permission +of the Royal Society, and I am also indebted to the Royal Society +and to Professor Fowler and Father Cortie for the privilege of +reproducing from the _Proceedings_ two illustrations of their +spectroscopic results. + +G. E. H. + +January, 1922. + + + + +CONTENTS + +CHAPTER + I. THE NEW HEAVENS + II. GIANT STARS + III. COSMIC CRUCIBLES + + + +ILLUSTRATIONS + +FIG. + 1. The Constellation of Orion (Hubble) + 2. The Great Nebula in Orion (Pease) + 3. Model by Ellerman of summit of Mount Wilson, showing the observatory + buildings among the trees and bushes + 4. The 100-inch Hooker telescope + 5. Erecting the polar axis of the 100-inch telescope + 6. Lowest section of tube of 100-inch telescope, ready to leave Pasadena + for Mount Wilson + 7. Section of a steel girder for dome covering the 100-inch telescope, + on its way up Mount Wilson + 8. Erecting the steel building and revolving dome that cover the Hooker + telescope + 9. Building and revolving dome, 100 feet in diameter, covering the + 100-inch Hooker telescope + 10. One-hundred-inch mirror, just silvered, rising out of the + silvering-room in pier before attachment to lower end of telescope + tube. (Seen above) + 11. The driving-clock and worm-gear that cause the 100-inch Hooker + telescope to follow the stars + 12. Large irregular nebula and star cluster in Sagittarius (Duncan) + 13. Faint spiral nebula in the constellation of the Hunting Dogs (Pease) + 14. Spiral nebula in Andromeda, seen edge on (Ritchey) + 15. Photograph of the moon made on September 15, 1919, with the 100-inch + Hooker telescope (Pease) + 16. Photograph of the moon made on September 15, 1919, with the 100-inch + Hooker telescope (Pease) + 17. Hubble's Variable Nebula. One of the few nebulę known to vary in + brightness and form + 18. Ring Nebula in Lyra, photographed with the 60-inch (Ritchey) and + 100-inch (Duncan) telescopes + 19. Gaseous prominence at the sun's limb, 140,000 miles high (Ellerman) + 20. The sun, 865,000 miles in diameter, from a direct photograph showing + many sun-spots (Whitney) + 21. Great sun-spot group, August 8, 1917 (Whitney) + 22. Photograph of the hydrogen atmosphere of the sun (Ellerman) + 23. Diagram showing outline of the 100-inch Hooker telescope, and path of + the two pencils of light from a star when under observation with the + 20-foot Michelson interferometer + 24. Twenty-foot Michelson interferometer for measuring star diameters, + attached to upper end of the skeleton tube of the 100-inch Hooker + telescope + 25. The giant Betelgeuse (within the circle), familiar as the conspicuous + red star in the right shoulder of Orion (Hubble) + 26. Arcturus (within the white circle), known to the Arabs as the "Lance + Bearer," and to the Chinese as the "Great Horn" or the "Palace of the + Emperors" (Hubble) + 27. The giant star Antares (within the white circle), notable for its red + color in the constellation Scorpio, and named by the Greeks "A Rival + of Mars" (Hubble) + 28. Diameters of the Sun, Arcturus, Betelgeuse, and Antares compared with + the orbit of Mars + 29. Aldebaran, the "leader" (of the Pleiades), was also known to the Arabs + as "The Eye of the Bull," "The Heart of the Bull," and "The Great + Camel" (Hubble) + 30. Solar prominences, photographed with the spectroheliograph without an + eclipse (Ellerman) + 31. The 150-foot tower telescope of the Mount Wilson Observatory + 32. Pasadena Laboratory of the Mount Wilson Observatory + 33. Sun-spot vortex in the upper hydrogen atmosphere (Benioff) + 34. Splitting of spectrum lines by a magnetic field (Bacock) + 35. Electric furnace in the Pasadena Laboratory of the Mount Wilson + Observatory + 36. Titanium oxide in red stars + 37. Titanium oxide in sun-spots + 38. The Cavendish experiment + 39. The Trifid Nebula in Sagittarius (Ritchey) + 40. Spiral nebula in Ursa Major (Ritchey) + 41. Mount San Antonio as seen from Mount Wilson + + + + +CHAPTER I + +THE NEW HEAVENS + +Go out under the open sky, on a clear and moon-less night, and try +to count the stars. If your station lies well beyond the glare of +cities, which is often strong enough to conceal all but the brighter +objects, you will find the task a difficult one. Ranging through +the six magnitudes of the Greek astronomers, from the brilliant +Sirius to the faintest perceptible points of light, the stars are +scattered in great profusion over the celestial vault. Their number +seems limitless, yet actual count will show that the eye has been +deceived. In a survey of the entire heavens, from pole to pole, +it would not be possible to detect more than from six to seven +thousand stars with the naked eye. From a single viewpoint, even +with the keenest vision, only two or three thousand can be seen. +So many of these are at the limit of visibility that Ptolemy's +"Almagest," a catalogue of all the stars whose places were measured +with the simple instruments of the Greek astronomers, contains +only 1,022 stars. + +Back of Ptolemy, through the speculations of the Greek philosophers, +the mysteries of the Egyptian sun-god, and the observations of the +ancient Chaldeans, the rich and varied traditions of astronomy stretch +far away into a shadowy past. All peoples, in the first stirrings +of their intellectual youth, drawn by the nightly splendor of the +skies and the ceaseless motions of the planets, have set up some +system of the heavens, in which the sense of wonder and the desire +for knowledge were no less concerned than the practical necessities +of life. The measurement of time and the needs of navigation have +always stimulated astronomical research, but the intellectual demand +has been keen from the first. Hipparchus and the Greek astronomers +of the Alexandrian school, shaking off the vagaries of magic and +divination, placed astronomy on a scientific basis, though the +reaction of the Middle Ages caused even such a great astronomer +as Tycho Brahe himself to revert for a time to the practice of +astrology. + +EARLY INSTRUMENTS + +The transparent sky of Egypt, rarely obscured by clouds, greatly +favored Ptolemy's observations. Here was prepared his great star +catalogue, based upon the earlier observations of Hipparchus, and +destined to remain alone in its field for more than twelve centuries, +until Ulugh Bey, Prince of Samarcand, repeated the work of his +Greek predecessor. Throughout this period the stars were looked +upon mainly as points of reference for the observation of planetary +motions, and the instruments of observation underwent little change. +The astrolabe, which consists of a circle divided into degrees, +with a rotating diametral arm for sighting purposes, embodies their +essential principle. In its simple form, the astrolabe was suspended +in a vertical plane, and the stars were observed by bringing the +sights on the movable diameter to bear upon them. Their altitude +was then read off on the circle. Ultimately, the circle of the +astrolabe, mounted with one of its diameters parallel to the earth's +axis, became the armillary sphere, the precursor of our modern +equatorial telescope. Great stone quadrants fixed in the meridian +were also employed from very early times. Out of such furnishings, +little modified by the lapse of centuries, was provided the elaborate +instrumental equipment of Uranibourg, the great observatory built +by Tycho Brahe on the Danish island of Huen in 1576. In this "City +of the Heavens," still dependent solely upon the unaided eye as a +collector of starlight, Tycho made those invaluable observations +that enabled Kepler to deduce the true laws of planetary motion. But +after all these centuries the sidereal world embraced no objects, +barring an occasional comet or temporary star, that lay beyond +the vision of the earliest astronomers. The conceptions of the +stellar universe, except those that ignored the solid ground of +observation, were limited by the small aperture of the human eye. +But the dawn of another age was at hand. + +[Illustration: Fig. 2. The Great Nebula in Orion (Pease). + +Photographed with the 100-inch telescope. This short-exposure photograph +shows only the bright central part of the nebula. A longer exposure +reveals a vast outlying region.] + +The dominance of the sun as the central body of the solar system, +recognized by Aristarchus of Samos nearly three centuries before +the Christian era, but subsequently denied under the authority of +Ptolemy and the teachings of the Church, was reaffirmed by the +Polish monk Copernicus in 1543. Kepler's laws of the motions of the +planets, showing them to revolve in ellipses instead of circles, +removed the last defect of the Copernican system, and left no room for +its rejection. But both the world and the Church clung to tradition, +and some visible demonstration was urgently needed. This was supplied +by Galileo through his invention of the telescope. + +[Illustration: Fig. 3. Model by Ellerman of summit of Mount Wilson, +showing the observatory buildings among the trees and bushes. + +The 60-foot tower on the extreme left, which is at the edge of +a precipitous cańon 1,500 feet deep, is the vertical telescope +of the Smithsonian Astrophysical Observatory. Above it are the +"Monastery" and other buildings used as quarters by the astronomers +of the Mount Wilson Observatory while at work on the mountain. (The +offices, computing-rooms, laboratories, and shops are in Pasadena.) +Following the ridge, we come successively to the dome of the 10-inch +photographic telescope, the power-house, laboratory, Snow horizontal +telescope, 60-foot-tower telescope, and 150-foot-tower telescope, +these last three used for the study of the sun. The dome of the +60-inch reflecting telescope is just below the 150-foot tower, +while that of the 100-inch telescope is farther to the right. The +altitude of Mount Wilson is about 5,900 feet.] + +The crystalline lens of the human eye, limited by the iris to a +maximum opening about one-quarter of an inch in diameter, was the +only collector of starlight available to the Greek and Arabian +astronomers. Galileo's telescope, which in 1610 suddenly pushed +out the boundaries of the known stellar universe and brought many +thousands of stars into range, had a lens about 2-1/4 inches in +diameter. The area of this lens, proportional to the square of +its diameter, was about eighty-one times that of the pupil of the +eye. This great increase in the amount of light collected should +bring to view stars down to magnitude 10.5, of which nearly half +a million are known to exist. + +It is not too much to say that Galileo's telescope revolutionized +human thought. Turned to the moon, it revealed mountains, plains, +and valleys, while the sun, previously supposed immaculate in its +perfection, was seen to be blemished with dark spots changing from +day to day. Jupiter, shown to be accompanied by four encircling +satellites, afforded a picture in miniature of the solar system, +and strongly supported the Copernican view of its organization, +which was conclusively demonstrated by Galileo's discovery of the +changing phases of Venus and the variation of its apparent diameter +during its revolution about the sun. Galileo's proof of the Copernican +theory marked the downfall of medięvalism and established astronomy on +a firm foundation. But while his telescope multiplied a hundredfold +the number of visible stars, more than a century elapsed before +the true possibilities of sidereal astronomy were perceived. + +[Illustration: Fig. 4. The 100-inch Hooker telescope.] + +STRUCTURE OF THE UNIVERSE + +Sir William Herschel was the first astronomer to make a serious +attack upon the problem of the structure of the stellar universe. +In his first memoir on the "Construction of the Heavens," read +before the Royal Society in 1784, he wrote as follows: + +"Hitherto the sidereal heavens have, not inadequately for the purpose +designed, been represented by the concave surface of a sphere in +the centre of which the eye of an observer might be supposed to be +placed.... In future we shall look upon those regions into which we +may now penetrate by means of such large telescopes, as a naturalist +regards a rich extent of ground or chain of mountains containing +strata variously inclined and directed as well as consisting of +very different materials." + +On turning his 18-inch reflecting telescope to a part of the Milky +Way in Orion, he found its whitish appearance to be completely +resolved into small stars, not separately seen with his former +telescopes. "The glorious multitude of stars of all possible sizes +that presented themselves here to my view are truly astonishing; but +as the dazzling brightness of glittering stars may easily mislead +us so far as to estimate their number greater than it really is, +I endeavored to ascertain this point by counting many fields, and +computing from a mean of them, what a certain given portion of +the Milky Way might contain." By this means, applied not only to +the Milky Way but to all parts of the heavens, Herschel determined +the approximate number and distribution of all the stars within +reach of his instrument. + +By comparing many hundred gauges or counts of stars visible in +a field of about one-quarter of the area of the moon, Herschel +found that the average number of stars increased toward the great +circle which most nearly conforms with the course of the Milky Way. +Ninety degrees from this plane, at the pole of the Milky Way, only +four stars, on the average, were seen in the field of the telescope. +In approaching the Milky Way this number increased slowly at first, +and then more and more rapidly, until it rose to an average of +122 stars per field. + +[Illustration: Fig. 5. Erecting the polar axis of the 100-inch +telescope.] + +These observations were made in the northern hemisphere, and +subsequently Sir John Herschel, using his father's telescope at +the Cape of Good Hope, found an almost exactly similar increase +of apparent star density for the southern hemisphere. According to +his estimates, the total number of stars in both hemispheres that +could be seen distinctly enough to be counted in this telescope +would probably be about five and one-half millions. + +The Herschels concluded that "the stars of our firmament, instead +of being scattered in all directions indifferently through space, +form a stratum of which the thickness is small, in comparison with +its length and breadth; and in which the earth occupies a place +somewhere about the middle of its thickness, between the point +where it subdivides into two principal laminę inclined at a small +angle to each other." This view does not differ essentially from our +modern conception of the form of the Galaxy; but as the Herschels +were unable to see stars fainter than the fifteenth magnitude, +it is evident that their conclusions apply only to a restricted +region surrounding the solar system, in the midst of the enormously +extended sidereal universe which modern instruments have brought +within our range. + +MODERN METHODS + +The remarkable progress of modern astronomy is mainly due to two +great instrumental advances: the rise and development of the +photographic telescope, and the application of the spectroscope to +the study of celestial objects. These new and powerful instruments, +supplemented by many accessories which have completely revolutionized +observatory equipment, have not only revealed a vastly greater +number of stars and nebulę: they have also rendered feasible +observations of a type formerly regarded as impossible. The chemical +analysis of a faint star is now so easy that it can be accomplished +in a very short time--as quickly, in fact, as an equally complex +substance can be analyzed in the laboratory. The spectroscope also +measures a star's velocity, the pressure at different levels in +its atmosphere, its approximate temperature, and now, by a new +and ingenious method, its distance from the earth. It determines +the velocity of rotation of the sun and of nebulę, the existence +and periods of orbital revolution of binary stars too close to +be separated by any telescope, the presence of magnetic fields +in sunspots, and the fact that the entire sun, like the earth, is +a magnet. + +[Illustration: Fig. 6. Lowest section of tube of 100-inch telescope, +ready to leave Pasadena for Mount Wilson.] + +Such new possibilities, with many others resulting from the application +of physical methods of the most diverse character, have greatly +enlarged the astronomer's outlook. He may now attack two great +problems: (1) The structure of the universe and the motions of +its constituent bodies, and (2) the evolution of the stars: their +nature, origin, growth, and decline. These two problems are intimately +related and must be studied as one.[*] + +[Footnote *: A third great problem open to the astronomer, the +study of the constitution of matter, is described in Chapter III.] + +If space permitted, it would be interesting to survey the progress +already accomplished by modern methods of astronomical research. +Hundreds of millions of stars have been photographed, and the boundaries +of the stellar universe have been pushed far into space, but have not +been attained. Globular star clusters, containing tens of thousands +of stars, are on so great a scale (according to Shapley) that light, +travelling at the rate of 186,000 miles per second, may take 500 +years to cross one of them, while the most distant of these objects +may be more than 200,000 light-years from the earth. The spiral +nebulę, more than a million in number, are vast whirling masses +in process of development, but we are not yet certain whether they +should be regarded as "island universes" or as subordinate to the +stellar system which includes our minute group of sun and planets, +the great star clouds of the Milky Way, and the distant globular +star clusters. + +[Illustration: Fig. 7. Section of a steel girder for dome covering +the 100-inch telescope, on its way up Mount Wilson.] + +These few particulars may give a slight conception of the scale +of the known universe, but a word must be added regarding some +of its most striking phenomena. The great majority of the stars +whose motions have been determined belong to one or the other of +two great star streams, but the part played by these streams in the +sidereal system as a whole is still obscure. The stars have been +grouped in classes, presumably in the order of their evolutional +development, as they pass from the early state of gaseous masses, of +low density, through the successive stages resulting from loss of +heat by radiation and increased density due to shrinkage. Strangely +enough, their velocities in space show a corresponding change, +increasing as they grow older or perhaps depending upon their mass. + +It is impossible within these limits to do more than to give some +indication of the scope of the new astronomy. Enough has been said, +however, to assist in appreciating the increased opportunity for +investigation, and the nature of the heavy demands made upon the +modern observatory. But before passing on to describe one of the +latest additions to the astronomer's instrumental equipment, a +word should be added regarding the chief classes of telescopes. + +REFRACTORS AND REFLECTORS + +Astronomical telescopes are of two types: refractors and reflectors. +A refracting telescope consists of an object-glass composed of +two or more lenses, mounted at the upper end of a tube, which is +pointed at the celestial object. The light, after passing through +the lenses, is brought to a focus at the lower end of the tube, where +the image is examined visually with an eyepiece, or photographed +upon a sensitive plate. The largest instruments of this type are +the 36-inch Lick telescope and the 40-inch refractor of the Yerkes +Observatory. + +[Illustration: Fig. 8. Erecting the steel building and revolving +dome that cover the Hooker telescope.] + +Reflecting telescopes, which are particularly adapted for photographic +work, though also excellent for visual observations, are very +differently constructed. No lens is used. The telescope tube is +usually built in skeleton form, open at its upper end, and with +a large concave mirror supported at its base. This mirror serves +in place of a lens. Its upper surface is paraboloidal in shape, +as a spherical surface will not unite in a sharp focus the rays +coming from a distant object. The light passes through no glass--a +great advantage, especially for photography, as the absorption +in lenses cuts out much of the blue and violet light, to which +photographic plates are most sensitive. The reflection occurs on +the _upper_ surface of the mirror, which is covered with a coat of +pure silver, renewed several times a year and always kept highly +burnished. Silvered glass is better than metals or other substances +for telescope mirrors, chiefly because of the perfection with which +glass can be ground and polished, and the ease of renewing its +silvered surface when tarnished. + +The great reflectors of Herschel and Lord Rosse, which were provided +with mirrors of speculum metal, were far inferior to much smaller +telescopes of the present day. With these instruments the star images +were watched as they were carried through the field of view by the +earth's rotation, or kept roughly in place by moving the telescope +with ropes or chains. Photographic plates, which reveal invisible +stars and nebulę when exposed for hours in modern instruments, were +not then available. In any case they could not have been used, +in the absence of the perfect mechanism required to keep the star +images accurately fixed in place upon the sensitive film. + +[Illustration: Fig. 9. Building and revolving dome, 100 feet in +diameter, covering the 100-inch Hooker telescope. + +Photographed from the summit of the 150-foot-tower telescope.] + +It would be interesting to trace the long contest for supremacy +between refracting and reflecting telescopes, each of which, at +certain stages in its development, appeared to be unrivalled. In +modern observatories both types are used, each for the purpose for +which it is best adapted. For the photography of nebulę and the +study of the fainter stars, the reflector has special advantages, +illustrated by the work of such instruments as the Crossley and Mills +reflectors of the Lick Observatory; the great 72-inch reflector, +recently brought into effective service at the Dominion Observatory +in Canada; and the 60-inch and 100-inch reflectors of the Mount +Wilson Observatory. + +The unaided eye, with an available area of one-twentieth of a square +inch, permits us to see stars of the sixth magnitude. Herschel's +18-inch reflector, with an area 5,000 times as great, rendered +visible stars of the fifteenth magnitude. The 60-inch reflector, +with an area 57,600 times that of the eye, reveals stars of the +eighteenth magnitude, while to reach stars of about the twentieth +magnitude, photographic exposures of four or five hours suffice +with this instrument. + +Every gain of a magnitude means a great gain in the number of stars +rendered visible. Stars of the second magnitude are 3.4 times as +numerous as those of the first, those of the eighth magnitude are +three times as numerous as those of the seventh, while the sixteenth +magnitude stars are only 1.7 as numerous as those of the fifteenth +magnitude. This steadily decreasing ratio is probably due to an +actual thinning out of the stars toward the boundaries of the stellar +universe, as the most exhaustive tests have failed to give any +evidence of absorption of light in its passage through space. But +in spite of this decrease, the gain of a single additional magnitude +may mean the addition of many millions of stars to the total of those +already shown by the 60-inch reflector. Here is one of the chief +sources of interest in the possibilities of a 100-inch reflecting +telescope. + +100-INCH TELESCOPE + +[Illustration: Fig. 10. One-hundred-inch mirror, just silvered, +rising out of the silvering-room in pier before attachment to lower +end of telescope tube. (Seen above.)] + +In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie +Institution of Washington a sum sufficient to construct a telescope +mirror 100 inches in diameter, and thus large enough to collect +160,000 times the light received by the eye. (Fig. 10.) The casting +and annealing of a suitable glass disk, 101 inches in diameter +and 13 inches thick, weighing four and one-half tons, was a most +difficult operation, finally accomplished by a great French glass +company at their factory in the Forest of St. Gobain. A special +optical laboratory was erected at the Pasadena headquarters of +the Mount Wilson Observatory, and here the long task of grinding, +figuring, and testing the mirror was successfully carried out by +the observatory opticians. This operation, which is one of great +delicacy, required years for its completion. Meanwhile the building, +dome, and mounting for the telescope were designed by members of +the observatory staff, and the working drawings were prepared. An +opportune addition by Mr. Carnegie to the endowment of the Carnegie +Institution of Washington, of which the observatory is a branch, +permitted the necessary appropriations to be made for the completion +and erection of the telescope. Though delayed by the war, during +which the mechanical and optical facilities of the observatory +shops were utilized for military and naval purposes, the telescope +is now in regular use on Mount Wilson. + +The instrument is mounted on a massive pier of reinforced concrete, +33 feet high and 52 feet in diameter at the top. A solid wall extends +south from this pier a distance of 50 feet, on the west side of +which a very powerful spectrograph, for photographing the spectra +of the brightest stars, will be mounted. Within the pier are a +photographic dark room, a room for silvering the large mirror (which +can be lowered into the pier), and the clock-room, where stands +the powerful driving-clock, with which the telescope is caused +to follow the apparent motion of the stars. (Fig. 11.) + +[Illustration: Fig. 11. The driving-clock and worm-gear that cause +the 100-inch Hooker telescope to follow the stars.] + +The telescope mounting is of the English type, in which the telescope +tube is supported by the declination trunnions between the arms of +the polar axis, built in the form of a rectangular yoke carried by +bearings on massive pedestals to the north and south. These bearings +must be aligned exactly parallel to the axis of the earth, and must +support the polar axis so freely that it can be rotated with perfect +precision by the driving-clock, which turns a worm-wheel 17 feet in +diameter, clamped to the lower end of the axis. As this motion +must be sufficiently uniform to counteract exactly the rotation +of the earth on its axis, and thus to maintain the star images +accurately in position in the field of view, the greatest care +had to be taken in the construction of the driving-clock and in +the spacing and cutting of the teeth in the large worm-wheel. Here, +as in the case of all of the more refined parts of the instrument, +the work was done by skilled machinists in the observatory shops in +Pasadena or on Mount Wilson after the assembling of the telescope. +The massive sections of the instrument, some of which weigh as +much as ten tons each, were constructed at Quincy, Mass., where +machinery sufficiently large to build battleships was available. +They were then shipped to California, and transported to the summit +of Mount Wilson over a road built for this purpose by the construction +division of the observatory, which also built the pier on which the +telescope stands, and erected the steel building and dome that +cover it. + +[Illustration: Fig. 12. Large irregular nebula and star cluster +in Sagittarius (Duncan). + +Photographed with the 60-inch telescope.] + +[Illustration: Fig. 13. Faint spiral nebula in the constellation +of the Hunting Dogs (Pease). + +Photographed with the 60-inch telescope.] + +The parts of the telescope which are moved by the driving-clock +weigh about 100 tons, and it was necessary to provide means of +reducing the great friction on the bearings of the polar axis. To +accomplish this, large hollow steel cylinders, floating in mercury +held in cast-iron tanks, were provided at the upper and lower ends +of the polar axis. Almost the entire weight of the instrument is +thus floated in mercury, and in this way the friction is so greatly +reduced that the driving-clock moves the instrument with perfect +ease and smoothness. + +The 100-inch mirror rests at the bottom of the telescope tube on +a special support system, so designed as to prevent any bending of +the glass under its own weight. Electric motors, forty in number, are +provided to move the telescope rapidly or slowly in right ascension +(east or west) and in declination (north or south), for focussing +the mirrors, and for many other purposes. They are also used for +rotating the dome, 100 feet in diameter, under which the telescope +is mounted, and for opening the shutter, 20 feet wide, through +which the observations are made. + +A telescope of this kind can be used in several different ways. +The 100-inch mirror has a focal length of about 42 feet, and in +one of the arrangements of the instrument, the photographic plate +is mounted at the centre of the telescope tube near its upper end, +where it receives directly the image formed by the large mirror. In +another arrangement, a silvered glass mirror, with plane surface, +is supported near the upper end of the tube at an angle of 45°, so +as to form the image at the side of the tube, where the photographic +plate can be placed. In this case, the observer stands on a platform, +which is moved up and down by electric motors in front of the opening +in the dome through which the observations are made. + +[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on +(Ritchey). + +Photographed with the 60-inch telescope.] + +Other arrangements of the telescope, for which auxiliary convex +mirrors carried near the upper end of the tube are required, permit +the image to be photographed at the side of the tube near its lower +end, either with or without a spectrograph; or with a very powerful +spectrograph mounted within a constant-temperature chamber south +of the telescope pier. In this last case, the light of a star is +so reflected by auxiliary mirrors that it passes down through a +hole in the south end of the polar axis and brings the star to +a focus on the slit of the fixed spectrograph. + +ATMOSPHERIC LIMITATIONS + +The huge dimensions of such a powerful engine of research as the +Hooker telescope are not in themselves a source of satisfaction to +the astronomer, for they involve a decided increase in the labor +of observation and entail very heavy expense, justifiable only in +case important results, beyond the reach of other instruments, +can be secured. The construction of a telescope of these dimensions +was necessarily an experiment, for it was by no means certain, after +the optical and mechanical difficulties had been overcome, that +even the favorable atmosphere of California would be sufficiently +tranquil to permit sharply defined celestial images to be obtained +with so large an aperture. It is therefore important to learn what +the telescope will actually accomplish under customary observing +conditions. + +Fortunately we are able to measure the performance of the instrument +with certainty. Close beside it on Mount Wilson stands the 60-inch +reflector, of similar type, erected in 1908. The two telescopes can +thus be rigorously compared under identical atmospheric conditions. + +The large mirror of the 100-inch telescope has an area about 2.8 +times that of the 60-inch, and therefore receives nearly three times +as much light from a star. Under atmospheric conditions perfect +enough to allow all of this light to be concentrated in a point, +it should be capable of recording on a photographic plate, with a +given exposure, stars about one magnitude fainter than the faintest +stars within reach of the 60-inch. The increased focal length, +permitting such objects as the moon to be photographed on a larger +scale, should also reveal smaller details of structure and render +possible higher accuracy of measurement. Finally, the greater +theoretical resolving power of the larger aperture, providing it +can be utilized, should permit the separation of the members of +close double stars beyond the range of the smaller instrument. + +CRITICAL TESTS + +The many tests already made indicate that the advantages expected +of the new telescope will be realized in practice. The increased +light-gathering power will mean the addition of many millions of +stars to those already known. Spectroscopic observations now in +regular progress have carried the range of these investigations +far beyond the possibilities of the 60-inch telescope. A great +class of red stars, for example, almost all the members of which +were inaccessible to the 60-inch, are now being made the subject +of special study. And in other fields of research equal advantages +have been gained. + +The increase in the scale of the images over those given by the +60-inch telescope is illustrated by two photographs of the Ring +Nebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion, +photographed with the 100-inch telescope with a comparatively short +exposure, sufficient to bring out the brighter regions, is reproduced +in Fig. 2. It is interesting to compare this picture with the +small-scale image of the same nebula shown in Fig. 1. + +[Illustration: Fig. 15. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease). + +The ring-like formations are the so-called craters, most of them +far larger than anything similar on the earth. That in the lower +left corner with an isolated mountain in the centre is Albategnius, +sixty-four miles in diameter. Peaks in the ring rise to a height +of fifteen thousand feet above the central plain. Note the long +sunset shadows cast by the mountains on the left. The level region +below on the right is an extensive plain, the Mare Nubium.] + +[Illustration: Fig. 16. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease). + +The mountains above and to the left are the lunar Apennines; those +on the left just below the centre are the Alps. Both ranges include +peaks from fifteen thousand to twenty thousand feet in height. In +the upper right corner is Copernicus, about fifty miles in diameter. +The largest of the conspicuous group of three just below the Apennines +is Archimedes and at the lower end of the Alps is Plato. Note the +long sunset shadows cast by the isolated peaks on the left. The +central portion of the picture is a vast plain, the Mare Imbrium.] + +The sharpness of the images given by the new telescope may be +illustrated by some recent photographs of the moon, obtained with +an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged +region of the moon, containing many ring-like mountains or craters. +Fig. 16 shows the great arc of the lunar Apennines (above) and the +Alps (below), to the left of the broad plain of the Mare Imbrium. +The starlike points along the moon's terminator, which separates +the dark area from the region upon which the sun (on the right) +shines, are the mountain peaks, about to disappear at sunset. The +long shadows cast by the mountains just within the illuminated +area are plainly seen. Some of the peaks of the lunar Apennines +attain a height of 20,000 feet. + +In less powerful telescopes the stars at the centre of the great +globular clusters are so closely crowded together that they cannot +be studied separately with the spectrograph. Moreover, most of +them are much too faint for examination with this instrument. At +the 134-foot focus the 100-inch telescope gives a large-scale image +of such clusters, and permits the spectra of stars as faint as +the fifteenth magnitude to be separately photographed. + +[Illustration: Fig. 17. Hubble's Variable Nebula. One of the few +nebulę known to vary in brightness and form. + +Photographed with the 100-inch telescope (Hubble).] + +CLOSE DOUBLE STARS + +A remarkable use of the 100-inch telescope, which permits its full +theoretical resolving power to be not merely attained but to be +doubled, has been made possible by the first application of Michelson's +interference method to the measurement of very close double stars. +When employing this, the 100-inch mirror is completely covered, +except for two slits. Beams of light from a star, entering by the +slits, unite at the focus of the telescope, where the image is +examined by an eyepiece magnifying about five thousand diameters. +Across the enlarged star image a series of fine, sharp fringes is +seen, even when the atmospheric conditions are poor. If the star is +single the fringes remain visible, whatever the distance between the +slits. But in the case of a star like Capella, previously inferred +to be double from the periodic displacement of the lines in its +spectrum, but with components too close together to be distinguished +separately, the fringes behave differently. As the slits are moved +apart a point is reached where the fringes completely disappear, +only to reappear as the separation is continued. This effect is +obtained when the slits are at right angles to the line joining +the two stars of the pair, found by this method to be 0.0418 of a +second of arc apart (on December 30, 1919). Subsequent measures, +of far greater precision than those obtainable by other methods in +the case of easily separated double stars, show the rapid orbital +motion of the components of the system. This device will be applied +to other close binaries, hitherto beyond the reach of measurement. + +[Illustration: Fig. 18. Ring Nebula in Lyra, photographed with the +60-inch (Ritchey) and 100-inch (Duncan) telescopes. + +Showing the increased scale of the images given by the larger +instrument.] + +Without entering into further details of the tests, it is evident +that the new telescope will afford boundless possibilities for +the study of the stellar universe.[*] The structure and extent of +the galactic system, and the motions of the stars comprising it; +the distribution, distances, and dimensions of the spiral nebulę, +their motions, rotation, and mode of development; the origin of +the stars and the successive stages in their life history: these +are some of the great questions which the new telescope must help +to answer. In such an embarrassment of riches the chief difficulty +is to withstand the temptation toward scattering of effort, and to +form an observing programme directed toward the solution of crucial +problems rather than the accumulation of vast stores of miscellaneous +data. This programme will be supplemented by an extensive study +of the sun, the only star near enough the earth to be examined +in detail, and by a series of laboratory investigations involving +the experimental imitation of solar and stellar conditions, thus +aiding in the interpretation of celestial phenomena. + +[Footnote *: It is not adapted for work on the sun, as the mirrors +would be distorted by its heat. Three other telescopes, especially +designed for solar observations, are in use on Mount Wilson.] + + + + +CHAPTER II + +GIANT STARS + +Our ancestral sun, as pictured by Laplace, originally extended +in a state of luminous vapor beyond the boundaries of the solar +system. Rotating upon its axis, it slowly contracted through loss +of heat by radiation, leaving behind it portions of its mass, which +condensed to form the planets. Still gaseous, though now denser than +water, it continues to pour out the heat on which our existence +depends, as it shrinks imperceptibly toward its ultimate condition +of a cold and darkened globe. + +Laplace's hypothesis has been subjected in recent years to much +criticism, and there is good reason to doubt whether his description +of the mode of evolution of our solar system is correct in every +particular. All critics agree, however, that the sun was once enormously +larger than it now is, and that the planets originally formed part +of its distended mass. + +Even in its present diminished state, the sun is huge beyond easy +conception. Our own earth, though so minute a fragment of the primeval +sun, is nevertheless so large that some parts of its surface have +not yet been explored. Seen beside the sun, by an observer on one +of the planets, the earth would appear as an insignificant speck, +which could be swallowed with ease by the whirling vortex of a +sun-spot. If the sun were hollow, with the earth at its centre, +the moon, though 240,000 miles from us, would have room and to +spare in which to describe its orbit, for the sun is 865,000 miles +in diameter, so that its volume is more than a million times that +of the earth. + +[Illustration: Fig. 19. Gaseous prominence at the sun's limb, 140,000 +miles high (Ellerman). + +Photographed with the spectroheliograph, using the light emitted +by glowing calcium vapor. The comparative size of the earth is +indicated by the white circle.] + +But what of the stars, proved by the spectroscope to be self-luminous, +intensely hot, and formed of the same chemical elements that constitute +the sun and the earth? Are they comparable in size with the sun? Do +they occur in all stages of development, from infancy to old age? +And if such stages can be detected, do they afford indications +of the gradual diminution in volume which Laplace imagined the +sun to experience? + +[Illustration: Fig. 20. The sun, 865,000 miles in diameter, from +a direct photograph showing many sun-spots (Whitney) + +The small black disk in the centre represents the comparative size +of the earth, while the circle surrounding it corresponds in diameter +to the orbit of the moon.] + +STAR IMAGES + +Prior to the application of the powerful new engine of research +described in this article we have had no means of measuring the +diameters of the stars. We have measured their distances and their +motions, determined their chemical composition, and obtained undeniable +evidence of progressive development, but even in the most powerful +telescopes their images are so minute that they appear as points +rather than as disks. In fact, the larger the telescope and the +more perfect the atmospheric conditions at the observer's command, +the smaller do these images appear. On the photographic plate, it is +true, the stars are recorded as measurable disks, but these are due +to the spreading of the light from their bright point-like images, +and their diameters increase as the exposure time is prolonged. +From the images of the brighter stars rays of light project in +straight lines, but these also are instrumental phenomena, due +to diffraction of light by the steel bars that support the small +mirror in the tube of reflecting telescopes. In a word, the stars +are so remote that the largest and most perfect telescopes show +them only as extremely minute needle-points of light, without any +trace of their true disks. + +[Illustration: Fig. 21. Great sun-spot group, August 8, 1917 (Whitney). + +The disk in the corner represents the comparative size of the earth.] + +How, then, may we hope to measure their diameters? By using, as +the man of science must so often do, indirect means when the direct +attack fails. Most of the remarkable progress of astronomy during +the last quarter-century has resulted from the application of new and +ingenious devices borrowed from the physicist. These have multiplied +to such a degree that some of our observatories are literally physical +laboratories, in which the sun and stars are examined by powerful +spectroscopes and other optical instruments that have recently advanced +our knowledge of physics by leaps and bounds. In the present case +we are indebted for our star-measuring device to the distinguished +physicist Professor Albert A. Michelson, who has contributed a long +array of novel apparatus and methods to physics and astronomy. + +THE INTERFEROMETER + +The instrument in question, known as the interferometer, had previously +yielded a remarkable series of results when applied in its various +forms to the solution of fundamental problems. To mention only a +few of those that have helped to establish Michelson's fame, we may +recall that our exact knowledge of the length of the international +metre at Sevres, the world's standard of measurement, was obtained +by him with an interferometer in terms of the invariable length of +light-waves. A different form of interferometer has more recently +enabled him to measure the minute tides within the solid body of the +earth--not the great tides of the ocean, but the slight deformations +of the earth's body, which is as rigid as steel, that are caused by +the varying attractions of the sun and moon. Finally, to mention +only one more case, it was the Michelson-Morley experiment, made +years ago with still another form of interferometer, that yielded +the basic idea from which the theory of relativity was developed +by Lorentz and Einstein. + +[Illustration: Fig. 22. Photograph of the hydrogen atmosphere of +the sun (Ellerman). + +Made with the spectroheliograph, showing the immense vortices, +or whirling storms like tornadoes, that centre in sun-spots. The +comparative size of the earth is shown by the white circle traced +on the largest sun-spot.] + +The history of the method of measuring star diameters is a very +curious one, showing how the most promising opportunities for scientific +progress may lie unused for decades. The fundamental principle +of the device was first suggested by the great French physicist +Fizeau in 1868. In 1874 the theory was developed by the French +astronomer Stéphan, who observed interference fringes given by a +large number of stars, and rightly concluded that their angular +diameters must be much smaller than 0.158 of a second of arc, the +smallest measurable with his instrument. In 1890 Michelson, unaware +of the earlier work, published in the _Philosophical Magazine_ a +complete description of an interferometer capable of determining +with surprising accuracy the distance between the components of +double stars so close together that no telescope can separate them. +He also showed how the same principle could be applied to the +measurement of star diameters if a sufficiently large interferometer +could be built for this purpose, and developed the theory much +more completely than Stéphan had done. A year later he measured +the diameters of Jupiter's satellites by this means at the Lick +Observatory. But nearly thirty years elapsed before the next step +was taken. Two causes have doubtless contributed to this delay. Both +theory and experiment have demonstrated the extreme sensitiveness +of the "interference fringes," on the observation of which the +method depends, and it was generally supposed by astronomers that +disturbances in the earth's atmosphere would prevent them from +being clearly seen with large telescopes. Furthermore, a very large +interferometer, too large to be carried by any existing telescope, +was required for the star-diameter work, though close double stars +could have been easily studied by this device with several of the +large telescopes of the early nineties. But whatever the reasons, +a powerful method of research lay unused. + +The approaching completion of the 100-inch telescope of the Mount +Wilson Observatory led me to suggest to Professor Michelson, before +the United States entered the war, that the method be thoroughly +tested under the favorable atmospheric conditions of Southern +California. He was at that time at work on a special form of +interferometer, designed to determine whether atmospheric disturbances +could be disregarded in planning large-scale experiments. But the +war intervened, and all of our efforts were concentrated for two +years on the solution of war problems.[*] In 1919, as soon as the +100-inch telescope had been completed and tested, the work was +resumed on Mount Wilson. + +[Footnote *: Professor Michelson's most important contribution during +the war period was a new and very efficient form of range-finder, +adopted for use by the U. S. Navy.] + +A LABORATORY EXPERIMENT + +The principle of the method can be most readily seen by the aid +of an experiment which any one can easily perform for himself with +simple apparatus. Make a narrow slit, a few thousandths of an inch +in width, in a sheet of black paper, and support it vertically +before a brilliant source of light. Observe this from a distance of +40 or 50 feet with a small telescope magnifying about 30 diameters. +The object-glass of the telescope should be covered with an opaque +cap, pierced by two circular holes about one-eighth of an inch in +diameter and half an inch apart. The holes should be on opposite +sides of the centre of the object-glass and equidistant from it, +and the line joining the holes should be horizontal. When this +cap is removed the slit appears as a narrow vertical band with +much fainter bands on both sides of it. With the cap in place, the +central bright band appears to be ruled with narrow vertical lines +or fringes produced by the "interference"[*] of the two pencils of +light coming through different parts of the object-glass from the +distant slit. Cover one of the holes, and the fringes instantly +disappear. Their production requires the joint effect of the two +light-pencils. + +[Footnote *: For an explanation of the phenomena of interference, +see any encyclopęedia or book on physics.] + +Now suppose the two holes over the object-glass to be in movable +plates, so that their distance apart can be varied. As they are +gradually separated the narrow vertical fringes become less and +less distinct, and finally vanish completely. Measure the distance +between the holes and divide this by the wavelength of light, which +we may call 1/50000 of an inch. The result is the angular width +of the distant slit. Knowing the distance of the slit, we can at +once calculate its linear width. If for the slit we substitute a +minute circular hole, the method of measurement remains the same, +but the angular diameter as calculated above must be multiplied +by 1.22.[*] + +[Footnote *: More complete details may be found in Michelson's Lowell +Lectures on "Light-Waves and Their Uses," University of Chicago +Press, 1907.] + +To measure the diameter of a star we proceed in a similar way, +but, as the angle it subtends is so small, we must use a very large +telescope, for the smaller the angle the farther apart must be the +two holes over the object-glass (or the mirror, in case a reflecting +telescope is employed). In fact, when the holes are moved apart to +the full aperture of the 100-inch Hooker telescope, the interference +fringes are still visible even with the star Betelgeuse, though its +angular diameter is perhaps as great as that of any other star. +Thus, we must build an attachment for the telescope, so arranged +as to permit us to move the openings still farther apart. + +[Illustration: Fig. 23. Diagram showing outline of the 100-inch +Hooker telescope, and path of the two pencils of light from a star +when under observation with the 20-foot Michelson interferometer. + +A photograph of the interferometer is shown in Fig. 24.] + +THE 20-FOOT INSTRUMENT + +The 20-foot interferometer designed by Messrs. Michelson and Pease, +and constructed in the Mount Wilson Observatory instrument-shop, +is shown in the diagram (Fig. 23) and in a photograph of the upper +end of the skeleton tube of the telescope (Fig. 24). The light from +the star is received by two flat mirrors (Ml, M4) which project +beyond the tube and can be moved apart along the supporting arm. +These take the place of the two holes over the object-glass in +our experiment. From these mirrors the light is reflected to a +second pair of flat mirrors (M2, M3), which send it toward the +100-inch concave mirror (M5) at the bottom of the telescope tube. +After this the course of the light is exactly as it would be if +the mirrors M2, M3 were replaced by two holes over the 100-inch +mirror. It is reflected to the convex mirror (M6), then back in +a less rapidly convergent beam toward the large mirror. Before +reaching it the light is caught by the plane mirror (M7) and reflected +through an opening at the side of the telescope tube to the eye-piece +E. Here the fringes are observed with a magnification ranging from +1,500 to 3,000 diameters. + +[Illustration: Fig. 24. Twenty-foot Michelson interferometer for +measuring star diameters, attached to upper end of the skeleton +tube of the 100-inch Hooker telescope. + +The path of the two pencils of light from the star is shown in +Fig. 23. For a photograph of the entire telescope, see Fig. 4.] + +In the practical application of this method to the measurement of +star diameters, the chief problem was whether the atmosphere would +be quiet enough to permit sharp interference fringes to be produced +with light-pencils more than 100 inches apart. After successful +preliminary tests with the 40-inch refracting telescope of the +Yerkes Observatory, Professor Michelson made the first attempt +to see the fringes with the 60-inch and 100-inch reflectors on +Mount Wilson in September, 1919. He was surprised and delighted to +find that the fringes were perfectly sharp and distinct with the +full aperture of both these instruments. Doctor Anderson, of the +observatory staff, then devised a special form of interferometer +for the measurement of close double stars, and applied it with +the 100-inch telescope to the measurement of the orbital motion +of the close components of Capella, with results of extraordinary +accuracy, far beyond anything attainable by previous methods. The +success of this work strongly encouraged the more ambitious project +of measuring the diameter of a star, and the 20-foot interferometer +was built for this purpose. + +The difficult and delicate problem of adjusting the mirrors of +this instrument with the necessary extreme accuracy was solved by +Professor Michelson during his visit to Mount Wilson in the summer +of 1920, and with the assistance of Mr. Pease, of the observatory +staff, interference fringes were observed in the case of certain +stars when the mirrors were as much as 18 feet apart. All was thus +in readiness for a decisive test as soon as a suitable star presented +itself. + +THE GIANT BETELGEUSE + +Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic +for "the giant's shoulder"), the bright red star in the constellation +of Orion (Fig. 25), as the most favorable of all stars for measurement, +and the last-named had given its angular diameter as 0.051 of a +second of arc. This deduction from theory appeared in his recent +presidential address before the British Association for the Advancement +of Science, in which Professor Eddington remarked: "Probably the +greatest need of stellar astronomy at the present day, in order +to make sure that our theoretical deductions are starting on the +right lines, is some means of measuring the apparent angular diameter +of stars." He then referred to the work already in progress on +Mount Wilson, but anticipated "that atmospheric disturbance will +ultimately set the limit to what can be accomplished." + +[Illustration: Fig. 25. The giant Betelgeuse (within the circle), +familiar as the conspicuous red star in the right shoulder of Orion +(Hubble). + +Measures with the interferometer show its angular diameter to be +0.047 of a second of arc, corresponding to a linear diameter of +215,000,000 miles, if the best available determination of its distance +can be relied upon. This determination shows Betelgeuse to be 160 +light-years from the earth. Light travels at the rate of 186,000 +miles per second, and yet spends 160 years on its journey to us +from this star.] + +On December 13, 1920, Mr. Pease successfully measured the diameter +of Betelgeuse with the 20-foot interferometer. As the outer mirrors +were separated the interference fringes gradually became less distinct, +as theory requires, and as Doctor Merrill had previously seen when +observing Betelgeuse with the interferometer used for Capella. At +a separation of 10 feet the fringes disappeared completely, giving +the data required for calculating the diameter of the star. To +test the perfection of the adjustment, the telescope was turned to +other stars, of smaller angular diameter, which showed the fringes +with perfect clearness. Turning back to Betelgeuse, they were seen +beyond doubt to be absent. Assuming the mean wave-length of the +light of this star to be 5750/10000000 of a millimetre, its angular +diameter comes out 0.047 of a second of arc, thus falling between +the values--0.051 and 0.031 of a second--predicted by Eddington and +Russell from slightly different assumptions. Subsequent corrections +and repeated measurement will change Mr. Pease's result somewhat, +but it is almost certainly within 10 or 15 per cent of the truth. +We may therefore conclude that the angular diameter of Betelgeuse +is very nearly the same as that of a ball one inch in diameter, +seen at a distance of seventy miles. + +[Illustration: Fig. 26. Arcturus (within the white circle), known +to the Arabs as the "Lance Bearer," and to the Chinese as the "Great +Horn" or the "Palace of the Emperors" (Hubble). + +Its angular diameter, measured at Mount Wilson by Pease with the +20-foot Michelson interferometer on April 15, 1921, is 0.022 of a +second, in close agreement with Russell's predicted value of 0.019 +of a second. The mean parallax of Arcturus, based upon several +determinations, is 0.095 of a second, corresponding to a distance of +34 light-years. The linear diameter, computed from Pease's measure +and this value of the distance is about 21 million miles.] + +But this represents only the angle subtended by the star's disk. +To learn its linear diameter, we must know its distance. Four +determinations of the parallax, which determines the distance, +have been made. Elkin, with the Yale heliometer, obtained 0.032 +of a second of arc. Schlesinger, from photographs taken with the +30-inch Allegheny refractor, derived 0.016. Adams, by his spectroscopic +method applied with the 60-inch Mount Wilson reflector, obtained +0.012. Lee's recent value, secured photographically with the 40-inch +Yerkes refractor, is 0.022. The heliometer parallax is doubtless +less reliable than the photographic ones, and Doctor Adams states +that the spectral type and luminosity of Betelgeuse make his value +less certain than in the case of most other stars. If we take a +(weighted) mean value of 0.020 of a second, we shall probably not +be far from the truth. This parallax represents the angle subtended +by the radius of the earth's orbit (93,000,000 miles) at the distance +of Betelgeuse. By comparing it with 0.047, the angular diameter of +the star, we see that the linear diameter is about two and one-third +times as great as the distance from the earth to the sun, or +approximately 215,000,000 miles. Thus, if this measure of its distance +is not considerably in error, Betelgeuse would nearly fill the +orbit of Mars. All methods of determining the distances of the +stars are subject to uncertainty, however, and subsequent measures +may reduce this figure very appreciably. But there can be no doubt +that the diameter of Betelgeuse exceeds 100,000,000 miles, and +it is probably much greater. + +The extremely small angle subtended by this enormous disk is explained +by the great distance of the star, which is about 160 light-years. +That is to say, light travelling at the rate of 186,000 miles per +second spends 160 years in crossing the space that lies between +us and Betelgeuse, whose tremendous proportions therefore seem +so minute even in the most powerful telescopes. + +STELLAR EVOLUTION + +This actual measure of the diameter of Betelgeuse supplies a new +and striking test of Russell's and Hertzsprung's theory of dwarf +and giant stars. Just before the war Russell showed that our old +methods of classifying the stars according to their spectra must +be radically changed. Stars in an early stage of their life history +may be regarded as diffuse gaseous masses, enormously larger than +our sun, and at a much lower temperature. Their density must be +very low, and their state that of a perfect gas. These are the +"giants." In the slow process of time they contract through constant +loss of heat by radiation. But, despite this loss, the heat produced +by contraction and from other sources (see p. 82) causes their +temperature to rise, while their color changes from red to bluish +white. The process of shrinkage and rise of temperature goes on so +long as they remain in the state of a perfect gas. But as soon as +contraction has increased the density of the gas beyond a certain +point the cycle reverses and the temperature begins to fall. The +bluish-white light of the star turns yellowish, and we enter the +dwarf stage, of which our own sun is a representative. The density +increases, surpassing that of water in the case of the sun, and +going far beyond this point in later stages. In the lapse of millions +of years a reddish hue appears, finally turning to deep red. The +falling temperature permits the chemical elements, existing in a +gaseous state in the outer atmosphere of the star, to unite into +compounds, which are rendered conspicuous by their characteristic +bands in the spectrum. Finally comes extinction of light, as the +star approaches its ultimate state of a cold and solid globe. + +[Illustration: Fig. 27. The giant star Antares (within the white +circle), notable for its red color in the constellation Scorpio, +and named by the Greeks "A Rival of Mars" (Hubble). + +The distance of Antares, though not very accurately known, is probably +not far from 350 light-years. Its angular diameter of 0.040 of a +second would thus correspond to a linear diameter of about 400 +million miles.] + +We may thus form a new picture of the two branches of the temperature +curve, long since suggested by Lockyer, on very different grounds, as +the outline of stellar life. On the ascending side are the giants, +of vast dimensions and more diffuse than the air we breathe. There +are good reasons for believing that the mass of Betelgeuse cannot +be more than ten times that of the sun, while its volume is at +least a million times as great and may exceed eight million times +the sun's volume. Therefore, its average density must be like that +of an attenuated gas in an electric vacuum tube. Three-quarters +of the naked-eye stars are in the giant stage, which comprises +such familiar objects as Betelgeuse, Antares, and Aldebaran, but +most of them are much denser than these greatly inflated bodies. +The pinnacle is reached in the intensely hot white stars of the +helium class, in whose spectra the lines of this gas are very +conspicuous. The density of these stars is perhaps one-tenth that +of the sun. Sirius, also very hot, is nearly twice as dense. Then +comes the cooling stage, characterized, as already remarked, by +increasing density, and also by increasing chemical complexity +resulting from falling temperature. This life cycle is probably +not followed by all stars, but it may hold true for millions of +them. + +The existence of giant and dwarf stars has been fully proved by +the remarkable work of Adams and his associates on Mount Wilson, +where his method of determining a star's distance and intrinsic +luminosity by spectroscopic observations has already been applied +to 2,000 stars. Discussion of the results leads at once to the +recognition of the two great classes of giants and dwarfs. Now +comes the work of Michelson and Pease to cap the climax, giving us +the actual diameter of a typical giant star, in close agreement with +predictions based upon theory. From this diameter we may conclude that +the density of Betelgeuse is extremely low, in harmony with Russell's +theory, which is further supported by spectroscopic analysis of +the star's light, revealing evidence of the comparatively low +temperature called for by the theory at this early stage of stellar +existence. + +TWO OTHER GIANTS + +The diameter of Arcturus was successfully measured by Mr. Pease +at Mount Wilson on April 15. As the mirrors of the interferometer +were moved apart, the fringes gradually decreased in visibility +until they finally disappeared at a mirror separation of 19.6 feet. +Adopting a mean wave-length of 5600/10000000 of a millimetre for +the light of Arcturus, this gives a value of 0.022 of a second of +arc for the angular diameter of the star. If we use a mean value +of 0.095 of a second for the parallax, the corresponding linear +diameter comes out 21,000,000 miles. The angular diameter, as in +the case of Betelgeuse, is in remarkably close agreement with the +diameter predicted from theory. Antares, the third star measured +by Mr. Pease, is the largest of all. If it is actually a member of +the Scorpius-Centaurus group, as we have strong reason to believe, +it is fully 350 light-years from the earth, and its diameter is +about 400,000,000 miles. + +[Illustration: Fig. 28. Diameters of the Sun, Arcturus, Betelgeuse, +and Antares compared with the orbit of Mars. + +Sun, diameter, 865,000 miles. + +Arcturus, diameter, 21,000,000 miles. + +Betelgeuse, diameter, 215,000,000 miles. + +Antares, diameter, 400,000,000 miles.] + +It now remains to make further measures of Betelgeuse, especially +because its marked changes in brightness suggest possible variations +in diameter. We must also apply the interferometer method to stars +of the various spectral types, in order to afford a sure basis for +future studies of stellar evolution. Unfortunately, only a few +giant stars are certain to fall within the range of our present +instrument. An interferometer of 70-feet aperture would be needed +to measure Sirius accurately, and one of twice this size to deal +with less brilliant white stars. A 100-foot instrument, if feasible +to build, would permit objects representing most of the chief stages +of stellar development to be measured, thus contributing in the +highest degree to the progress of our knowledge of the life history +of the stars. Fortunately, though the mechanical difficulties are +great, the optical problem is insignificant, and the cost of the +entire apparatus, though necessarily high, would be only a small +fraction of that of a telescope of corresponding aperture, if such +could be built. A 100-foot interferometer might be designed in +many different forms, and one of these may ultimately be found +to be within the range of possibility. Meanwhile the 20-foot +interferometer has been improved so materially that it now promises +to yield approximate measures of stars at first supposed to be +beyond its capacity. + +[Illustration: Fig. 29. Aldebaran, the "leader" (of the Pleiades), +was also known to the Arabs as "The Eye of the Bull," "The Heart +of the Bull," and "The Great Camel" (Hubble). + +Like Betelgeuse and Antares, it is notable for its red color, which +accounts for the fact that its image on this photograph is hardly +more conspicuous than the images of stars which are actually much +fainter but contain a larger proportion of blue light, to which +the photographic plates here employed are more sensitive than to +red or yellow. Aldebaran is about 50 light-years from the earth. +Interferometer measures, now in progress on Mount Wilson, indicate +that its angular diameter is about 0.020 of a second.] + +While the theory of dwarf and giant stars and the measurements just +described afford no direct evidence bearing on Laplace's explanation +of the formation of planets, they show that stars exist which are +comparable in diameter with our solar system, and suggest that the +sun must have shrunk from vast dimensions. The mode of formation +of systems like our own, and of other systems numerously illustrated +in the heavens, is one of the most fascinating problems of astronomy. +Much light has been thrown on it by recent investigations, rendered +possible by the development of new and powerful instruments and by +advances in physics of the most fundamental character. All the +evidence confirms the existence of dwarf and giant stars, but much +work must be done before the entire course of stellar evolution +can be explained. + + + + +CHAPTER III + +COSMIC CRUCIBLES + +"Shelter during Raids," marking the entrance to underground passages, +was a sign of common occurrence and sinister suggestion throughout +London during the war. With characteristic ingenuity and craftiness, +ostensibly for purposes of peace but with bomb-carrying capacity +as a prime specification, the Zeppelin had been developed by the +Germans to a point where it seriously threatened both London and +Paris. Searchlights, range-finders, and anti-aircraft guns, surpassed +by the daring ventures of British and French airmen, would have +served but little against the night invader except for its one +fatal defect--the inflammable nature of the hydrogen gas that kept +it aloft. A single explosive bullet served to transform a Zeppelin +into a heap of scorched and twisted metal. This characteristic +of hydrogen caused the failure of the Zeppelin raids. + +Had the war lasted a few months longer, however, the work of American +scientists would have made our counter-attack in the air a formidable +one. At the signing of the armistice hundreds of cylinders of compressed +helium lay at the docks ready for shipment abroad. Extracted from +the natural gas of Texas wells by new and ingenious processes, +this substitute for hydrogen, almost as light and absolutely +uninflammable, produced in quantities of millions of cubic feet, +would have made the dirigibles of the Allies masters of the air. The +special properties of this remarkable gas, previously obtainable only +in minute quantities, would have sufficed to reverse the situation. + +SOLAR HELIUM + +Helium, as its name implies, is of solar origin. In 1868, when +Lockyer first directed his spectroscope to the great flames or +prominences that rise thousands of miles, sometimes hundreds of +thousands, above the surface of the sun, he instantly identified +the characteristic red and blue radiations of hydrogen. In the +yellow, close to the position of the well-known double line of +sodium, but not quite coincident with it, he detected a new line, +of great brilliancy, extending to the highest levels. Its similarity +in this respect with the lines of hydrogen led him to recognize +the existence of a new and very light gas, unknown to terrestrial +chemistry. + +Many years passed before any chemical laboratory on earth was able +to match this product of the great laboratory of the sun. In 1896 +Ramsay at last succeeded in separating helium, recognized by the same +yellow line in its spectrum, in minute quantities from the mineral +uraninite. Once available for study under electrical excitation in +vacuum tubes, helium was found to have many other lines in its +spectrum, which have been identified in the spectra of solar +prominences, gaseous nebulę, and hot stars. Indeed, there is a +stellar class known as helium stars, because of the dominance of +this gas in their atmospheres. + +[Illustration: Fig. 30. Solar prominences, photographed with the +spectroheliograph without an eclipse (Ellerman). + +In these luminous gaseous clouds, which sometimes rise to elevations +exceeding half the sun's diameter, the new gas helium was discovered +by Lockyer in 1868. Helium was not found on the earth until 1896. +Since then it has been shown to be a prominent constituent of nebulę +and hot stars.] + +The chief importance of helium lies in the clue it has afforded to +the constitution of matter and the transmutation of the elements. +Radium and other radioactive substances, such as uranium, spontaneously +emit negatively charged particles of extremely small mass (electrons), +and also positively charged particles of much greater mass, known +as alpha particles. Rutherford and Geiger actually succeeded in +counting the number of alpha particles emitted per second by a +known mass of radium, and showed that these were charged helium +atoms. + +To discuss more at length the extraordinary characteristics of +helium, which plays so large a part in celestial affairs, would +take us too far afield. Let us therefore pass to another case in +which a fundamental discovery, this time in physics, was first +foreshadowed by astronomical observation. + +SUN-SPOTS AS MAGNETS + +No archęologist, whether Young or Champollion deciphering the Rosetta +Stone, or Rawlinson copying the cuneiform inscription on the cliff +of Behistun, was ever faced by a more fascinating problem than that +which confronts the solar physicist engaged in the interpretation +of the hieroglyphic lines of sun-spot spectra. The colossal whirling +storms that constitute sun-spots, so vast that the earth would make +but a moment's scant mouthful for them, differ materially from +the general light of the sun when examined with the spectroscope. +Observing them visually many years ago, the late Professor Young, +of Princeton, found among their complex features a number of double +lines which he naturally attributed, in harmony with the physical +knowledge of the time, to the effect of "reversal" by superposed +layers of vapors of different density and temperature. What he +actually saw, however, as was proved at the Mount Wilson Observatory +in 1908, was the effect of a powerful magnetic field on radiation, +now known as the Zeeman effect. + +[Illustration: Fig. 31. The 150-foot tower telescope of the Mount +Wilson Observatory. + +An image of the sun about 16 inches in diameter is formed in the +laboratory at the base of the tower. Below this, in a well extending +80 feet into the earth, is the powerful spectroscope with which +the magnetic fields in sun-spots and the general magnetic field +of the sun are studied.] + +Faraday was the first to detect the influence of magnetism on light. +Between the poles of a large electromagnet, powerful for those +days (1845), he placed a block of very dense glass. The plane of +polarization of a beam of light, which passed unaffected through +the glass before the switch was closed, was seen to rotate when the +magnetic field was produced by the flow of the current. A similar +rotation is now familiar in the well-known tests of sugars--lęvulose +and dextrose--which rotate plane-polarized light to left and right, +respectively. + +But in this first discovery of a relationship between light and +magnetism Faraday had not taken the more important step that he +coveted--to determine whether the vibration period of a light-emitting +particle is subject to change in a magnetic field. He attempted +this in 1862--the last experiment of his life. A sodium flame was +placed between the poles of a magnet, and the yellow lines were +watched in a spectroscope when the magnet was excited. No change +could be detected, and none was found by subsequent investigators +until Zeeman, of Leiden, with more powerful instruments made his +famous discovery, the twenty-fifth anniversary of which has recently +been celebrated. + +[Illustration: Fig. 32. Pasadena Laboratory of the Mount Wilson +Observatory. + +Showing the large magnet (on the left) and the spectroscopes used +for the study of the effect of magnetism on radiation. A single line +in the spectrum is split by the magnetic field into from three to +twenty-one components, as illustrated in Fig. 34. The corresponding +lines in the spectra of sun-spots are split up in precisely the +same way, thus indicating the presence of powerful magnetic fields +in the sun.] + +His method of procedure was similar to Faraday's, but his magnet and +spectroscope were much more powerful, and a theory due to Lorentz, +predicting the nature of the change to be expected, was available +as a check on his results. When the current was applied the lines +were seen to widen. In a still more powerful magnetic field each +of them split into two components (when the observation was made +along the lines of force), and the light of the components of each +line was found to be circularly polarized in opposite directions. +Strictly in harmony with Lorentz's theory, this splitting and +polarization proved the presence in the luminous vapor of exactly such +negatively charged electrons as had been indicated there previously +by very different experimental methods. + +In 1908 great cyclonic storms, or vortices, were discovered at +the Mount Wilson Observatory centring in sun-spots. Such whirling +masses of hot vapors, inferred from Sir Joseph Thomson's results +to contain electrically charged particles, should give rise to a +magnetic field. This hypothesis at once suggested that the double +lines observed by Young might really represent the Zeeman effect. +The test was made, and all the characteristic phenomena of radiation +in a magnetic field were found. + +Thus a great physical experiment is constantly being performed +for us in the sun. Every large sunspot contains a magnetic field +covering many thousands of square miles, within which the spectrum +lines of iron, manganese, chromium, titanium, vanadium, calcium, +and other metallic vapors are so powerfully affected that their +widening and splitting can be seen with telescopes and spectroscopes +of moderate size. + +THE TOWER TELESCOPE + +Both of these illustrations show how the physicist and chemist, +when adequately armed for astronomical attack, can take advantage +in their studies of the stupendous processes visible in cosmic +crucibles, heated to high temperatures and influenced, as in the +case of sun-spots, by intense magnetic fields. Certain modern +instruments, like the 60-foot and 150-foot tower telescopes on +Mount Wilson, are especially designed for observing the course +of these experiments. The second of these telescopes produces at +a fixed point in a laboratory an image of the sun about 16 inches +in diameter, thus enlarging the sun-spots to such a scale that +the magnetic phenomena of their various parts can be separately +studied. This analysis is accomplished with a spectroscope 80 feet +in length, mounted in a subterranean chamber beneath the tower. The +varied results of such investigations cannot be described here. +Only one of them may be mentioned--the discovery that the entire sun, +rotating on its axis, is a great magnet. Hence we may reasonably +infer that every star, and probably every planet, is also a magnet, +as the earth has been known to be since the days of Gilbert's "De +Magnete." Here lies one of the best clues for the physicist who +seeks the cause of magnetism, and attempts to produce it, as Barnett +has recently succeeded in doing, by rapidly whirling masses of +metal in the laboratory. + +[Illustration: Fig. 33. Sun-spot vortex in the upper hydrogen +atmosphere. (Benioff). + +Photographed with the spectroheliograph. The electric vortex that +causes the magnetic field of the spot lies at a lower level, and +is not shown by such photographs.] + +Perhaps a word of caution should be interpolated at this point. +Solar magnetism in no wise accounts for the sun's gravitational +power. Indeed, its attraction cannot be felt by the most delicate +instruments at the distance of the earth, and would still be unknown +were it not for the influence of magnetism on light. + +Auroras, magnetic storms, and such electric currents as those that +recently deranged several Atlantic cables are due, not to the magnetism +of the sun or its spots, but probably to streams of electrons, shot +out from highly disturbed areas of the solar surface surrounding +great sun-spots, traversing ninety-three million miles of the ether +of space, and penetrating deep into the earth's atmosphere. These +striking phenomena lead us into another chapter of physics, which +limitations of space forbid us to pursue. + +STELLAR CHEMISTRY + +Let us turn again to chemistry, and see where experiments performed +in cosmic laboratories can serve as a guide to the investigator. +A spinning solar tornado, incomparably greater in scale than the +devastating whirlwinds that so often cut narrow paths of destruction +through town and country in the Middle West, gradually gives rise +to a sun-spot. The expansion produced by the centrifugal force at +the centre of the storm cools the intensely hot gases of the solar +atmosphere to a point where chemical union can occur. Titanium +and oxygen, too hot to combine in most regions of the sun, join +to form the vapor of titanium oxide, characterized in the sunspot +spectrum by fluted bands, made up of hundreds of regularly spaced +lines. Similarly magnesium and hydrogen combine as magnesium hydride +and calcium and hydrogen form calcium hydride. None of these compounds, +stable at the high temperatures of sun-spots, has been much studied +in the laboratory. The regions in which they exist, though cooler +than the general atmosphere of the sun, are at temperatures of +several thousand degrees, attained in our laboratories only with +the aid of such devices as powerful electric furnaces. + +[Illustration: Fig. 34. Splitting of spectrum lines by a magnetic +field (Babcock). + +The upper and lower strips show lines in the spectrum of chromium, +observed without a magnetic field. When subjected to the influence +of magnetism, these single lines are split into several components. +Thus the first line on the right is resolved by the field into +three components, one of which (plane polarized) appears in the +second strip, while the other two, which are polarized in a plane +at right angles to that of the middle component, are shown on the +third strip. The next line is split by the magnetic field into +twelve components, four of which appear in the second strip and +eight in the third. The magnetic fields in sun-spots affect these +lines in precisely the same way.] + +It is interesting to follow our line of reasoning to the stars, +which differ widely in temperature at various stages in their +life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely +hot solar vapors are cooled by expansion, giving rise to the compounds +already named. A red star, in Russell's scheme of stellar evolution, +is a cooler sun, vast in volume and far more tenuous than atmospheric +air when in the initial period of the "giant" stage, but compressed +and denser than water in the "dwarf" stage, into which our sun has +already entered as it gradually approaches the last phases of its +existence. Therefore we should find, throughout the entire atmosphere +of such stars, some of the same compounds that are produced within +the comparatively small limits of a sun-spot. This, of course, +on the correct assumption that sun and stars are made of the same +substances. Fowler has already identified the bands of titanium +oxide in such red stars as the giant Betelgeuse, and in others +of its class. It is safe to predict that an interesting chapter +in the chemistry of the future will be based upon the study of +such compounds, both in the laboratory and under the progressive +temperature conditions afforded by the countless stellar "giants" +and "dwarfs" that precede and follow the solar state. + +[Footnote *: See Chapter II.] + +[Illustration: Fig. 35. Electric furnace in the Pasadena laboratory +of the Mount Wilson Observatory. + +With which the chemical phenomena observed in sun-spots and red +stars are experimentally imitated.] + +ASTROPHYSICAL LABORATORIES + +It is precisely in this long sequence of physical and chemical +changes that the astrophysicist and the astrochemist can find the +means of pushing home their attack. It is true, of course, that +the laboratory investigator has a great advantage in his ability +to control his experiments, and to vary their progress at will. +But by judicious use of the transcendental temperatures, far out +ranging those of his furnaces, and extreme conditions, which he +can only partially imitate, afforded by the sun, stars, and nebulę, +he may greatly widen the range of his inquiries. The sequence of +phenomena seen during the growth of a sun-spot, or the observation +of spots of different sizes, and the long series of successive +steps that mark the rise and decay of stellar life, resemble the +changes that the experimenter brings about as he increases and +diminishes the current in the coils of his magnet or raises and +lowers the temperature of his electric furnace, examining from +time to time the spectrum of the glowing vapors, and noting the +changes shown by the varying appearance of their lines. + +[Illustration: Fig. 36. Titanium oxide in red stars. + +The upper spectrum is that of titanium in the flame of the electric +arc, where its combination with oxygen gives rise to the bands of +titanium oxide (Fowler). The lower strip shows the spectrum of +the red star Mira (Omicron Ceti), as drawn by Cortie at Stonyhurst. +The bands of titanium oxide are clearly present in the star.] + +[Illustration: Fig. 37. Titanium oxide in sun-spots. + +The upper strip shows a portion of the spectrum of a sun-spot +(Ellerman); the lower one the corresponding region of the spectrum +of titanium oxide (King). The fluted bands of the oxide spectrum +are easily identified in the spot, where they indicate that titanium +and oxygen, too hot to combine in the solar atmosphere, unite in the +spot because of the cooling produced by expansion in the vortex.] + +Astronomical observations of this character, it should be noted, are +most effective when constantly tested and interpreted by laboratory +experiment. Indeed, a modern astrophysical observatory should be +equipped like a great physical laboratory, provided on the one hand +with telescopes and accessory apparatus of the greatest attainable +power, and on the other with every device known to the investigator +of radiation and the related physical and chemical phenomena. Its +telescopes, especially designed with the aims of the physicist and +chemist in view, bring images of sun, stars, nebulę, and other +heavenly bodies within the reach of powerful spectroscopes, sensitive +bolometers and thermopiles, and the long array of other appliances +available for the measurement and analysis of radiation. Its electric +furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing +and decreasing pressure, varying chemical conditions, and subjecting +luminous gases and vapors to the influence of electric and magnetic +fields, provide the means of imitating celestial phenomena, and of +repeating and interpreting the experiments observed at the telescope. +And the advantage thus derived, as we have seen, is not confined +to the astronomer, who has often been able, by making fundamental +physical and chemical discoveries, to repay his debt to the physicist +and chemist for the apparatus and methods which he owes to them. + +NEWTON AND EINSTEIN + +Take, for another example, the greatest law of physics--Newton's +law of gravitation. Huge balls of lead, as used by Cavendish, produce +by their gravitational effect a minute rotation of a delicately +suspended bar, carrying smaller balls at its extremities. But no +such feeble means sufficed for Newton's purpose. To prove the law +of gravitation he had recourse to the tremendous pull on the moon +of the entire mass of the earth, and then extended his researches +to the mutual attractions of all the bodies of the solar system. +Later Herschel applied this law to the suns which constitute double +stars, and to-day Adams observes from Mount Wilson stars falling +with great velocity toward the centre of the galactic system under +the combined pull of the millions of objects that compose it. Thus +full advantage has been taken of the possibility of utilizing the +great masses of the heavenly bodies for the discovery and application +of a law of physics and its reciprocal use in explaining celestial +motions. + +[Illustration: Fig. 38. The Cavendish experiment. + +Two lead balls, each two inches in diameter, are attached to the +ends of a torsion rod six feet long, which is suspended by a fine +wire. The experiment consists in measuring the rotation of the +suspended system, caused by the gravitational attraction of two +lead spheres, each twelve inches in diameter, acting on the two +small lead balls.] + +Or consider the Einstein theory of relativity, the truth or falsity +of which is no less fundamental to physics. Its inception sprang from +the Michelson-Morley experiment, made in a laboratory in Cleveland, +which showed that motion of the earth through the ether of space could +not be detected. All of the three chief tests of Einstein's general +theory are astronomical--because of the great masses required to +produce the minute effects predicted: the motion of the perihelion +of Mercury, the deflection of the light of a star by the attraction +of the sun, and the shift of the lines of the solar spectrum toward +the red--questions not yet completely answered. + +But it is in the study of the constitution of matter and the evolution +of the elements, the deepest and most critical problem of physics +and chemistry, that the extremes of pressure and temperature in the +heavenly bodies, and the prevalence of other physical conditions not +yet successfully imitated on earth, promise the greatest progress. +It fortunately happens that astrophysical research is now at the +very apex of its development, founded as it is upon many centuries +of astronomical investigation, rejuvenated by the introduction +into the observatory of all the modern devices of the physicist, +and strengthened with instruments of truly extraordinary range +and power. These instruments bring within reach experiments that +are in progress on some minute region of the sun's disk, or in +some star too distant even to be glimpsed with ordinary telescopes. +Indeed, the huge astronomical lenses and mirrors now available +serve for these remote light-sources exactly the purpose of the +lens or mirror employed by the physicist to project upon the slit +of his spectroscope the image of a spark or arc or vacuum tube +within which atoms and molecules are exposed to the influence of +the electric discharge. The physicist has the advantage of complete +control over the experimental conditions, while the astrophysicist +must observe and interpret the experiments performed for him in +remote laboratories. In actual practice, the two classes of work +must be done in the closest conjunction, if adequate utilization +is to be made of either. And this is only natural, for the trend +of recent research has made clear the fact that one of the three +greatest problems of modern astronomy and astrophysics, ranking +with the structure of the universe and the evolution of celestial +bodies, is the constitution of matter. Let us see why this is so. + +TRANSMUTATION OF THE ELEMENTS + +The dream of the alchemist was to transmute one element into another, +with the prime object of producing gold. Such transmutation has been +actually accomplished within the last few years, but the process +is invariably one of disintegration--the more complex elements +being broken up into simpler constituents. Much remains to be done +in this same direction; and here the stars and nebulę, which show +the spectra of the elements under a great variety of conditions, +should help to point the way. The progressive changes in spectra, +from the exclusive indications of the simple elements hydrogen, +helium, nitrogen, possibly carbon, and the terrestrially unknown +gas nebulium in the gaseous nebulę, to the long list of familiar +substances, including several chemical compounds, in the red stars, +may prove to be fundamentally significant when adequately studied +from the standpoint of the investigator of atomic structure. The +existing evidence seems to favor the view, recently expressed by +Saha, that many of these differences are due to varying degrees +of ionization, the outer electrons of the atoms being split off +by high temperature or electrical excitation. It is even possible +that cosmic crucibles, unrivalled by terrestrial ones, may help +materially to reveal the secret of the formation of complex elements +from simpler ones. Physicists now believe that all of the elements are +compounded of hydrogen atoms, bound together by negative electrons. +Thus helium is made up of four hydrogen atoms, yet the atomic weight +of helium (4) is less than four times that of hydrogen (1.008). +The difference may represent the mass of the electrical energy +released when the transmutation occurred. + +[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey). + +The gas "nebulium," not yet found on the earth, is the most +characteristic constituent of irregular nebulę. Nebulium is recognized +by two green lines in its spectrum, which cause the green color of +nebulę of the gaseous type.] + +Eddington has speculated in a most interesting way on this possible +source of stellar heat in his recent presidential address before the +British Association for the Advancement of Science (see _Nature_, +September 2, 1920). He points out that the old contraction hypothesis, +according to which the source of solar and stellar heat was supposed +to reside in the slow condensation of a radiating mass of gas under +the action of gravity, is wholly inadequate to explain the observed +phenomena. If the old view were correct, the earlier history of +a star, from the giant stage of a cool and diaphanous gas to the +period of highest temperature, would be run through within eighty +thousand years, whereas we have the best of evidence that many +thousands of centuries would not suffice. Some other source of energy +is imperatively needed. If 5 per cent of a star's mass consists +originally of hydrogen atoms, which gradually combine in the slow +process of time to form more complex elements, the total heat thus +liberated would more than suffice to account for all demands, and +it would be unnecessary to assume the existence of any other source +of heat. + +[Illustration: Fig. 40. Spiral nebula in Ursa Major (Ritchey). + +Luminous matter, in every variety of physical and chemical state, +is available for study in the most diverse celestial objects, from +the spiral and irregular nebulę through all the types of stars. +Doctor van Maanen's measures of the Mount Wilson photographs indicate +outward motion along the arms of spiral nebulę, while the spectroscope +shows them to be whirling at enormous velocities.] + +COSMIC PRESSURES + +This, it may fairly be said, is very speculative, but the fact +remains that celestial bodies appear to be the only places in which +the complex elements may be in actual process of formation from their +known source--hydrogen. At least we may see what a vast variety +of physical conditions these cosmic crucibles afford. At one end of +the scale we have the excessively tenuous nebulę, the luminosity of +which, mysterious in its origin, resembles the electric glow in our +vacuum tubes. Here we can detect only the lightest and simplest of +the elements. In the giant stars, also extremely tenuous (the density +of Betelgeuse can hardly exceed one-thousandth of an atmosphere) we +observe the spectra of iron, manganese, titanium, calcium, chromium, +magnesium, vanadium, and sodium, in addition to titanium oxide. +The outer part of these bodies, from which light reaches us, must +therefore be at a temperature of only a few thousand degrees, but +vastly higher temperatures must prevail at their centres. In passing +up the temperature curve more and more elements appear, the surface +temperature rises, and the internal temperature may reach millions +of degrees. At the same time the pressure within must also rise, +reaching enormous figures in the last stages of stellar life. Cook +has calculated that the pressure at the centre of the earth is +between 4,000 and 10,000 tons per square inch, and this must be +only a very small fraction of that attained within larger celestial +bodies. Jeans has computed the pressure at the centre of two colliding +stars as they strike and flatten, and finds it may be of the order +of 1,000,000,000 tons per square inch--sufficient, if their diameter +be equal to that of the sun--to vaporize them 100,000 times over. + +Compare these pressures with the highest that can be produced on +earth. If the German gun that bombarded Paris were loaded with a +solid steel projectile of suitable dimensions, a muzzle velocity +of 6,000 feet per second could be reached. Suppose this to be fired +into a tapered hole in a great block of steel. The instantaneous +pressure, according to Cook, would be about 7,000 tons per square +inch, only 1/150000 of that possible through the collision of the +largest stars. + +[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson. + +Michelson is measuring the velocity of light between stations on +Mount Wilson and Mount San Antonio. Astronomical observations afford +the best means, however, of detecting any possible difference between +the velocities of light of different colors. From studies of variable +stars in the cluster Messier 5 Shapley concludes that if there is +any difference between the velocities of blue and yellow light +in free space it cannot exceed two inches in one second, the time +in which light travels 186,000 miles.] + +Finally, we may compare the effects of light pressure on the earth +and stars. Twenty years ago Nichols and Hull succeeded, with the +aid of the most sensitive apparatus, in measuring the minute +displacements produced by the pressure of light. The effect is +so slight, even with the brightest light-sources available, that +great experimental skill is required to measure it. Yet in the +case of some of the larger stars Eddington calculates that one-half +of their mass is supported by radiation pressure, and this against +their enormous gravitational attraction. In fact, if their mass +were as great as ten times that of the sun, the radiation pressure +would so nearly overcome the pull of gravitation that they would +be likely to break up. + +But enough has been said to illustrate the wide variety of experimental +devices that stand at our service in the laboratories of the heavens. +Here the physicist and chemist of the future will more and more +frequently supplement their terrestrial apparatus, and find new +clues to the complex problems which the amazing progress of recent +years has already done so much to solve. + +PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER + +The layman has no difficulty in recognizing the practical value +of researches directed toward the improvement of the incandescent +lamp or the increased efficiency of the telephone. He can see the +results in the greatly decreased cost of electric illumination +and the rapid extension of the range of the human voice. But the +very men who have made these advances, those who have succeeded +beyond all expectation in accomplishing the economic purposes in +view, are most emphatic in their insistence upon the importance +of research of a more fundamental character. Thus Vice-President +J. J. Carty, of the American Telephone and Telegraph Company, who +directs its great Department of Development and Research, and Doctor +W. J. Whitney, Director of the Research Laboratory of the General +Electric Company, have repeatedly expressed their indebtedness +to the investigations of the physicist, made with no thought of +immediate practical return. Faraday, studying the laws of electricity, +discovered the principle which rendered the dynamo possible. Maxwell, +Henry, and Hertz, equally unconcerned with material advantage, +made wireless telegraphy practicable. In fact, all truly great +advances are thus derived from fundamental science, and the future +progress of the world will be largely dependent upon the provision +made for scientific research, especially in the fields of physics +and chemistry, which underlie all branches of engineering. + +The constitution of matter, therefore, instead of appealing as +a subject to research only to the natural philosopher or to the +general student of science, is a question of the greatest practical +concern. Already the by-products of investigations directed toward +its elucidation have been numerous and useful in the highest degree. +Helium has been already cited; X-rays hardly require mention; radium, +which has so materially aided sufferers from cancer, is still better +known. Wireless telephony and transcontinental telephony with wires +were both rendered possible by studies of the nature of the electric +discharge in vacuum tubes. Thus the "practical man," with his distrust +of "pure" science, need not resent investments made for the purpose +of advancing our knowledge of such fundamental subjects as physics +and chemistry. On the contrary, if true to his name, he should +help to multiply them many fold in the interest of economic and +commercial development. + + + + + + +End of the Project Gutenberg EBook of The New Heavens, by George Ellery Hale + +*** END OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + +***** This file should be named 19395-8.txt or 19395-8.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/1/9/3/9/19395/ + +Produced by Robert J. 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You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The New Heavens + +Author: George Ellery Hale + +Release Date: September 28, 2006 [EBook #19395] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + + + + +Produced by Robert J. Hall + + + + + +</pre> + + +<div class="image" style="width: 404px;"> +<a name="fig_01"> +<img src="images/fig_01.jpg" width="404" height="553" alt="Fig. 1"></a> +<p class="image">Fig. 1. The Constellation of Orion (Hubble).</p> + +<p class="smaller"> +Photographed with a small camera lens of 1 inch aperture and 5 +inches focal length. The three bright stars in the centre of the +picture form the belt of Orion. Just below, in the sword handle, is +an irregular white patch about one-eighth of an inch in diameter. +This is a small-scale image of the great nebula in Orion, shown +on a larger scale in Fig. 2.</p> +</div> + +<h1>THE NEW HEAVENS</h1> + +<p class="center" style="font-size: smaller;">BY</p> + +<p class="author">GEORGE ELLERY HALE</p> + +<p class="center" style="font-size: smaller; margin-left: 10%; + margin-right: 10%;">DIRECTOR OF THE MOUNT + WILSON OBSERVATORY OF THE CARNEGIE INSTITUTION OF WASHINGTON</p> + +<p class="center" style="margin: 2em;">WITH NUMEROUS ILLUSTRATIONS</p> + +<p class="center"> +NEW YORK<br /> +CHARLES SCRIBNER'S SONS<br /> +1922 +</p> + +<p class="dedication">TO MY WIFE</p> + +<h2>PREFACE</h2> + +<p class="indent"> +Fourteen years ago, in a book entitled "The Study of Stellar Evolution" +(University of Chicago Press, 1908), I attempted to give in untechnical +language an account of some modern methods of astrophysical research. +This book is now out of print, and the rapid progress of science has +left it completely out of date. As I have found no opportunity to +prepare a new edition, or to write another book of similar purpose, +I have adopted the simpler expedient of contributing occasional +articles on recent developments to <i>Scribner's Magazine</i>, +three of which are included in the present volume. +</p> + +<p class="indent"> +I am chiefly indebted, for the illustrations, to the Mount Wilson +Observatory and the present and former members of its staff whose +names appear in the captions. Special thanks are due to Mr. Ferdinand +Ellerman, who made all of the photographs of the observatory buildings +and instruments, and prepared all material for reproduction. The +cut of the original Cavendish apparatus is copied from the +<i>Philosophical Transactions for 1798</i> with the kind permission +of the Royal Society, and I am also indebted to the Royal Society +and to Professor Fowler and Father Cortie for the privilege of +reproducing from the <i>Proceedings</i> two illustrations of their +spectroscopic results. +</p> + +<p class="indent">G. E. H.</p> + +<p>January, 1922.</p> + +<h2>CONTENTS</h2> + +<table> + <tr><td>CHAPTER</td></tr> + <tr><td class="right">I.</td> + <td><a href="#chapter_I">THE NEW HEAVENS</a></td></tr> + <tr><td class="right">II.</td> + <td><a href="#chapter_II">GIANT STARS</a></td></tr> + <tr><td class="right">III.</td> + <td><a href="#chapter_III">COSMIC CRUCIBLES</a></td></tr> +</table> + +<h2>ILLUSTRATIONS</h2> + +<p>FIG.</p> +<ol> + <li><a href="#fig_01">The Constellation of Orion (Hubble)</a></li> + <li><a href="#fig_02">The Great Nebula in Orion (Pease)</a></li> + <li><a href="#fig_03">Model by Ellerman of summit of Mount Wilson, + showing the observatory buildings among the trees and bushes</a></li> + <li><a href="#fig_04">The 100-inch Hooker telescope</a></li> + <li><a href="#fig_05">Erecting the polar axis of the 100-inch + telescope</a></li> + <li><a href="#fig_06">Lowest section of tube of 100-inch telescope, + ready to leave Pasadena for Mount Wilson</a></li> + <li><a href="#fig_07">Section of a steel girder for dome covering + the 100-inch telescope, on its way up Mount Wilson</a></li> + <li><a href="#fig_08">Erecting the steel building and revolving dome + that cover the Hooker telescope</a></li> + <li><a href="#fig_09">Building and revolving dome, 100 feet in + diameter, covering the 100-inch Hooker telescope</a></li> + <li><a href="#fig_10">One-hundred-inch mirror, just silvered, rising + out of the silvering-room in pier before attachment to lower end + of telescope tube. (Seen above)</a></li> + <li><a href="#fig_11">The driving-clock and worm-gear that cause the + 100-inch Hooker telescope to follow the stars</a></li> + <li><a href="#fig_12">Large irregular nebula and star cluster in + Sagittarius (Duncan)</a></li> + <li><a href="#fig_13">Faint spiral nebula in the constellation of the + Hunting Dogs (Pease)</a></li> + <li><a href="#fig_14">Spiral nebula in Andromeda, seen edge on + (Ritchey)</a></li> + <li><a href="#fig_15">Photograph of the moon made on September 15, + 1919, with the 100-inch Hooker telescope (Pease)</a></li> + <li><a href="#fig_16">Photograph of the moon made on September 15, + 1919, with the 100-inch Hooker telescope (Pease)</a></li> + <li><a href="#fig_17">Hubble's Variable Nebula. One of the few + nebulæ known to vary in brightness and form</a></li> + <li><a href="#fig_18">Ring Nebula in Lyra, photographed with the + 60-inch (Ritchey) and 100-inch (Duncan) telescopes</a></li> + <li><a href="#fig_19">Gaseous prominence at the sun's limb, 140,000 + miles high (Ellerman)</a></li> + <li><a href="#fig_20">The sun, 865,000 miles in diameter, from a + direct photograph showing many sun-spots (Whitney)</a></li> + <li><a href="#fig_21">Great sun-spot group, August 8, 1917 + (Whitney)</a></li> + <li><a href="#fig_22">Photograph of the hydrogen atmosphere of the + sun (Ellerman)</a></li> + <li><a href="#fig_23">Diagram showing outline of the 100-inch Hooker + telescope, and path of the two pencils of light from a star when + under observation with the 20-foot Michelson interferometer</a></li> + <li><a href="#fig_24">Twenty-foot Michelson interferometer for + measuring star diameters, attached to upper end of the skeleton + tube of the 100-inch Hooker telescope</a></li> + <li><a href="#fig_25">The giant Betelgeuse (within the circle), + familiar as the conspicuous red star in the right shoulder of Orion + (Hubble)</a></li> + <li><a href="#fig_26">Arcturus (within the white circle), known to + the Arabs as the "Lance Bearer," and to the Chinese as the "Great + Horn" or the "Palace of the Emperors" (Hubble)</a></li> + <li><a href="#fig_27">The giant star Antares (within the white circle), + notable for its red color in the constellation Scorpio, and named by + the Greeks "A Rival of Mars" (Hubble)</a></li> + <li><a href="#fig_28">Diameters of the Sun, Arcturus, Betelgeuse, and + Antares compared with the orbit of Mars</a></li> + <li><a href="#fig_29">Aldebaran, the "leader" (of the Pleiades), was + also known to the Arabs as "The Eye of the Bull," "The Heart of the + Bull," and "The Great Camel" (Hubble)</a></li> + <li><a href="#fig_30">Solar prominences, photographed with the + spectroheliograph without an eclipse (Ellerman)</a></li> + <li><a href="#fig_31">The 150-foot tower telescope of the Mount Wilson + Observatory</a></li> + <li><a href="#fig_32">Pasadena Laboratory of the Mount Wilson + Observatory</a></li> + <li><a href="#fig_33">Sun-spot vortex in the upper hydrogen atmosphere + (Benioff)</a></li> + <li><a href="#fig_34">Splitting of spectrum lines by a magnetic field + (Bacock)</a></li> + <li><a href="#fig_35">Electric furnace in the Pasadena Laboratory of + the Mount Wilson Observatory</a></li> + <li><a href="#fig_36">Titanium oxide in red stars</a></li> + <li><a href="#fig_37">Titanium oxide in sun-spots</a></li> + <li><a href="#fig_38">The Cavendish experiment</a></li> + <li><a href="#fig_39">The Trifid Nebula in Sagittarius (Ritchey)</a></li> + <li><a href="#fig_40">Spiral nebula in Ursa Major (Ritchey)</a></li> + <li><a href="#fig_41">Mount San Antonio as seen from Mount + Wilson</a></li> +</ol> + +<h2><a name="chapter_I">CHAPTER I</a></h2> + +<p class="subtitle">THE NEW HEAVENS</p> + +<p class="indent"> +Go out under the open sky, on a clear and moon-less night, and try +to count the stars. If your station lies well beyond the glare of +cities, which is often strong enough to conceal all but the brighter +objects, you will find the task a difficult one. Ranging through +the six magnitudes of the Greek astronomers, from the brilliant +Sirius to the faintest perceptible points of light, the stars are +scattered in great profusion over the celestial vault. Their number +seems limitless, yet actual count will show that the eye has been +deceived. In a survey of the entire heavens, from pole to pole, +it would not be possible to detect more than from six to seven +thousand stars with the naked eye. From a single viewpoint, even +with the keenest vision, only two or three thousand can be seen. +So many of these are at the limit of visibility that Ptolemy's +"Almagest," a catalogue of all the stars whose places were measured +with the simple instruments of the Greek astronomers, contains +only 1,022 stars. +</p> + +<p class="indent"> +Back of Ptolemy, through the speculations of the Greek philosophers, +the mysteries of the Egyptian sun-god, and the observations of the +ancient Chaldeans, the rich and varied traditions of astronomy stretch +far away into a shadowy past. All peoples, in the first stirrings +of their intellectual youth, drawn by the nightly splendor of the +skies and the ceaseless motions of the planets, have set up some +system of the heavens, in which the sense of wonder and the desire +for knowledge were no less concerned than the practical necessities +of life. The measurement of time and the needs of navigation have +always stimulated astronomical research, but the intellectual demand +has been keen from the first. Hipparchus and the Greek astronomers +of the Alexandrian school, shaking off the vagaries of magic and +divination, placed astronomy on a scientific basis, though the +reaction of the Middle Ages caused even such a great astronomer +as Tycho Brahe himself to revert for a time to the practice of +astrology. +</p> + +<h3>EARLY INSTRUMENTS</h3> + +<p class="indent"> +The transparent sky of Egypt, rarely obscured by clouds, greatly +favored Ptolemy's observations. Here was prepared his great star +catalogue, based upon the earlier observations of Hipparchus, and +destined to remain alone in its field for more than twelve centuries, +until Ulugh Bey, Prince of Samarcand, repeated the work of his +Greek predecessor. Throughout this period the stars were looked +upon mainly as points of reference for the observation of planetary +motions, and the instruments of observation underwent little change. +The astrolabe, which consists of a circle divided into degrees, +with a rotating diametral arm for sighting purposes, embodies their +essential principle. In its simple form, the astrolabe was suspended +in a vertical plane, and the stars were observed by bringing the +sights on the movable diameter to bear upon them. Their altitude +was then read off on the circle. Ultimately, the circle of the +astrolabe, mounted with one of its diameters parallel to the earth's +axis, became the armillary sphere, the precursor of our modern +equatorial telescope. Great stone quadrants fixed in the meridian +were also employed from very early times. Out of such furnishings, +little modified by the lapse of centuries, was provided the elaborate +instrumental equipment of Uranibourg, the great observatory built +by Tycho Brahe on the Danish island of Huen in 1576. In this "City +of the Heavens," still dependent solely upon the unaided eye as a +collector of starlight, Tycho made those invaluable observations +that enabled Kepler to deduce the true laws of planetary motion. But +after all these centuries the sidereal world embraced no objects, +barring an occasional comet or temporary star, that lay beyond +the vision of the earliest astronomers. The conceptions of the +stellar universe, except those that ignored the solid ground of +observation, were limited by the small aperture of the human eye. +But the dawn of another age was at hand. +</p> + +<div class="image" style="width: 419px;"> +<a name="fig_02"> +<img src="images/fig_02.jpg" width="419" height="362" alt="Fig. 2"></a> +<p class="image">Fig. 2. The Great Nebula in Orion (Pease).</p> + +<p class="smaller"> +Photographed with the 100-inch telescope. This short-exposure photograph +shows only the bright central part of the nebula. A longer exposure +reveals a vast outlying region. +</p> +</div> + +<p class="indent"> +The dominance of the sun as the central body of the solar system, +recognized by Aristarchus of Samos nearly three centuries before +the Christian era, but subsequently denied under the authority of +Ptolemy and the teachings of the Church, was reaffirmed by the +Polish monk Copernicus in 1543. Kepler's laws of the motions of the +planets, showing them to revolve in ellipses instead of circles, +removed the last defect of the Copernican system, and left no room for +its rejection. But both the world and the Church clung to tradition, +and some visible demonstration was urgently needed. This was supplied +by Galileo through his invention of the telescope. +</p> + +<div class="image" style="width: 689px;"> +<a name="fig_03"> +<img src="images/fig_03.jpg" width="689" height="239" alt="Fig. 3"></a> +<p class="image">Fig. 3. Model by Ellerman of summit of Mount Wilson, +showing the observatory buildings among the trees and bushes.</p> + +<p class="smaller"> +The 60-foot tower on the extreme left, which is at the edge of a +precipitous cañon 1,500 feet deep, is the vertical telescope +of the Smithsonian Astrophysical Observatory. Above it are the +"Monastery" and other buildings used as quarters by the astronomers +of the Mount Wilson Observatory while at work on the mountain. (The +offices, computing-rooms, laboratories, and shops are in Pasadena.) +Following the ridge, we come successively to the dome of the 10-inch +photographic telescope, the power-house, laboratory, Snow horizontal +telescope, 60-foot-tower telescope, and 150-foot-tower telescope, +these last three used for the study of the sun. The dome of the +60-inch reflecting telescope is just below the 150-foot tower, +while that of the 100-inch telescope is farther to the right. The +altitude of Mount Wilson is about 5,900 feet.</p> +</div> + +<p class="indent"> +The crystalline lens of the human eye, limited by the iris to a +maximum opening about one-quarter of an inch in diameter, was the +only collector of starlight available to the Greek and Arabian +astronomers. Galileo's telescope, which in 1610 suddenly pushed +out the boundaries of the known stellar universe and brought many +thousands of stars into range, had a lens about 2-1/4 inches in +diameter. The area of this lens, proportional to the square of +its diameter, was about eighty-one times that of the pupil of the +eye. This great increase in the amount of light collected should +bring to view stars down to magnitude 10.5, of which nearly half +a million are known to exist. +</p> + +<p class="indent"> +It is not too much to say that Galileo's telescope revolutionized +human thought. Turned to the moon, it revealed mountains, plains, +and valleys, while the sun, previously supposed immaculate in its +perfection, was seen to be blemished with dark spots changing from +day to day. Jupiter, shown to be accompanied by four encircling +satellites, afforded a picture in miniature of the solar system, +and strongly supported the Copernican view of its organization, +which was conclusively demonstrated by Galileo's discovery of the +changing phases of Venus and the variation of its apparent diameter +during its revolution about the sun. Galileo's proof of the Copernican +theory marked the downfall of mediævalism and established +astronomy on a firm foundation. But while his telescope multiplied +a hundredfold the number of visible stars, more than a century +elapsed before the true possibilities of sidereal astronomy were +perceived. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_04"> +<img src="images/fig_04.jpg" width="417" height="531" alt="Fig. 4"></a> +<p class="image">Fig. 4. The 100-inch Hooker telescope.</p> +</div> + +<h3>STRUCTURE OF THE UNIVERSE</h3> + +<p class="indent"> +Sir William Herschel was the first astronomer to make a serious +attack upon the problem of the structure of the stellar universe. +In his first memoir on the "Construction of the Heavens," read +before the Royal Society in 1784, he wrote as follows: +</p> + +<p class="indent"> +"Hitherto the sidereal heavens have, not inadequately for the purpose +designed, been represented by the concave surface of a sphere in +the centre of which the eye of an observer might be supposed to be +placed.... In future we shall look upon those regions into which we +may now penetrate by means of such large telescopes, as a naturalist +regards a rich extent of ground or chain of mountains containing +strata variously inclined and directed as well as consisting of +very different materials." +</p> + +<p class="indent"> +On turning his 18-inch reflecting telescope to a part of the Milky +Way in Orion, he found its whitish appearance to be completely +resolved into small stars, not separately seen with his former +telescopes. "The glorious multitude of stars of all possible sizes +that presented themselves here to my view are truly astonishing; but +as the dazzling brightness of glittering stars may easily mislead +us so far as to estimate their number greater than it really is, +I endeavored to ascertain this point by counting many fields, and +computing from a mean of them, what a certain given portion of +the Milky Way might contain." By this means, applied not only to +the Milky Way but to all parts of the heavens, Herschel determined +the approximate number and distribution of all the stars within +reach of his instrument. +</p> + +<p class="indent"> +By comparing many hundred gauges or counts of stars visible in +a field of about one-quarter of the area of the moon, Herschel +found that the average number of stars increased toward the great +circle which most nearly conforms with the course of the Milky Way. +Ninety degrees from this plane, at the pole of the Milky Way, only +four stars, on the average, were seen in the field of the telescope. +In approaching the Milky Way this number increased slowly at first, +and then more and more rapidly, until it rose to an average of +122 stars per field. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_05"> +<img src="images/fig_05.jpg" width="417" height="336" alt="Fig. 5"></a> +<p class="image">Fig. 5. Erecting the polar axis of the 100-inch +telescope.</p> +</div> + +<p class="indent"> +These observations were made in the northern hemisphere, and +subsequently Sir John Herschel, using his father's telescope at +the Cape of Good Hope, found an almost exactly similar increase +of apparent star density for the southern hemisphere. According to +his estimates, the total number of stars in both hemispheres that +could be seen distinctly enough to be counted in this telescope +would probably be about five and one-half millions. +</p> + +<p class="indent"> +The Herschels concluded that "the stars of our firmament, instead +of being scattered in all directions indifferently through space, +form a stratum of which the thickness is small, in comparison with +its length and breadth; and in which the earth occupies a place +somewhere about the middle of its thickness, between the point where +it subdivides into two principal laminæ inclined at a small +angle to each other." This view does not differ essentially from our +modern conception of the form of the Galaxy; but as the Herschels +were unable to see stars fainter than the fifteenth magnitude, +it is evident that their conclusions apply only to a restricted +region surrounding the solar system, in the midst of the enormously +extended sidereal universe which modern instruments have brought +within our range. +</p> + +<h3>MODERN METHODS</h3> + +<p class="indent"> +The remarkable progress of modern astronomy is mainly due to two +great instrumental advances: the rise and development of the +photographic telescope, and the application of the spectroscope to +the study of celestial objects. These new and powerful instruments, +supplemented by many accessories which have completely revolutionized +observatory equipment, have not only revealed a vastly greater +number of stars and nebulæ: they have also rendered feasible +observations of a type formerly regarded as impossible. The chemical +analysis of a faint star is now so easy that it can be accomplished +in a very short time—as quickly, in fact, as an equally complex +substance can be analyzed in the laboratory. The spectroscope also +measures a star's velocity, the pressure at different levels in +its atmosphere, its approximate temperature, and now, by a new and +ingenious method, its distance from the earth. It determines the +velocity of rotation of the sun and of nebulæ, the existence +and periods of orbital revolution of binary stars too close to be +separated by any telescope, the presence of magnetic fields in +sunspots, and the fact that the entire sun, like the earth, is +a magnet. +</p> + +<div class="image" style="width: 414px;"> +<a name="fig_06"> +<img src="images/fig_06.jpg" width="414" height="328" alt="Fig. 6"></a> +<p class="image">Fig. 6. Lowest section of tube of 100-inch telescope, +ready to leave Pasadena for Mount Wilson.</p> +</div> + +<p class="indent"> +Such new possibilities, with many others resulting from the application +of physical methods of the most diverse character, have greatly +enlarged the astronomer's outlook. He may now attack two great +problems: (1) The structure of the universe and the motions of +its constituent bodies, and (2) the evolution of the stars: their +nature, origin, growth, and decline. These two problems are intimately +related and must be studied as one.[*] +</p> + +<p class="footnote"> +[Footnote *: A third great problem open to the astronomer, the +study of the constitution of matter, is described in Chapter III.] +</p> + +<p class="indent"> +If space permitted, it would be interesting to survey the progress +already accomplished by modern methods of astronomical research. +Hundreds of millions of stars have been photographed, and the boundaries +of the stellar universe have been pushed far into space, but have not +been attained. Globular star clusters, containing tens of thousands +of stars, are on so great a scale (according to Shapley) that light, +travelling at the rate of 186,000 miles per second, may take 500 +years to cross one of them, while the most distant of these objects +may be more than 200,000 light-years from the earth. The spiral +nebulæ, more than a million in number, are vast whirling +masses in process of development, but we are not yet certain whether +they should be regarded as "island universes" or as subordinate +to the stellar system which includes our minute group of sun and +planets, the great star clouds of the Milky Way, and the distant +globular star clusters. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_07"> +<img src="images/fig_07.jpg" width="417" height="343" alt="Fig. 7"></a> +<p class="image">Fig. 7. Section of a steel girder for dome covering +the 100-inch telescope, on its way up Mount Wilson.</p> +</div> + +<p class="indent"> +These few particulars may give a slight conception of the scale +of the known universe, but a word must be added regarding some +of its most striking phenomena. The great majority of the stars +whose motions have been determined belong to one or the other of +two great star streams, but the part played by these streams in the +sidereal system as a whole is still obscure. The stars have been +grouped in classes, presumably in the order of their evolutional +development, as they pass from the early state of gaseous masses, of +low density, through the successive stages resulting from loss of +heat by radiation and increased density due to shrinkage. Strangely +enough, their velocities in space show a corresponding change, +increasing as they grow older or perhaps depending upon their mass. +</p> + +<p class="indent"> +It is impossible within these limits to do more than to give some +indication of the scope of the new astronomy. Enough has been said, +however, to assist in appreciating the increased opportunity for +investigation, and the nature of the heavy demands made upon the +modern observatory. But before passing on to describe one of the +latest additions to the astronomer's instrumental equipment, a +word should be added regarding the chief classes of telescopes. +</p> + +<h3>REFRACTORS AND REFLECTORS</h3> + +<p class="indent"> +Astronomical telescopes are of two types: refractors and reflectors. +A refracting telescope consists of an object-glass composed of +two or more lenses, mounted at the upper end of a tube, which is +pointed at the celestial object. The light, after passing through +the lenses, is brought to a focus at the lower end of the tube, where +the image is examined visually with an eyepiece, or photographed +upon a sensitive plate. The largest instruments of this type are +the 36-inch Lick telescope and the 40-inch refractor of the Yerkes +Observatory. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_08"> +<img src="images/fig_08.jpg" width="417" height="537" alt="Fig. 8"></a> +<p class="image">Fig. 8. Erecting the steel building and revolving +dome that cover the Hooker telescope.</p> +</div> + +<p class="indent"> +Reflecting telescopes, which are particularly adapted for photographic +work, though also excellent for visual observations, are very +differently constructed. No lens is used. The telescope tube is +usually built in skeleton form, open at its upper end, and with a +large concave mirror supported at its base. This mirror serves in +place of a lens. Its upper surface is paraboloidal in shape, as a +spherical surface will not unite in a sharp focus the rays coming +from a distant object. The light passes through no glass—a +great advantage, especially for photography, as the absorption +in lenses cuts out much of the blue and violet light, to which +photographic plates are most sensitive. The reflection occurs on +the <i>upper</i> surface of the mirror, which is covered with a +coat of pure silver, renewed several times a year and always kept +highly burnished. Silvered glass is better than metals or other +substances for telescope mirrors, chiefly because of the perfection +with which glass can be ground and polished, and the ease of renewing +its silvered surface when tarnished. +</p> + +<p class="indent"> +The great reflectors of Herschel and Lord Rosse, which were provided +with mirrors of speculum metal, were far inferior to much smaller +telescopes of the present day. With these instruments the star +images were watched as they were carried through the field of view +by the earth's rotation, or kept roughly in place by moving the +telescope with ropes or chains. Photographic plates, which reveal +invisible stars and nebulæ when exposed for hours in modern +instruments, were not then available. In any case they could not +have been used, in the absence of the perfect mechanism required to +keep the star images accurately fixed in place upon the sensitive +film. +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_09"> +<img src="images/fig_09.jpg" width="415" height="329" alt="Fig. 9"></a> +<p class="image">Fig. 9. Building and revolving dome, 100 feet in +diameter, covering the 100-inch Hooker telescope.</p> + +<p class="smaller"> +Photographed from the summit of the 150-foot-tower telescope.</p> +</div> + +<p class="indent"> +It would be interesting to trace the long contest for supremacy +between refracting and reflecting telescopes, each of which, at +certain stages in its development, appeared to be unrivalled. In +modern observatories both types are used, each for the purpose +for which it is best adapted. For the photography of nebulæ +and the study of the fainter stars, the reflector has special +advantages, illustrated by the work of such instruments as the +Crossley and Mills reflectors of the Lick Observatory; the great +72-inch reflector, recently brought into effective service at the +Dominion Observatory in Canada; and the 60-inch and 100-inch reflectors +of the Mount Wilson Observatory. +</p> + +<p class="indent"> +The unaided eye, with an available area of one-twentieth of a square +inch, permits us to see stars of the sixth magnitude. Herschel's +18-inch reflector, with an area 5,000 times as great, rendered +visible stars of the fifteenth magnitude. The 60-inch reflector, +with an area 57,600 times that of the eye, reveals stars of the +eighteenth magnitude, while to reach stars of about the twentieth +magnitude, photographic exposures of four or five hours suffice +with this instrument. +</p> + +<p class="indent"> +Every gain of a magnitude means a great gain in the number of stars +rendered visible. Stars of the second magnitude are 3.4 times as +numerous as those of the first, those of the eighth magnitude are +three times as numerous as those of the seventh, while the sixteenth +magnitude stars are only 1.7 as numerous as those of the fifteenth +magnitude. This steadily decreasing ratio is probably due to an +actual thinning out of the stars toward the boundaries of the stellar +universe, as the most exhaustive tests have failed to give any +evidence of absorption of light in its passage through space. But +in spite of this decrease, the gain of a single additional magnitude +may mean the addition of many millions of stars to the total of those +already shown by the 60-inch reflector. Here is one of the chief +sources of interest in the possibilities of a 100-inch reflecting +telescope. +</p> + +<h3>100-INCH TELESCOPE</h3> + +<div class="image" style="width: 416px;"> +<a name="fig_10"> +<img src="images/fig_10.jpg" width="416" height="335" alt="Fig. 10"></a> +<p class="image">Fig. 10. One-hundred-inch mirror, just silvered, +rising out of the silvering-room in pier before attachment to lower +end of telescope tube. (Seen above.)</p> +</div> + +<p class="indent"> +In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie +Institution of Washington a sum sufficient to construct a telescope +mirror 100 inches in diameter, and thus large enough to collect +160,000 times the light received by the eye. (Fig. 10.) The casting +and annealing of a suitable glass disk, 101 inches in diameter +and 13 inches thick, weighing four and one-half tons, was a most +difficult operation, finally accomplished by a great French glass +company at their factory in the Forest of St. Gobain. A special +optical laboratory was erected at the Pasadena headquarters of +the Mount Wilson Observatory, and here the long task of grinding, +figuring, and testing the mirror was successfully carried out by +the observatory opticians. This operation, which is one of great +delicacy, required years for its completion. Meanwhile the building, +dome, and mounting for the telescope were designed by members of +the observatory staff, and the working drawings were prepared. An +opportune addition by Mr. Carnegie to the endowment of the Carnegie +Institution of Washington, of which the observatory is a branch, +permitted the necessary appropriations to be made for the completion +and erection of the telescope. Though delayed by the war, during +which the mechanical and optical facilities of the observatory +shops were utilized for military and naval purposes, the telescope +is now in regular use on Mount Wilson. +</p> + +<p class="indent"> +The instrument is mounted on a massive pier of reinforced concrete, +33 feet high and 52 feet in diameter at the top. A solid wall extends +south from this pier a distance of 50 feet, on the west side of +which a very powerful spectrograph, for photographing the spectra +of the brightest stars, will be mounted. Within the pier are a +photographic dark room, a room for silvering the large mirror (which +can be lowered into the pier), and the clock-room, where stands +the powerful driving-clock, with which the telescope is caused +to follow the apparent motion of the stars. (Fig. 11.) +</p> + +<div class="image" style="width: 419px;"> +<a name="fig_11"> +<img src="images/fig_11.jpg" width="419" height="359" alt="Fig. 11"></a> +<p class="image">Fig. 11. The driving-clock and worm-gear that cause +the 100-inch Hooker telescope to follow the stars.</p> +</div> + +<p class="indent"> +The telescope mounting is of the English type, in which the telescope +tube is supported by the declination trunnions between the arms of +the polar axis, built in the form of a rectangular yoke carried by +bearings on massive pedestals to the north and south. These bearings +must be aligned exactly parallel to the axis of the earth, and must +support the polar axis so freely that it can be rotated with perfect +precision by the driving-clock, which turns a worm-wheel 17 feet in +diameter, clamped to the lower end of the axis. As this motion +must be sufficiently uniform to counteract exactly the rotation +of the earth on its axis, and thus to maintain the star images +accurately in position in the field of view, the greatest care +had to be taken in the construction of the driving-clock and in +the spacing and cutting of the teeth in the large worm-wheel. Here, +as in the case of all of the more refined parts of the instrument, +the work was done by skilled machinists in the observatory shops in +Pasadena or on Mount Wilson after the assembling of the telescope. +The massive sections of the instrument, some of which weigh as +much as ten tons each, were constructed at Quincy, Mass., where +machinery sufficiently large to build battleships was available. +They were then shipped to California, and transported to the summit +of Mount Wilson over a road built for this purpose by the construction +division of the observatory, which also built the pier on which the +telescope stands, and erected the steel building and dome that +cover it. +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_12"> +<img src="images/fig_12.jpg" width="415" height="387" alt="Fig. 12"></a> +<p class="image">Fig. 12. Large irregular nebula and star cluster +in Sagittarius (Duncan).</p> + +<p class="smaller">Photographed with the 60-inch telescope.</p> +</div> + +<div class="image" style="width: 417px; margin-top: 1em;"> +<a name="fig_13"> +<img src="images/fig_13.jpg" width="417" height="332" alt="Fig. 13"></a> +<p class="image">Fig. 13. Faint spiral nebula in the constellation +of the Hunting Dogs (Pease).</p> + +<p class="smaller">Photographed with the 60-inch telescope.</p> +</div> + +<p class="indent"> +The parts of the telescope which are moved by the driving-clock +weigh about 100 tons, and it was necessary to provide means of +reducing the great friction on the bearings of the polar axis. To +accomplish this, large hollow steel cylinders, floating in mercury +held in cast-iron tanks, were provided at the upper and lower ends +of the polar axis. Almost the entire weight of the instrument is +thus floated in mercury, and in this way the friction is so greatly +reduced that the driving-clock moves the instrument with perfect +ease and smoothness. +</p> + +<p class="indent"> +The 100-inch mirror rests at the bottom of the telescope tube on +a special support system, so designed as to prevent any bending of +the glass under its own weight. Electric motors, forty in number, are +provided to move the telescope rapidly or slowly in right ascension +(east or west) and in declination (north or south), for focussing +the mirrors, and for many other purposes. They are also used for +rotating the dome, 100 feet in diameter, under which the telescope +is mounted, and for opening the shutter, 20 feet wide, through +which the observations are made. +</p> + +<p class="indent"> +A telescope of this kind can be used in several different ways. +The 100-inch mirror has a focal length of about 42 feet, and in +one of the arrangements of the instrument, the photographic plate +is mounted at the centre of the telescope tube near its upper end, +where it receives directly the image formed by the large mirror. In +another arrangement, a silvered glass mirror, with plane surface, is +supported near the upper end of the tube at an angle of 45°, so +as to form the image at the side of the tube, where the photographic +plate can be placed. In this case, the observer stands on a platform, +which is moved up and down by electric motors in front of the opening +in the dome through which the observations are made. +</p> + +<div class="image" style="width: 419px;"> +<a name="fig_14"> +<img src="images/fig_14.jpg" width="419" height="333" alt="Fig. 14"></a> +<p class="image">Fig. 14. Spiral nebula in Andromeda, seen edge on +(Ritchey).</p> + +<p class="smaller">Photographed with the 60-inch telescope.</p> +</div> + +<p class="indent"> +Other arrangements of the telescope, for which auxiliary convex +mirrors carried near the upper end of the tube are required, permit +the image to be photographed at the side of the tube near its lower +end, either with or without a spectrograph; or with a very powerful +spectrograph mounted within a constant-temperature chamber south +of the telescope pier. In this last case, the light of a star is +so reflected by auxiliary mirrors that it passes down through a +hole in the south end of the polar axis and brings the star to +a focus on the slit of the fixed spectrograph. +</p> + +<h3>ATMOSPHERIC LIMITATIONS</h3> + +<p class="indent"> +The huge dimensions of such a powerful engine of research as the +Hooker telescope are not in themselves a source of satisfaction to +the astronomer, for they involve a decided increase in the labor +of observation and entail very heavy expense, justifiable only in +case important results, beyond the reach of other instruments, +can be secured. The construction of a telescope of these dimensions +was necessarily an experiment, for it was by no means certain, after +the optical and mechanical difficulties had been overcome, that +even the favorable atmosphere of California would be sufficiently +tranquil to permit sharply defined celestial images to be obtained +with so large an aperture. It is therefore important to learn what +the telescope will actually accomplish under customary observing +conditions. +</p> + +<p class="indent"> +Fortunately we are able to measure the performance of the instrument +with certainty. Close beside it on Mount Wilson stands the 60-inch +reflector, of similar type, erected in 1908. The two telescopes can +thus be rigorously compared under identical atmospheric conditions. +</p> + +<p class="indent"> +The large mirror of the 100-inch telescope has an area about 2.8 +times that of the 60-inch, and therefore receives nearly three times +as much light from a star. Under atmospheric conditions perfect +enough to allow all of this light to be concentrated in a point, +it should be capable of recording on a photographic plate, with a +given exposure, stars about one magnitude fainter than the faintest +stars within reach of the 60-inch. The increased focal length, +permitting such objects as the moon to be photographed on a larger +scale, should also reveal smaller details of structure and render +possible higher accuracy of measurement. Finally, the greater +theoretical resolving power of the larger aperture, providing it +can be utilized, should permit the separation of the members of +close double stars beyond the range of the smaller instrument. +</p> + +<h3>CRITICAL TESTS</h3> + +<p class="indent"> +The many tests already made indicate that the advantages expected +of the new telescope will be realized in practice. The increased +light-gathering power will mean the addition of many millions of +stars to those already known. Spectroscopic observations now in +regular progress have carried the range of these investigations +far beyond the possibilities of the 60-inch telescope. A great +class of red stars, for example, almost all the members of which +were inaccessible to the 60-inch, are now being made the subject +of special study. And in other fields of research equal advantages +have been gained. +</p> + +<p class="indent"> +The increase in the scale of the images over those given by the +60-inch telescope is illustrated by two photographs of the Ring +Nebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion, +photographed with the 100-inch telescope with a comparatively short +exposure, sufficient to bring out the brighter regions, is reproduced +in Fig. 2. It is interesting to compare this picture with the +small-scale image of the same nebula shown in Fig. 1. +</p> + +<div class="image" style="width: 414px;"> +<a name="fig_15"> +<img src="images/fig_15.jpg" width="414" height="521" alt="Fig. 15"></a> +<p class="image">Fig. 15. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease).</p> + +<p class="smaller"> +The ring-like formations are the so-called craters, most of them +far larger than anything similar on the earth. That in the lower +left corner with an isolated mountain in the centre is Albategnius, +sixty-four miles in diameter. Peaks in the ring rise to a height +of fifteen thousand feet above the central plain. Note the long +sunset shadows cast by the mountains on the left. The level region +below on the right is an extensive plain, the Mare Nubium.</p> +</div> + +<div class="image" style="width: 412px; margin-top: 1em;"> +<a name="fig_16"> +<img src="images/fig_16.jpg" width="412" height="519" alt="Fig. 16"></a> +<p class="image">Fig. 16. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease).</p> + +<p class="smaller"> +The mountains above and to the left are the lunar Apennines; those +on the left just below the centre are the Alps. Both ranges include +peaks from fifteen thousand to twenty thousand feet in height. In +the upper right corner is Copernicus, about fifty miles in diameter. +The largest of the conspicuous group of three just below the Apennines +is Archimedes and at the lower end of the Alps is Plato. Note the +long sunset shadows cast by the isolated peaks on the left. The +central portion of the picture is a vast plain, the Mare Imbrium.</p> +</div> + +<p class="indent"> +The sharpness of the images given by the new telescope may be +illustrated by some recent photographs of the moon, obtained with +an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged +region of the moon, containing many ring-like mountains or craters. +Fig. 16 shows the great arc of the lunar Apennines (above) and the +Alps (below), to the left of the broad plain of the Mare Imbrium. +The starlike points along the moon's terminator, which separates +the dark area from the region upon which the sun (on the right) +shines, are the mountain peaks, about to disappear at sunset. The +long shadows cast by the mountains just within the illuminated +area are plainly seen. Some of the peaks of the lunar Apennines +attain a height of 20,000 feet. +</p> + +<p class="indent"> +In less powerful telescopes the stars at the centre of the great +globular clusters are so closely crowded together that they cannot +be studied separately with the spectrograph. Moreover, most of +them are much too faint for examination with this instrument. At +the 134-foot focus the 100-inch telescope gives a large-scale image +of such clusters, and permits the spectra of stars as faint as +the fifteenth magnitude to be separately photographed. +</p> + +<div class="image" style="width: 337px;"> +<a name="fig_17"> +<img src="images/fig_17.jpg" width="337" height="487" alt="Fig. 17"></a> +<p class="image">Fig. 17. Hubble's Variable Nebula. One of the few +nebulæ known to vary in brightness and form.</p> + +<p class="smaller">Photographed with the 100-inch telescope +(Hubble).</p> +</div> + +<h3>CLOSE DOUBLE STARS</h3> + +<p class="indent"> +A remarkable use of the 100-inch telescope, which permits its full +theoretical resolving power to be not merely attained but to be +doubled, has been made possible by the first application of Michelson's +interference method to the measurement of very close double stars. +When employing this, the 100-inch mirror is completely covered, +except for two slits. Beams of light from a star, entering by the +slits, unite at the focus of the telescope, where the image is +examined by an eyepiece magnifying about five thousand diameters. +Across the enlarged star image a series of fine, sharp fringes is +seen, even when the atmospheric conditions are poor. If the star is +single the fringes remain visible, whatever the distance between the +slits. But in the case of a star like Capella, previously inferred +to be double from the periodic displacement of the lines in its +spectrum, but with components too close together to be distinguished +separately, the fringes behave differently. As the slits are moved +apart a point is reached where the fringes completely disappear, +only to reappear as the separation is continued. This effect is +obtained when the slits are at right angles to the line joining +the two stars of the pair, found by this method to be 0.0418 of a +second of arc apart (on December 30, 1919). Subsequent measures, +of far greater precision than those obtainable by other methods in +the case of easily separated double stars, show the rapid orbital +motion of the components of the system. This device will be applied +to other close binaries, hitherto beyond the reach of measurement. +</p> + +<div class="image" style="width: 414px;"> +<a name="fig_18"> +<img src="images/fig_18.jpg" width="414" height="230" alt="Fig. 18"></a> +<p class="image">Fig. 18. Ring Nebula in Lyra, photographed with +the 60-inch (Ritchey) and 100-inch (Duncan) telescopes.</p> + +<p class="smaller"> +Showing the increased scale of the images given by the larger +instrument.</p> +</div> + +<p class="indent"> +Without entering into further details of the tests, it is evident +that the new telescope will afford boundless possibilities for +the study of the stellar universe.[*] The structure and extent +of the galactic system, and the motions of the stars comprising +it; the distribution, distances, and dimensions of the spiral +nebulæ, their motions, rotation, and mode of development; +the origin of the stars and the successive stages in their life +history: these are some of the great questions which the new telescope +must help to answer. In such an embarrassment of riches the chief +difficulty is to withstand the temptation toward scattering of +effort, and to form an observing programme directed toward the +solution of crucial problems rather than the accumulation of vast +stores of miscellaneous data. This programme will be supplemented +by an extensive study of the sun, the only star near enough the +earth to be examined in detail, and by a series of laboratory +investigations involving the experimental imitation of solar and +stellar conditions, thus aiding in the interpretation of celestial +phenomena. +</p> + +<p class="footnote"> +[Footnote *: It is not adapted for work on the sun, as the mirrors +would be distorted by its heat. Three other telescopes, especially +designed for solar observations, are in use on Mount Wilson.] +</p> + +<h2><a name="chapter_II">CHAPTER II</a></h2> + +<p class="subtitle">GIANT STARS</p> + +<p class="indent"> +Our ancestral sun, as pictured by Laplace, originally extended +in a state of luminous vapor beyond the boundaries of the solar +system. Rotating upon its axis, it slowly contracted through loss +of heat by radiation, leaving behind it portions of its mass, which +condensed to form the planets. Still gaseous, though now denser than +water, it continues to pour out the heat on which our existence +depends, as it shrinks imperceptibly toward its ultimate condition +of a cold and darkened globe. +</p> + +<p class="indent"> +Laplace's hypothesis has been subjected in recent years to much +criticism, and there is good reason to doubt whether his description +of the mode of evolution of our solar system is correct in every +particular. All critics agree, however, that the sun was once enormously +larger than it now is, and that the planets originally formed part +of its distended mass. +</p> + +<p class="indent"> +Even in its present diminished state, the sun is huge beyond easy +conception. Our own earth, though so minute a fragment of the primeval +sun, is nevertheless so large that some parts of its surface have +not yet been explored. Seen beside the sun, by an observer on one +of the planets, the earth would appear as an insignificant speck, +which could be swallowed with ease by the whirling vortex of a +sun-spot. If the sun were hollow, with the earth at its centre, +the moon, though 240,000 miles from us, would have room and to +spare in which to describe its orbit, for the sun is 865,000 miles +in diameter, so that its volume is more than a million times that +of the earth. +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_19"> +<img src="images/fig_19.jpg" width="415" height="320" alt="Fig. 19"></a> +<p class="image">Fig. 19. Gaseous prominence at the sun's limb, +140,000 miles high (Ellerman).</p> + +<p class="smaller"> +Photographed with the spectroheliograph, using the light emitted +by glowing calcium vapor. The comparative size of the earth is +indicated by the white circle.</p> +</div> + +<p class="indent"> +But what of the stars, proved by the spectroscope to be self-luminous, +intensely hot, and formed of the same chemical elements that constitute +the sun and the earth? Are they comparable in size with the sun? Do +they occur in all stages of development, from infancy to old age? +And if such stages can be detected, do they afford indications +of the gradual diminution in volume which Laplace imagined the +sun to experience? +</p> + +<div class="image" style="width: 420px;"> +<a name="fig_20"> +<img src="images/fig_20.jpg" width="420" height="413" alt="Fig. 20"></a> +<p class="image">Fig. 20. The sun, 865,000 miles in diameter, from a +direct photograph showing many sun-spots (Whitney)</p> + +<p class="smaller"> +The small black disk in the centre represents the comparative size +of the earth, while the circle surrounding it corresponds in diameter +to the orbit of the moon.</p> +</div> + +<h3>STAR IMAGES</h3> + +<p class="indent"> +Prior to the application of the powerful new engine of research +described in this article we have had no means of measuring the +diameters of the stars. We have measured their distances and their +motions, determined their chemical composition, and obtained undeniable +evidence of progressive development, but even in the most powerful +telescopes their images are so minute that they appear as points +rather than as disks. In fact, the larger the telescope and the +more perfect the atmospheric conditions at the observer's command, +the smaller do these images appear. On the photographic plate, it is +true, the stars are recorded as measurable disks, but these are due +to the spreading of the light from their bright point-like images, +and their diameters increase as the exposure time is prolonged. +From the images of the brighter stars rays of light project in +straight lines, but these also are instrumental phenomena, due +to diffraction of light by the steel bars that support the small +mirror in the tube of reflecting telescopes. In a word, the stars +are so remote that the largest and most perfect telescopes show +them only as extremely minute needle-points of light, without any +trace of their true disks. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_21"> +<img src="images/fig_21.jpg" width="417" height="368" alt="Fig. 21"></a> +<p class="image">Fig. 21. Great sun-spot group, August 8, 1917 +(Whitney).</p> + +<p class="smaller"> +The disk in the corner represents the comparative size of the +earth.</p> +</div> + +<p class="indent"> +How, then, may we hope to measure their diameters? By using, as +the man of science must so often do, indirect means when the direct +attack fails. Most of the remarkable progress of astronomy during +the last quarter-century has resulted from the application of new and +ingenious devices borrowed from the physicist. These have multiplied +to such a degree that some of our observatories are literally physical +laboratories, in which the sun and stars are examined by powerful +spectroscopes and other optical instruments that have recently advanced +our knowledge of physics by leaps and bounds. In the present case +we are indebted for our star-measuring device to the distinguished +physicist Professor Albert A. Michelson, who has contributed a long +array of novel apparatus and methods to physics and astronomy. +</p> + +<h3>THE INTERFEROMETER</h3> + +<p class="indent"> +The instrument in question, known as the interferometer, had previously +yielded a remarkable series of results when applied in its various +forms to the solution of fundamental problems. To mention only a +few of those that have helped to establish Michelson's fame, we may +recall that our exact knowledge of the length of the international +metre at Sevres, the world's standard of measurement, was obtained +by him with an interferometer in terms of the invariable length of +light-waves. A different form of interferometer has more recently +enabled him to measure the minute tides within the solid body of +the earth—not the great tides of the ocean, but the slight +deformations of the earth's body, which is as rigid as steel, that are +caused by the varying attractions of the sun and moon. Finally, to +mention only one more case, it was the Michelson-Morley experiment, +made years ago with still another form of interferometer, that yielded +the basic idea from which the theory of relativity was developed +by Lorentz and Einstein. +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_22"> +<img src="images/fig_22.jpg" width="415" height="403" alt="Fig. 22"></a> +<p class="image">Fig. 22. Photograph of the hydrogen atmosphere of +the sun (Ellerman).</p> + +<p class="smaller"> +Made with the spectroheliograph, showing the immense vortices, +or whirling storms like tornadoes, that centre in sun-spots. The +comparative size of the earth is shown by the white circle traced +on the largest sun-spot.</p> +</div> + +<p class="indent"> +The history of the method of measuring star diameters is a very +curious one, showing how the most promising opportunities for scientific +progress may lie unused for decades. The fundamental principle of +the device was first suggested by the great French physicist Fizeau +in 1868. In 1874 the theory was developed by the French astronomer +Stéphan, who observed interference fringes given by a large +number of stars, and rightly concluded that their angular diameters +must be much smaller than 0.158 of a second of arc, the smallest +measurable with his instrument. In 1890 Michelson, unaware of the +earlier work, published in the <i>Philosophical Magazine</i> a +complete description of an interferometer capable of determining +with surprising accuracy the distance between the components of +double stars so close together that no telescope can separate them. +He also showed how the same principle could be applied to the +measurement of star diameters if a sufficiently large interferometer +could be built for this purpose, and developed the theory much more +completely than Stéphan had done. A year later he measured +the diameters of Jupiter's satellites by this means at the Lick +Observatory. But nearly thirty years elapsed before the next step +was taken. Two causes have doubtless contributed to this delay. Both +theory and experiment have demonstrated the extreme sensitiveness +of the "interference fringes," on the observation of which the +method depends, and it was generally supposed by astronomers that +disturbances in the earth's atmosphere would prevent them from +being clearly seen with large telescopes. Furthermore, a very large +interferometer, too large to be carried by any existing telescope, +was required for the star-diameter work, though close double stars +could have been easily studied by this device with several of the +large telescopes of the early nineties. But whatever the reasons, +a powerful method of research lay unused. +</p> + +<p class="indent"> +The approaching completion of the 100-inch telescope of the Mount +Wilson Observatory led me to suggest to Professor Michelson, before +the United States entered the war, that the method be thoroughly +tested under the favorable atmospheric conditions of Southern +California. He was at that time at work on a special form of +interferometer, designed to determine whether atmospheric disturbances +could be disregarded in planning large-scale experiments. But the +war intervened, and all of our efforts were concentrated for two +years on the solution of war problems.[*] In 1919, as soon as the +100-inch telescope had been completed and tested, the work was +resumed on Mount Wilson. +</p> + +<p class="footnote"> +[Footnote *: Professor Michelson's most important contribution during +the war period was a new and very efficient form of range-finder, +adopted for use by the U. S. Navy.] +</p> + +<h3>A LABORATORY EXPERIMENT</h3> + +<p class="indent"> +The principle of the method can be most readily seen by the aid +of an experiment which any one can easily perform for himself with +simple apparatus. Make a narrow slit, a few thousandths of an inch +in width, in a sheet of black paper, and support it vertically +before a brilliant source of light. Observe this from a distance of +40 or 50 feet with a small telescope magnifying about 30 diameters. +The object-glass of the telescope should be covered with an opaque +cap, pierced by two circular holes about one-eighth of an inch in +diameter and half an inch apart. The holes should be on opposite +sides of the centre of the object-glass and equidistant from it, +and the line joining the holes should be horizontal. When this +cap is removed the slit appears as a narrow vertical band with +much fainter bands on both sides of it. With the cap in place, the +central bright band appears to be ruled with narrow vertical lines +or fringes produced by the "interference"[*] of the two pencils of +light coming through different parts of the object-glass from the +distant slit. Cover one of the holes, and the fringes instantly +disappear. Their production requires the joint effect of the two +light-pencils. +</p> + +<p class="footnote"> +[Footnote *: For an explanation of the phenomena of interference, +see any encyclopæedia or book on physics.] +</p> + +<p class="indent"> +Now suppose the two holes over the object-glass to be in movable +plates, so that their distance apart can be varied. As they are +gradually separated the narrow vertical fringes become less and +less distinct, and finally vanish completely. Measure the distance +between the holes and divide this by the wavelength of light, which +we may call 1/50000 of an inch. The result is the angular width +of the distant slit. Knowing the distance of the slit, we can at +once calculate its linear width. If for the slit we substitute a +minute circular hole, the method of measurement remains the same, +but the angular diameter as calculated above must be multiplied +by 1.22.[*] +</p> + +<p class="footnote"> +[Footnote *: More complete details may be found in Michelson's Lowell +Lectures on "Light-Waves and Their Uses," University of Chicago +Press, 1907.] +</p> + +<p class="indent"> +To measure the diameter of a star we proceed in a similar way, +but, as the angle it subtends is so small, we must use a very large +telescope, for the smaller the angle the farther apart must be the +two holes over the object-glass (or the mirror, in case a reflecting +telescope is employed). In fact, when the holes are moved apart to +the full aperture of the 100-inch Hooker telescope, the interference +fringes are still visible even with the star Betelgeuse, though its +angular diameter is perhaps as great as that of any other star. +Thus, we must build an attachment for the telescope, so arranged +as to permit us to move the openings still farther apart. +</p> + +<div class="image" style="width: 322px;"> +<a name="fig_23"> +<img src="images/fig_23.jpg" width="322" height="339" alt="Fig. 23"></a> +<p class="image">Fig. 23. Diagram showing outline of the 100-inch +Hooker telescope, and path of the two pencils of light from a star +when under observation with the 20-foot Michelson interferometer.</p> + +<p class="smaller"> +A photograph of the interferometer is shown in Fig. 24.</p> +</div> + +<h3>THE 20-FOOT INSTRUMENT</h3> + +<p class="indent"> +The 20-foot interferometer designed by Messrs. Michelson and Pease, +and constructed in the Mount Wilson Observatory instrument-shop, +is shown in the diagram (Fig. 23) and in a photograph of the upper +end of the skeleton tube of the telescope (Fig. 24). The light from +the star is received by two flat mirrors (Ml, M4) which project +beyond the tube and can be moved apart along the supporting arm. +These take the place of the two holes over the object-glass in +our experiment. From these mirrors the light is reflected to a +second pair of flat mirrors (M2, M3), which send it toward the +100-inch concave mirror (M5) at the bottom of the telescope tube. +After this the course of the light is exactly as it would be if +the mirrors M2, M3 were replaced by two holes over the 100-inch +mirror. It is reflected to the convex mirror (M6), then back in +a less rapidly convergent beam toward the large mirror. Before +reaching it the light is caught by the plane mirror (M7) and reflected +through an opening at the side of the telescope tube to the eye-piece +E. Here the fringes are observed with a magnification ranging from +1,500 to 3,000 diameters. +</p> + +<div class="image" style="width: 416px;"> +<a name="fig_24"> +<img src="images/fig_24.jpg" width="416" height="306" alt="Fig. 24"></a> +<p class="image">Fig. 24. Twenty-foot Michelson interferometer for +measuring star diameters, attached to upper end of the skeleton tube +of the 100-inch Hooker telescope.</p> + +<p class="smaller"> +The path of the two pencils of light from the star is shown in +Fig. 23. For a photograph of the entire telescope, see Fig. 4.</p> +</div> + +<p class="indent"> +In the practical application of this method to the measurement of +star diameters, the chief problem was whether the atmosphere would +be quiet enough to permit sharp interference fringes to be produced +with light-pencils more than 100 inches apart. After successful +preliminary tests with the 40-inch refracting telescope of the +Yerkes Observatory, Professor Michelson made the first attempt +to see the fringes with the 60-inch and 100-inch reflectors on +Mount Wilson in September, 1919. He was surprised and delighted to +find that the fringes were perfectly sharp and distinct with the +full aperture of both these instruments. Doctor Anderson, of the +observatory staff, then devised a special form of interferometer +for the measurement of close double stars, and applied it with +the 100-inch telescope to the measurement of the orbital motion +of the close components of Capella, with results of extraordinary +accuracy, far beyond anything attainable by previous methods. The +success of this work strongly encouraged the more ambitious project +of measuring the diameter of a star, and the 20-foot interferometer +was built for this purpose. +</p> + +<p class="indent"> +The difficult and delicate problem of adjusting the mirrors of +this instrument with the necessary extreme accuracy was solved by +Professor Michelson during his visit to Mount Wilson in the summer +of 1920, and with the assistance of Mr. Pease, of the observatory +staff, interference fringes were observed in the case of certain +stars when the mirrors were as much as 18 feet apart. All was thus +in readiness for a decisive test as soon as a suitable star presented +itself. +</p> + +<h3>THE GIANT BETELGEUSE</h3> + +<p class="indent"> +Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic +for "the giant's shoulder"), the bright red star in the constellation +of Orion (Fig. 25), as the most favorable of all stars for measurement, +and the last-named had given its angular diameter as 0.051 of a +second of arc. This deduction from theory appeared in his recent +presidential address before the British Association for the Advancement +of Science, in which Professor Eddington remarked: "Probably the +greatest need of stellar astronomy at the present day, in order +to make sure that our theoretical deductions are starting on the +right lines, is some means of measuring the apparent angular diameter +of stars." He then referred to the work already in progress on +Mount Wilson, but anticipated "that atmospheric disturbance will +ultimately set the limit to what can be accomplished." +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_25"> +<img src="images/fig_25.jpg" width="415" height="473" alt="Fig. 25"></a> +<p class="image">Fig. 25. The giant Betelgeuse (within the circle), +familiar as the conspicuous red star in the right shoulder of Orion +(Hubble).</p> + +<p class="smaller"> +Measures with the interferometer show its angular diameter to be +0.047 of a second of arc, corresponding to a linear diameter of +215,000,000 miles, if the best available determination of its distance +can be relied upon. This determination shows Betelgeuse to be 160 +light-years from the earth. Light travels at the rate of 186,000 +miles per second, and yet spends 160 years on its journey to us +from this star.</p> +</div> + +<p class="indent"> +On December 13, 1920, Mr. Pease successfully measured the diameter +of Betelgeuse with the 20-foot interferometer. As the outer mirrors +were separated the interference fringes gradually became less distinct, +as theory requires, and as Doctor Merrill had previously seen when +observing Betelgeuse with the interferometer used for Capella. At +a separation of 10 feet the fringes disappeared completely, giving +the data required for calculating the diameter of the star. To +test the perfection of the adjustment, the telescope was turned to +other stars, of smaller angular diameter, which showed the fringes +with perfect clearness. Turning back to Betelgeuse, they were seen +beyond doubt to be absent. Assuming the mean wave-length of the +light of this star to be 5750/10000000 of a millimetre, its angular +diameter comes out 0.047 of a second of arc, thus falling between +the values—0.051 and 0.031 of a second—predicted by +Eddington and Russell from slightly different assumptions. Subsequent +corrections and repeated measurement will change Mr. Pease's result +somewhat, but it is almost certainly within 10 or 15 per cent of +the truth. We may therefore conclude that the angular diameter +of Betelgeuse is very nearly the same as that of a ball one inch +in diameter, seen at a distance of seventy miles. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_26"> +<img src="images/fig_26.jpg" width="417" height="431" alt="Fig. 26"></a> +<p class="image">Fig. 26. Arcturus (within the white circle), known +to the Arabs as the "Lance Bearer," and to the Chinese as the "Great +Horn" or the "Palace of the Emperors" (Hubble).</p> + +<p class="smaller"> +Its angular diameter, measured at Mount Wilson by Pease with the +20-foot Michelson interferometer on April 15, 1921, is 0.022 of a +second, in close agreement with Russell's predicted value of 0.019 +of a second. The mean parallax of Arcturus, based upon several +determinations, is 0.095 of a second, corresponding to a distance of +34 light-years. The linear diameter, computed from Pease's measure +and this value of the distance is about 21 million miles.</p> +</div> + +<p class="indent"> +But this represents only the angle subtended by the star's disk. +To learn its linear diameter, we must know its distance. Four +determinations of the parallax, which determines the distance, +have been made. Elkin, with the Yale heliometer, obtained 0.032 +of a second of arc. Schlesinger, from photographs taken with the +30-inch Allegheny refractor, derived 0.016. Adams, by his spectroscopic +method applied with the 60-inch Mount Wilson reflector, obtained +0.012. Lee's recent value, secured photographically with the 40-inch +Yerkes refractor, is 0.022. The heliometer parallax is doubtless +less reliable than the photographic ones, and Doctor Adams states +that the spectral type and luminosity of Betelgeuse make his value +less certain than in the case of most other stars. If we take a +(weighted) mean value of 0.020 of a second, we shall probably not +be far from the truth. This parallax represents the angle subtended +by the radius of the earth's orbit (93,000,000 miles) at the distance +of Betelgeuse. By comparing it with 0.047, the angular diameter of +the star, we see that the linear diameter is about two and one-third +times as great as the distance from the earth to the sun, or +approximately 215,000,000 miles. Thus, if this measure of its distance +is not considerably in error, Betelgeuse would nearly fill the +orbit of Mars. All methods of determining the distances of the +stars are subject to uncertainty, however, and subsequent measures +may reduce this figure very appreciably. But there can be no doubt +that the diameter of Betelgeuse exceeds 100,000,000 miles, and +it is probably much greater. +</p> + +<p class="indent"> +The extremely small angle subtended by this enormous disk is explained +by the great distance of the star, which is about 160 light-years. +That is to say, light travelling at the rate of 186,000 miles per +second spends 160 years in crossing the space that lies between +us and Betelgeuse, whose tremendous proportions therefore seem +so minute even in the most powerful telescopes. +</p> + +<h3>STELLAR EVOLUTION</h3> + +<p class="indent"> +This actual measure of the diameter of Betelgeuse supplies a new +and striking test of Russell's and Hertzsprung's theory of dwarf +and giant stars. Just before the war Russell showed that our old +methods of classifying the stars according to their spectra must +be radically changed. Stars in an early stage of their life history +may be regarded as diffuse gaseous masses, enormously larger than +our sun, and at a much lower temperature. Their density must be +very low, and their state that of a perfect gas. These are the +"giants." In the slow process of time they contract through constant +loss of heat by radiation. But, despite this loss, the heat produced +by contraction and from other sources (see p. 82) causes their +temperature to rise, while their color changes from red to bluish +white. The process of shrinkage and rise of temperature goes on so +long as they remain in the state of a perfect gas. But as soon as +contraction has increased the density of the gas beyond a certain +point the cycle reverses and the temperature begins to fall. The +bluish-white light of the star turns yellowish, and we enter the +dwarf stage, of which our own sun is a representative. The density +increases, surpassing that of water in the case of the sun, and +going far beyond this point in later stages. In the lapse of millions +of years a reddish hue appears, finally turning to deep red. The +falling temperature permits the chemical elements, existing in a +gaseous state in the outer atmosphere of the star, to unite into +compounds, which are rendered conspicuous by their characteristic +bands in the spectrum. Finally comes extinction of light, as the +star approaches its ultimate state of a cold and solid globe. +</p> + +<div class="image" style="width: 419px;"> +<a name="fig_27"> +<img src="images/fig_27.jpg" width="419" height="385" alt="Fig. 27"></a> +<p class="image">Fig. 27. The giant star Antares (within the white +circle), notable for its red color in the constellation Scorpio, +and named by the Greeks "A Rival of Mars" (Hubble).</p> + +<p class="smaller"> +The distance of Antares, though not very accurately known, is probably +not far from 350 light-years. Its angular diameter of 0.040 of a +second would thus correspond to a linear diameter of about 400 +million miles.</p> +</div> + +<p class="indent"> +We may thus form a new picture of the two branches of the temperature +curve, long since suggested by Lockyer, on very different grounds, as +the outline of stellar life. On the ascending side are the giants, +of vast dimensions and more diffuse than the air we breathe. There +are good reasons for believing that the mass of Betelgeuse cannot +be more than ten times that of the sun, while its volume is at +least a million times as great and may exceed eight million times +the sun's volume. Therefore, its average density must be like that +of an attenuated gas in an electric vacuum tube. Three-quarters +of the naked-eye stars are in the giant stage, which comprises +such familiar objects as Betelgeuse, Antares, and Aldebaran, but +most of them are much denser than these greatly inflated bodies. +The pinnacle is reached in the intensely hot white stars of the +helium class, in whose spectra the lines of this gas are very +conspicuous. The density of these stars is perhaps one-tenth that +of the sun. Sirius, also very hot, is nearly twice as dense. Then +comes the cooling stage, characterized, as already remarked, by +increasing density, and also by increasing chemical complexity +resulting from falling temperature. This life cycle is probably +not followed by all stars, but it may hold true for millions of +them. +</p> + +<p class="indent"> +The existence of giant and dwarf stars has been fully proved by +the remarkable work of Adams and his associates on Mount Wilson, +where his method of determining a star's distance and intrinsic +luminosity by spectroscopic observations has already been applied +to 2,000 stars. Discussion of the results leads at once to the +recognition of the two great classes of giants and dwarfs. Now +comes the work of Michelson and Pease to cap the climax, giving us +the actual diameter of a typical giant star, in close agreement with +predictions based upon theory. From this diameter we may conclude that +the density of Betelgeuse is extremely low, in harmony with Russell's +theory, which is further supported by spectroscopic analysis of +the star's light, revealing evidence of the comparatively low +temperature called for by the theory at this early stage of stellar +existence. +</p> + +<h3>TWO OTHER GIANTS</h3> + +<p class="indent"> +The diameter of Arcturus was successfully measured by Mr. Pease +at Mount Wilson on April 15. As the mirrors of the interferometer +were moved apart, the fringes gradually decreased in visibility +until they finally disappeared at a mirror separation of 19.6 feet. +Adopting a mean wave-length of 5600/10000000 of a millimetre for +the light of Arcturus, this gives a value of 0.022 of a second of +arc for the angular diameter of the star. If we use a mean value +of 0.095 of a second for the parallax, the corresponding linear +diameter comes out 21,000,000 miles. The angular diameter, as in +the case of Betelgeuse, is in remarkably close agreement with the +diameter predicted from theory. Antares, the third star measured +by Mr. Pease, is the largest of all. If it is actually a member of +the Scorpius-Centaurus group, as we have strong reason to believe, +it is fully 350 light-years from the earth, and its diameter is +about 400,000,000 miles. +</p> + +<div class="image" style="width: 440px;"> +<a name="fig_28"> +<img src="images/fig_28.jpg" width="440" height="572" alt="Fig. 28"></a> +</div> + +<p class="indent"> +It now remains to make further measures of Betelgeuse, especially +because its marked changes in brightness suggest possible variations +in diameter. We must also apply the interferometer method to stars +of the various spectral types, in order to afford a sure basis for +future studies of stellar evolution. Unfortunately, only a few +giant stars are certain to fall within the range of our present +instrument. An interferometer of 70-feet aperture would be needed +to measure Sirius accurately, and one of twice this size to deal +with less brilliant white stars. A 100-foot instrument, if feasible +to build, would permit objects representing most of the chief stages +of stellar development to be measured, thus contributing in the +highest degree to the progress of our knowledge of the life history +of the stars. Fortunately, though the mechanical difficulties are +great, the optical problem is insignificant, and the cost of the +entire apparatus, though necessarily high, would be only a small +fraction of that of a telescope of corresponding aperture, if such +could be built. A 100-foot interferometer might be designed in +many different forms, and one of these may ultimately be found +to be within the range of possibility. Meanwhile the 20-foot +interferometer has been improved so materially that it now promises +to yield approximate measures of stars at first supposed to be +beyond its capacity. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_29"> +<img src="images/fig_29.jpg" width="417" height="482" alt="Fig. 29"></a> +<p class="image">Fig. 29. Aldebaran, the "leader" (of the Pleiades), +was also known to the Arabs as "The Eye of the Bull," "The Heart of +the Bull," and "The Great Camel" (Hubble).</p> + +<p class="smaller"> +Like Betelgeuse and Antares, it is notable for its red color, which +accounts for the fact that its image on this photograph is hardly +more conspicuous than the images of stars which are actually much +fainter but contain a larger proportion of blue light, to which +the photographic plates here employed are more sensitive than to +red or yellow. Aldebaran is about 50 light-years from the earth. +Interferometer measures, now in progress on Mount Wilson, indicate +that its angular diameter is about 0.020 of a second.</p> +</div> + +<p class="indent"> +While the theory of dwarf and giant stars and the measurements just +described afford no direct evidence bearing on Laplace's explanation +of the formation of planets, they show that stars exist which are +comparable in diameter with our solar system, and suggest that the +sun must have shrunk from vast dimensions. The mode of formation +of systems like our own, and of other systems numerously illustrated +in the heavens, is one of the most fascinating problems of astronomy. +Much light has been thrown on it by recent investigations, rendered +possible by the development of new and powerful instruments and by +advances in physics of the most fundamental character. All the +evidence confirms the existence of dwarf and giant stars, but much +work must be done before the entire course of stellar evolution +can be explained. +</p> + +<h2><a name="chapter_III">CHAPTER III</a></h2> + +<p class="subtitle">COSMIC CRUCIBLES</p> + +<p class="indent"> +"Shelter during Raids," marking the entrance to underground passages, +was a sign of common occurrence and sinister suggestion throughout +London during the war. With characteristic ingenuity and craftiness, +ostensibly for purposes of peace but with bomb-carrying capacity +as a prime specification, the Zeppelin had been developed by the +Germans to a point where it seriously threatened both London and +Paris. Searchlights, range-finders, and anti-aircraft guns, surpassed +by the daring ventures of British and French airmen, would have +served but little against the night invader except for its one fatal +defect—the inflammable nature of the hydrogen gas that kept +it aloft. A single explosive bullet served to transform a Zeppelin +into a heap of scorched and twisted metal. This characteristic of +hydrogen caused the failure of the Zeppelin raids. +</p> + +<p class="indent"> +Had the war lasted a few months longer, however, the work of American +scientists would have made our counter-attack in the air a formidable +one. At the signing of the armistice hundreds of cylinders of compressed +helium lay at the docks ready for shipment abroad. Extracted from +the natural gas of Texas wells by new and ingenious processes, +this substitute for hydrogen, almost as light and absolutely +uninflammable, produced in quantities of millions of cubic feet, +would have made the dirigibles of the Allies masters of the air. The +special properties of this remarkable gas, previously obtainable only +in minute quantities, would have sufficed to reverse the situation. +</p> + +<h3>SOLAR HELIUM</h3> + +<p class="indent"> +Helium, as its name implies, is of solar origin. In 1868, when +Lockyer first directed his spectroscope to the great flames or +prominences that rise thousands of miles, sometimes hundreds of +thousands, above the surface of the sun, he instantly identified +the characteristic red and blue radiations of hydrogen. In the +yellow, close to the position of the well-known double line of +sodium, but not quite coincident with it, he detected a new line, +of great brilliancy, extending to the highest levels. Its similarity +in this respect with the lines of hydrogen led him to recognize +the existence of a new and very light gas, unknown to terrestrial +chemistry. +</p> + +<p class="indent"> +Many years passed before any chemical laboratory on earth was able +to match this product of the great laboratory of the sun. In 1896 +Ramsay at last succeeded in separating helium, recognized by the same +yellow line in its spectrum, in minute quantities from the mineral +uraninite. Once available for study under electrical excitation in +vacuum tubes, helium was found to have many other lines in its +spectrum, which have been identified in the spectra of solar +prominences, gaseous nebulæ, and hot stars. Indeed, there +is a stellar class known as helium stars, because of the dominance +of this gas in their atmospheres. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_30"> +<img src="images/fig_30.jpg" width="417" height="401" alt="Fig. 30"></a> +<p class="image">Fig. 30. Solar prominences, photographed with the +spectroheliograph without an eclipse (Ellerman).</p> + +<p class="smaller"> +In these luminous gaseous clouds, which sometimes rise to elevations +exceeding half the sun's diameter, the new gas helium was discovered +by Lockyer in 1868. Helium was not found on the earth until 1896. +Since then it has been shown to be a prominent constituent of +nebulæ and hot stars.</p> +</div> + +<p class="indent"> +The chief importance of helium lies in the clue it has afforded to +the constitution of matter and the transmutation of the elements. +Radium and other radioactive substances, such as uranium, spontaneously +emit negatively charged particles of extremely small mass (electrons), +and also positively charged particles of much greater mass, known +as alpha particles. Rutherford and Geiger actually succeeded in +counting the number of alpha particles emitted per second by a +known mass of radium, and showed that these were charged helium +atoms. +</p> + +<p class="indent"> +To discuss more at length the extraordinary characteristics of +helium, which plays so large a part in celestial affairs, would +take us too far afield. Let us therefore pass to another case in +which a fundamental discovery, this time in physics, was first +foreshadowed by astronomical observation. +</p> + +<h3>SUN-SPOTS AS MAGNETS</h3> + +<p class="indent"> +No archæologist, whether Young or Champollion deciphering +the Rosetta Stone, or Rawlinson copying the cuneiform inscription +on the cliff of Behistun, was ever faced by a more fascinating +problem than that which confronts the solar physicist engaged in +the interpretation of the hieroglyphic lines of sun-spot spectra. +The colossal whirling storms that constitute sun-spots, so vast +that the earth would make but a moment's scant mouthful for them, +differ materially from the general light of the sun when examined +with the spectroscope. Observing them visually many years ago, +the late Professor Young, of Princeton, found among their complex +features a number of double lines which he naturally attributed, +in harmony with the physical knowledge of the time, to the effect +of "reversal" by superposed layers of vapors of different density +and temperature. What he actually saw, however, as was proved at +the Mount Wilson Observatory in 1908, was the effect of a powerful +magnetic field on radiation, now known as the Zeeman effect. +</p> + +<div class="image" style="width: 418px;"> +<a name="fig_31"> +<img src="images/fig_31.jpg" width="418" height="494" alt="Fig. 31"></a> +<p class="image">Fig. 31. The 150-foot tower telescope of the +Mount Wilson Observatory.</p> + +<p class="smaller"> +An image of the sun about 16 inches in diameter is formed in the +laboratory at the base of the tower. Below this, in a well extending +80 feet into the earth, is the powerful spectroscope with which +the magnetic fields in sun-spots and the general magnetic field +of the sun are studied.</p> +</div> + +<p class="indent"> +Faraday was the first to detect the influence of magnetism on light. +Between the poles of a large electromagnet, powerful for those days +(1845), he placed a block of very dense glass. The plane of polarization +of a beam of light, which passed unaffected through the glass before +the switch was closed, was seen to rotate when the magnetic field +was produced by the flow of the current. A similar rotation is +now familiar in the well-known tests of sugars—lævulose +and dextrose—which rotate plane-polarized light to left and +right, respectively. +</p> + +<p class="indent"> +But in this first discovery of a relationship between light and +magnetism Faraday had not taken the more important step that he +coveted—to determine whether the vibration period of a +light-emitting particle is subject to change in a magnetic field. +He attempted this in 1862—the last experiment of his life. +A sodium flame was placed between the poles of a magnet, and the +yellow lines were watched in a spectroscope when the magnet was +excited. No change could be detected, and none was found by subsequent +investigators until Zeeman, of Leiden, with more powerful instruments +made his famous discovery, the twenty-fifth anniversary of which +has recently been celebrated. +</p> + +<div class="image" style="width: 414px;"> +<a name="fig_32"> +<img src="images/fig_32.jpg" width="414" height="336" alt="Fig. 32"></a> +<p class="image">Fig. 32. Pasadena Laboratory of the Mount Wilson +Observatory.</p> + +<p class="smaller"> +Showing the large magnet (on the left) and the spectroscopes used +for the study of the effect of magnetism on radiation. A single line +in the spectrum is split by the magnetic field into from three to +twenty-one components, as illustrated in Fig. 34. The corresponding +lines in the spectra of sun-spots are split up in precisely the +same way, thus indicating the presence of powerful magnetic fields +in the sun. +</p> +</div> + +<p class="indent"> +His method of procedure was similar to Faraday's, but his magnet and +spectroscope were much more powerful, and a theory due to Lorentz, +predicting the nature of the change to be expected, was available +as a check on his results. When the current was applied the lines +were seen to widen. In a still more powerful magnetic field each +of them split into two components (when the observation was made +along the lines of force), and the light of the components of each +line was found to be circularly polarized in opposite directions. +Strictly in harmony with Lorentz's theory, this splitting and +polarization proved the presence in the luminous vapor of exactly such +negatively charged electrons as had been indicated there previously +by very different experimental methods. +</p> + +<p class="indent"> +In 1908 great cyclonic storms, or vortices, were discovered at +the Mount Wilson Observatory centring in sun-spots. Such whirling +masses of hot vapors, inferred from Sir Joseph Thomson's results +to contain electrically charged particles, should give rise to a +magnetic field. This hypothesis at once suggested that the double +lines observed by Young might really represent the Zeeman effect. +The test was made, and all the characteristic phenomena of radiation +in a magnetic field were found. +</p> + +<p class="indent"> +Thus a great physical experiment is constantly being performed +for us in the sun. Every large sunspot contains a magnetic field +covering many thousands of square miles, within which the spectrum +lines of iron, manganese, chromium, titanium, vanadium, calcium, +and other metallic vapors are so powerfully affected that their +widening and splitting can be seen with telescopes and spectroscopes +of moderate size. +</p> + +<h3>THE TOWER TELESCOPE</h3> + +<p class="indent"> +Both of these illustrations show how the physicist and chemist, +when adequately armed for astronomical attack, can take advantage +in their studies of the stupendous processes visible in cosmic +crucibles, heated to high temperatures and influenced, as in the +case of sun-spots, by intense magnetic fields. Certain modern +instruments, like the 60-foot and 150-foot tower telescopes on +Mount Wilson, are especially designed for observing the course +of these experiments. The second of these telescopes produces at +a fixed point in a laboratory an image of the sun about 16 inches +in diameter, thus enlarging the sun-spots to such a scale that +the magnetic phenomena of their various parts can be separately +studied. This analysis is accomplished with a spectroscope 80 feet +in length, mounted in a subterranean chamber beneath the tower. The +varied results of such investigations cannot be described here. +Only one of them may be mentioned—the discovery that the +entire sun, rotating on its axis, is a great magnet. Hence we may +reasonably infer that every star, and probably every planet, is +also a magnet, as the earth has been known to be since the days +of Gilbert's "De Magnete." Here lies one of the best clues for +the physicist who seeks the cause of magnetism, and attempts to +produce it, as Barnett has recently succeeded in doing, by rapidly +whirling masses of metal in the laboratory. +</p> + +<div class="image" style="width: 421px;"> +<a name="fig_33"> +<img src="images/fig_33.jpg" width="421" height="305" alt="Fig. 33"></a> +<p class="image">Fig. 33. Sun-spot vortex in the upper hydrogen +atmosphere. (Benioff).</p> + +<p class="smaller"> +Photographed with the spectroheliograph. The electric vortex that +causes the magnetic field of the spot lies at a lower level, and +is not shown by such photographs.</p> +</div> + +<p class="indent"> +Perhaps a word of caution should be interpolated at this point. +Solar magnetism in no wise accounts for the sun's gravitational +power. Indeed, its attraction cannot be felt by the most delicate +instruments at the distance of the earth, and would still be unknown +were it not for the influence of magnetism on light. +</p> + +<p class="indent"> +Auroras, magnetic storms, and such electric currents as those that +recently deranged several Atlantic cables are due, not to the magnetism +of the sun or its spots, but probably to streams of electrons, shot +out from highly disturbed areas of the solar surface surrounding +great sun-spots, traversing ninety-three million miles of the ether +of space, and penetrating deep into the earth's atmosphere. These +striking phenomena lead us into another chapter of physics, which +limitations of space forbid us to pursue. +</p> + +<h3>STELLAR CHEMISTRY</h3> + +<p class="indent"> +Let us turn again to chemistry, and see where experiments performed +in cosmic laboratories can serve as a guide to the investigator. +A spinning solar tornado, incomparably greater in scale than the +devastating whirlwinds that so often cut narrow paths of destruction +through town and country in the Middle West, gradually gives rise +to a sun-spot. The expansion produced by the centrifugal force at +the centre of the storm cools the intensely hot gases of the solar +atmosphere to a point where chemical union can occur. Titanium +and oxygen, too hot to combine in most regions of the sun, join +to form the vapor of titanium oxide, characterized in the sunspot +spectrum by fluted bands, made up of hundreds of regularly spaced +lines. Similarly magnesium and hydrogen combine as magnesium hydride +and calcium and hydrogen form calcium hydride. None of these compounds, +stable at the high temperatures of sun-spots, has been much studied +in the laboratory. The regions in which they exist, though cooler +than the general atmosphere of the sun, are at temperatures of +several thousand degrees, attained in our laboratories only with +the aid of such devices as powerful electric furnaces. +</p> + +<div class="image" style="width: 413px;"> +<a name="fig_34"> +<img src="images/fig_34.jpg" width="413" height="324" alt="Fig. 34"></a> +<p class="image">Fig. 34. Splitting of spectrum lines by a magnetic +field (Babcock).</p> + +<p class="smaller"> +The upper and lower strips show lines in the spectrum of chromium, +observed without a magnetic field. When subjected to the influence +of magnetism, these single lines are split into several components. +Thus the first line on the right is resolved by the field into +three components, one of which (plane polarized) appears in the +second strip, while the other two, which are polarized in a plane +at right angles to that of the middle component, are shown on the +third strip. The next line is split by the magnetic field into +twelve components, four of which appear in the second strip and +eight in the third. The magnetic fields in sun-spots affect these +lines in precisely the same way.</p> +</div> + +<p class="indent"> +It is interesting to follow our line of reasoning to the stars, +which differ widely in temperature at various stages in their +life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely +hot solar vapors are cooled by expansion, giving rise to the compounds +already named. A red star, in Russell's scheme of stellar evolution, +is a cooler sun, vast in volume and far more tenuous than atmospheric +air when in the initial period of the "giant" stage, but compressed +and denser than water in the "dwarf" stage, into which our sun has +already entered as it gradually approaches the last phases of its +existence. Therefore we should find, throughout the entire atmosphere +of such stars, some of the same compounds that are produced within +the comparatively small limits of a sun-spot. This, of course, +on the correct assumption that sun and stars are made of the same +substances. Fowler has already identified the bands of titanium +oxide in such red stars as the giant Betelgeuse, and in others +of its class. It is safe to predict that an interesting chapter +in the chemistry of the future will be based upon the study of +such compounds, both in the laboratory and under the progressive +temperature conditions afforded by the countless stellar "giants" +and "dwarfs" that precede and follow the solar state. +</p> + +<p class="footnote"> +[Footnote *: See Chapter II.] +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_35"> +<img src="images/fig_35.jpg" width="417" height="318" alt="Fig. 35"></a> +<p class="image">Fig. 35. Electric furnace in the Pasadena +laboratory of the Mount Wilson Observatory.</p> + +<p class="smaller"> +With which the chemical phenomena observed in sun-spots and red +stars are experimentally imitated.</p> +</div> + +<h3>ASTROPHYSICAL LABORATORIES</h3> + +<p class="indent"> +It is precisely in this long sequence of physical and chemical +changes that the astrophysicist and the astrochemist can find the +means of pushing home their attack. It is true, of course, that +the laboratory investigator has a great advantage in his ability +to control his experiments, and to vary their progress at will. +But by judicious use of the transcendental temperatures, far out +ranging those of his furnaces, and extreme conditions, which he can +only partially imitate, afforded by the sun, stars, and nebulæ, +he may greatly widen the range of his inquiries. The sequence of +phenomena seen during the growth of a sun-spot, or the observation +of spots of different sizes, and the long series of successive +steps that mark the rise and decay of stellar life, resemble the +changes that the experimenter brings about as he increases and +diminishes the current in the coils of his magnet or raises and +lowers the temperature of his electric furnace, examining from +time to time the spectrum of the glowing vapors, and noting the +changes shown by the varying appearance of their lines. +</p> + +<div class="image" style="width: 690px;"> +<a name="fig_36"> +<img src="images/fig_36.jpg" width="690" height="135" alt="Fig. 36"></a> +<p class="image">Fig. 36. Titanium oxide in red stars.</p> + +<p class="smaller"> +The upper spectrum is that of titanium in the flame of the electric +arc, where its combination with oxygen gives rise to the bands of +titanium oxide (Fowler). The lower strip shows the spectrum of +the red star Mira (Omicron Ceti), as drawn by Cortie at Stonyhurst. +The bands of titanium oxide are clearly present in the star.</p> +</div> + +<div class="image" style="width: 691px; margin-top: 1em;"> +<a name="fig_37"> +<img src="images/fig_37.jpg" width="691" height="220" alt="Fig. 37"></a> +<p class="image">Fig. 37. Titanium oxide in sun-spots.</p> + +<p class="smaller"> +The upper strip shows a portion of the spectrum of a sun-spot +(Ellerman); the lower one the corresponding region of the spectrum +of titanium oxide (King). The fluted bands of the oxide spectrum +are easily identified in the spot, where they indicate that titanium +and oxygen, too hot to combine in the solar atmosphere, unite in the +spot because of the cooling produced by expansion in the vortex.</p> +</div> + +<p class="indent"> +Astronomical observations of this character, it should be noted, are +most effective when constantly tested and interpreted by laboratory +experiment. Indeed, a modern astrophysical observatory should be +equipped like a great physical laboratory, provided on the one hand +with telescopes and accessory apparatus of the greatest attainable +power, and on the other with every device known to the investigator +of radiation and the related physical and chemical phenomena. Its +telescopes, especially designed with the aims of the physicist and +chemist in view, bring images of sun, stars, nebulæ, and +other heavenly bodies within the reach of powerful spectroscopes, +sensitive bolometers and thermopiles, and the long array of other +appliances available for the measurement and analysis of radiation. +Its electric furnaces, arcs, sparks, and vacuum tubes, its apparatus +for increasing and decreasing pressure, varying chemical conditions, +and subjecting luminous gases and vapors to the influence of electric +and magnetic fields, provide the means of imitating celestial phenomena, +and of repeating and interpreting the experiments observed at the +telescope. And the advantage thus derived, as we have seen, is not +confined to the astronomer, who has often been able, by making +fundamental physical and chemical discoveries, to repay his debt +to the physicist and chemist for the apparatus and methods which +he owes to them. +</p> + +<h3>NEWTON AND EINSTEIN</h3> + +<p class="indent"> +Take, for another example, the greatest law of physics—Newton's +law of gravitation. Huge balls of lead, as used by Cavendish, produce +by their gravitational effect a minute rotation of a delicately +suspended bar, carrying smaller balls at its extremities. But no +such feeble means sufficed for Newton's purpose. To prove the law +of gravitation he had recourse to the tremendous pull on the moon +of the entire mass of the earth, and then extended his researches +to the mutual attractions of all the bodies of the solar system. +Later Herschel applied this law to the suns which constitute double +stars, and to-day Adams observes from Mount Wilson stars falling +with great velocity toward the centre of the galactic system under +the combined pull of the millions of objects that compose it. Thus +full advantage has been taken of the possibility of utilizing the +great masses of the heavenly bodies for the discovery and application +of a law of physics and its reciprocal use in explaining celestial +motions. +</p> + +<div class="image" style="width: 414px;"> +<a name="fig_38"> +<img src="images/fig_38.jpg" width="414" height="297" alt="Fig. 38"></a> +<p class="image">Fig. 38. The Cavendish experiment.</p> + +<p class="smaller"> +Two lead balls, each two inches in diameter, are attached to the +ends of a torsion rod six feet long, which is suspended by a fine +wire. The experiment consists in measuring the rotation of the +suspended system, caused by the gravitational attraction of two +lead spheres, each twelve inches in diameter, acting on the two +small lead balls.</p> +</div> + +<p class="indent"> +Or consider the Einstein theory of relativity, the truth or falsity +of which is no less fundamental to physics. Its inception sprang from +the Michelson-Morley experiment, made in a laboratory in Cleveland, +which showed that motion of the earth through the ether of space +could not be detected. All of the three chief tests of Einstein's +general theory are astronomical—because of the great masses +required to produce the minute effects predicted: the motion of +the perihelion of Mercury, the deflection of the light of a star +by the attraction of the sun, and the shift of the lines of the +solar spectrum toward the red—questions not yet completely +answered. +</p> + +<p class="indent"> +But it is in the study of the constitution of matter and the evolution +of the elements, the deepest and most critical problem of physics +and chemistry, that the extremes of pressure and temperature in the +heavenly bodies, and the prevalence of other physical conditions not +yet successfully imitated on earth, promise the greatest progress. +It fortunately happens that astrophysical research is now at the +very apex of its development, founded as it is upon many centuries +of astronomical investigation, rejuvenated by the introduction +into the observatory of all the modern devices of the physicist, +and strengthened with instruments of truly extraordinary range +and power. These instruments bring within reach experiments that +are in progress on some minute region of the sun's disk, or in +some star too distant even to be glimpsed with ordinary telescopes. +Indeed, the huge astronomical lenses and mirrors now available +serve for these remote light-sources exactly the purpose of the +lens or mirror employed by the physicist to project upon the slit +of his spectroscope the image of a spark or arc or vacuum tube +within which atoms and molecules are exposed to the influence of +the electric discharge. The physicist has the advantage of complete +control over the experimental conditions, while the astrophysicist +must observe and interpret the experiments performed for him in +remote laboratories. In actual practice, the two classes of work +must be done in the closest conjunction, if adequate utilization +is to be made of either. And this is only natural, for the trend +of recent research has made clear the fact that one of the three +greatest problems of modern astronomy and astrophysics, ranking +with the structure of the universe and the evolution of celestial +bodies, is the constitution of matter. Let us see why this is so. +</p> + +<h3>TRANSMUTATION OF THE ELEMENTS</h3> + +<p class="indent"> +The dream of the alchemist was to transmute one element into another, +with the prime object of producing gold. Such transmutation has been +actually accomplished within the last few years, but the process +is invariably one of disintegration—the more complex elements +being broken up into simpler constituents. Much remains to be done +in this same direction; and here the stars and nebulæ, which +show the spectra of the elements under a great variety of conditions, +should help to point the way. The progressive changes in spectra, +from the exclusive indications of the simple elements hydrogen, +helium, nitrogen, possibly carbon, and the terrestrially unknown gas +nebulium in the gaseous nebulæ, to the long list of familiar +substances, including several chemical compounds, in the red stars, +may prove to be fundamentally significant when adequately studied +from the standpoint of the investigator of atomic structure. The +existing evidence seems to favor the view, recently expressed by +Saha, that many of these differences are due to varying degrees +of ionization, the outer electrons of the atoms being split off +by high temperature or electrical excitation. It is even possible +that cosmic crucibles, unrivalled by terrestrial ones, may help +materially to reveal the secret of the formation of complex elements +from simpler ones. Physicists now believe that all of the elements are +compounded of hydrogen atoms, bound together by negative electrons. +Thus helium is made up of four hydrogen atoms, yet the atomic weight +of helium (4) is less than four times that of hydrogen (1.008). +The difference may represent the mass of the electrical energy +released when the transmutation occurred. +</p> + +<div class="image" style="width: 417px;"> +<a name="fig_39"> +<img src="images/fig_39.jpg" width="417" height="533" alt="Fig. 39"></a> +<p class="image">Fig. 39. The Trifid Nebula in Sagittarius +(Ritchey).</p> + +<p class="smaller"> +The gas "nebulium," not yet found on the earth, is the most +characteristic constituent of irregular nebulæ. Nebulium +is recognized by two green lines in its spectrum, which cause the +green color of nebulæ of the gaseous type.</p> +</div> + +<p class="indent"> +Eddington has speculated in a most interesting way on this possible +source of stellar heat in his recent presidential address before +the British Association for the Advancement of Science (see +<i>Nature</i>, September 2, 1920). He points out that the old +contraction hypothesis, according to which the source of solar +and stellar heat was supposed to reside in the slow condensation +of a radiating mass of gas under the action of gravity, is wholly +inadequate to explain the observed phenomena. If the old view were +correct, the earlier history of a star, from the giant stage of +a cool and diaphanous gas to the period of highest temperature, +would be run through within eighty thousand years, whereas we have +the best of evidence that many thousands of centuries would not +suffice. Some other source of energy is imperatively needed. If 5 +per cent of a star's mass consists originally of hydrogen atoms, +which gradually combine in the slow process of time to form more +complex elements, the total heat thus liberated would more than +suffice to account for all demands, and it would be unnecessary +to assume the existence of any other source of heat. +</p> + +<div class="image" style="width: 421px;"> +<a name="fig_40"> +<img src="images/fig_40.jpg" width="421" height="537" alt="Fig. 40"></a> +<p class="image">Fig. 40. Spiral nebula in Ursa Major (Ritchey).</p> + +<p class="smaller"> +Luminous matter, in every variety of physical and chemical state, +is available for study in the most diverse celestial objects, from +the spiral and irregular nebulæ through all the types of +stars. Doctor van Maanen's measures of the Mount Wilson photographs +indicate outward motion along the arms of spiral nebulæ, +while the spectroscope shows them to be whirling at enormous +velocities.</p> +</div> + +<h3>COSMIC PRESSURES</h3> + +<p class="indent"> +This, it may fairly be said, is very speculative, but the fact +remains that celestial bodies appear to be the only places in which +the complex elements may be in actual process of formation from +their known source—hydrogen. At least we may see what a vast +variety of physical conditions these cosmic crucibles afford. At +one end of the scale we have the excessively tenuous nebulæ, +the luminosity of which, mysterious in its origin, resembles the +electric glow in our vacuum tubes. Here we can detect only the +lightest and simplest of the elements. In the giant stars, also +extremely tenuous (the density of Betelgeuse can hardly exceed +one-thousandth of an atmosphere) we observe the spectra of iron, +manganese, titanium, calcium, chromium, magnesium, vanadium, and +sodium, in addition to titanium oxide. The outer part of these bodies, +from which light reaches us, must therefore be at a temperature +of only a few thousand degrees, but vastly higher temperatures +must prevail at their centres. In passing up the temperature curve +more and more elements appear, the surface temperature rises, and +the internal temperature may reach millions of degrees. At the +same time the pressure within must also rise, reaching enormous +figures in the last stages of stellar life. Cook has calculated +that the pressure at the centre of the earth is between 4,000 and +10,000 tons per square inch, and this must be only a very small +fraction of that attained within larger celestial bodies. Jeans +has computed the pressure at the centre of two colliding stars +as they strike and flatten, and finds it may be of the order of +1,000,000,000 tons per square inch—sufficient, if their diameter +be equal to that of the sun—to vaporize them 100,000 times +over. +</p> + +<p class="indent"> +Compare these pressures with the highest that can be produced on +earth. If the German gun that bombarded Paris were loaded with a +solid steel projectile of suitable dimensions, a muzzle velocity +of 6,000 feet per second could be reached. Suppose this to be fired +into a tapered hole in a great block of steel. The instantaneous +pressure, according to Cook, would be about 7,000 tons per square +inch, only 1/150000 of that possible through the collision of the +largest stars. +</p> + +<div class="image" style="width: 415px;"> +<a name="fig_41"> +<img src="images/fig_41.jpg" width="415" height="327" alt="Fig. 41"></a> +<p>Fig. 41. Mount San Antonio as seen from Mount Wilson.</p> + +<p class="smaller"> +Michelson is measuring the velocity of light between stations on +Mount Wilson and Mount San Antonio. Astronomical observations afford +the best means, however, of detecting any possible difference between +the velocities of light of different colors. From studies of variable +stars in the cluster Messier 5 Shapley concludes that if there is +any difference between the velocities of blue and yellow light +in free space it cannot exceed two inches in one second, the time +in which light travels 186,000 miles.</p> +</div> + +<p class="indent"> +Finally, we may compare the effects of light pressure on the earth +and stars. Twenty years ago Nichols and Hull succeeded, with the +aid of the most sensitive apparatus, in measuring the minute +displacements produced by the pressure of light. The effect is +so slight, even with the brightest light-sources available, that +great experimental skill is required to measure it. Yet in the +case of some of the larger stars Eddington calculates that one-half +of their mass is supported by radiation pressure, and this against +their enormous gravitational attraction. In fact, if their mass +were as great as ten times that of the sun, the radiation pressure +would so nearly overcome the pull of gravitation that they would +be likely to break up. +</p> + +<p class="indent"> +But enough has been said to illustrate the wide variety of experimental +devices that stand at our service in the laboratories of the heavens. +Here the physicist and chemist of the future will more and more +frequently supplement their terrestrial apparatus, and find new +clues to the complex problems which the amazing progress of recent +years has already done so much to solve. +</p> + +<h3>PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER</h3> + +<p class="indent"> +The layman has no difficulty in recognizing the practical value +of researches directed toward the improvement of the incandescent +lamp or the increased efficiency of the telephone. He can see the +results in the greatly decreased cost of electric illumination +and the rapid extension of the range of the human voice. But the +very men who have made these advances, those who have succeeded +beyond all expectation in accomplishing the economic purposes in +view, are most emphatic in their insistence upon the importance +of research of a more fundamental character. Thus Vice-President +J. J. Carty, of the American Telephone and Telegraph Company, who +directs its great Department of Development and Research, and Doctor +W. J. Whitney, Director of the Research Laboratory of the General +Electric Company, have repeatedly expressed their indebtedness +to the investigations of the physicist, made with no thought of +immediate practical return. Faraday, studying the laws of electricity, +discovered the principle which rendered the dynamo possible. Maxwell, +Henry, and Hertz, equally unconcerned with material advantage, +made wireless telegraphy practicable. In fact, all truly great +advances are thus derived from fundamental science, and the future +progress of the world will be largely dependent upon the provision +made for scientific research, especially in the fields of physics +and chemistry, which underlie all branches of engineering. +</p> + +<p class="indent"> +The constitution of matter, therefore, instead of appealing as +a subject to research only to the natural philosopher or to the +general student of science, is a question of the greatest practical +concern. Already the by-products of investigations directed toward +its elucidation have been numerous and useful in the highest degree. +Helium has been already cited; X-rays hardly require mention; radium, +which has so materially aided sufferers from cancer, is still better +known. Wireless telephony and transcontinental telephony with wires +were both rendered possible by studies of the nature of the electric +discharge in vacuum tubes. Thus the "practical man," with his distrust +of "pure" science, need not resent investments made for the purpose +of advancing our knowledge of such fundamental subjects as physics +and chemistry. On the contrary, if true to his name, he should +help to multiply them many fold in the interest of economic and +commercial development. +</p> + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of The New Heavens, by George Ellery Hale + +*** END OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + +***** This file should be named 19395-h.htm or 19395-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/1/9/3/9/19395/ + +Produced by Robert J. 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--- /dev/null +++ b/19395-h/images/fig_37.jpg diff --git a/19395-h/images/fig_38.jpg b/19395-h/images/fig_38.jpg Binary files differnew file mode 100644 index 0000000..2cea391 --- /dev/null +++ b/19395-h/images/fig_38.jpg diff --git a/19395-h/images/fig_39.jpg b/19395-h/images/fig_39.jpg Binary files differnew file mode 100644 index 0000000..7bd654a --- /dev/null +++ b/19395-h/images/fig_39.jpg diff --git a/19395-h/images/fig_40.jpg b/19395-h/images/fig_40.jpg Binary files differnew file mode 100644 index 0000000..1a02803 --- /dev/null +++ b/19395-h/images/fig_40.jpg diff --git a/19395-h/images/fig_41.jpg b/19395-h/images/fig_41.jpg Binary files differnew file mode 100644 index 0000000..8d9d5e9 --- /dev/null +++ b/19395-h/images/fig_41.jpg diff --git a/19395.txt b/19395.txt new file mode 100644 index 0000000..8923e6d --- /dev/null +++ b/19395.txt @@ -0,0 +1,2310 @@ +The Project Gutenberg EBook of The New Heavens, by George Ellery Hale + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The New Heavens + +Author: George Ellery Hale + +Release Date: September 28, 2006 [EBook #19395] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + + + + +Produced by Robert J. Hall + + + + +[Illustration: Fig. 1. The Constellation of Orion (Hubble). + +Photographed with a small camera lens of 1 inch aperture and 5 +inches focal length. The three bright stars in the centre of the +picture form the belt of Orion. Just below, in the sword handle, is +an irregular white patch about one-eighth of an inch in diameter. +This is a small-scale image of the great nebula in Orion, shown +on a larger scale in Fig. 2.] + + + + +THE NEW HEAVENS + + +BY + +GEORGE ELLERY HALE + +DIRECTOR OF THE MOUNT WILSON OBSERVATORY OF THE CARNEGIE INSTITUTION +OF WASHINGTON + +WITH NUMEROUS ILLUSTRATIONS + + +NEW YORK + +CHARLES SCRIBNER'S SONS + +1922 + + + + +TO MY WIFE + + + + +PREFACE + +Fourteen years ago, in a book entitled "The Study of Stellar Evolution" +(University of Chicago Press, 1908), I attempted to give in untechnical +language an account of some modern methods of astrophysical research. +This book is now out of print, and the rapid progress of science has +left it completely out of date. As I have found no opportunity to +prepare a new edition, or to write another book of similar purpose, +I have adopted the simpler expedient of contributing occasional +articles on recent developments to _Scribner's Magazine_, three +of which are included in the present volume. + +I am chiefly indebted, for the illustrations, to the Mount Wilson +Observatory and the present and former members of its staff whose +names appear in the captions. Special thanks are due to Mr. Ferdinand +Ellerman, who made all of the photographs of the observatory buildings +and instruments, and prepared all material for reproduction. The +cut of the original Cavendish apparatus is copied from the +_Philosophical Transactions for 1798_ with the kind permission +of the Royal Society, and I am also indebted to the Royal Society +and to Professor Fowler and Father Cortie for the privilege of +reproducing from the _Proceedings_ two illustrations of their +spectroscopic results. + +G. E. H. + +January, 1922. + + + + +CONTENTS + +CHAPTER + I. THE NEW HEAVENS + II. GIANT STARS + III. COSMIC CRUCIBLES + + + +ILLUSTRATIONS + +FIG. + 1. The Constellation of Orion (Hubble) + 2. The Great Nebula in Orion (Pease) + 3. Model by Ellerman of summit of Mount Wilson, showing the observatory + buildings among the trees and bushes + 4. The 100-inch Hooker telescope + 5. Erecting the polar axis of the 100-inch telescope + 6. Lowest section of tube of 100-inch telescope, ready to leave Pasadena + for Mount Wilson + 7. Section of a steel girder for dome covering the 100-inch telescope, + on its way up Mount Wilson + 8. Erecting the steel building and revolving dome that cover the Hooker + telescope + 9. Building and revolving dome, 100 feet in diameter, covering the + 100-inch Hooker telescope + 10. One-hundred-inch mirror, just silvered, rising out of the + silvering-room in pier before attachment to lower end of telescope + tube. (Seen above) + 11. The driving-clock and worm-gear that cause the 100-inch Hooker + telescope to follow the stars + 12. Large irregular nebula and star cluster in Sagittarius (Duncan) + 13. Faint spiral nebula in the constellation of the Hunting Dogs (Pease) + 14. Spiral nebula in Andromeda, seen edge on (Ritchey) + 15. Photograph of the moon made on September 15, 1919, with the 100-inch + Hooker telescope (Pease) + 16. Photograph of the moon made on September 15, 1919, with the 100-inch + Hooker telescope (Pease) + 17. Hubble's Variable Nebula. One of the few nebulae known to vary in + brightness and form + 18. Ring Nebula in Lyra, photographed with the 60-inch (Ritchey) and + 100-inch (Duncan) telescopes + 19. Gaseous prominence at the sun's limb, 140,000 miles high (Ellerman) + 20. The sun, 865,000 miles in diameter, from a direct photograph showing + many sun-spots (Whitney) + 21. Great sun-spot group, August 8, 1917 (Whitney) + 22. Photograph of the hydrogen atmosphere of the sun (Ellerman) + 23. Diagram showing outline of the 100-inch Hooker telescope, and path of + the two pencils of light from a star when under observation with the + 20-foot Michelson interferometer + 24. Twenty-foot Michelson interferometer for measuring star diameters, + attached to upper end of the skeleton tube of the 100-inch Hooker + telescope + 25. The giant Betelgeuse (within the circle), familiar as the conspicuous + red star in the right shoulder of Orion (Hubble) + 26. Arcturus (within the white circle), known to the Arabs as the "Lance + Bearer," and to the Chinese as the "Great Horn" or the "Palace of the + Emperors" (Hubble) + 27. The giant star Antares (within the white circle), notable for its red + color in the constellation Scorpio, and named by the Greeks "A Rival + of Mars" (Hubble) + 28. Diameters of the Sun, Arcturus, Betelgeuse, and Antares compared with + the orbit of Mars + 29. Aldebaran, the "leader" (of the Pleiades), was also known to the Arabs + as "The Eye of the Bull," "The Heart of the Bull," and "The Great + Camel" (Hubble) + 30. Solar prominences, photographed with the spectroheliograph without an + eclipse (Ellerman) + 31. The 150-foot tower telescope of the Mount Wilson Observatory + 32. Pasadena Laboratory of the Mount Wilson Observatory + 33. Sun-spot vortex in the upper hydrogen atmosphere (Benioff) + 34. Splitting of spectrum lines by a magnetic field (Bacock) + 35. Electric furnace in the Pasadena Laboratory of the Mount Wilson + Observatory + 36. Titanium oxide in red stars + 37. Titanium oxide in sun-spots + 38. The Cavendish experiment + 39. The Trifid Nebula in Sagittarius (Ritchey) + 40. Spiral nebula in Ursa Major (Ritchey) + 41. Mount San Antonio as seen from Mount Wilson + + + + +CHAPTER I + +THE NEW HEAVENS + +Go out under the open sky, on a clear and moon-less night, and try +to count the stars. If your station lies well beyond the glare of +cities, which is often strong enough to conceal all but the brighter +objects, you will find the task a difficult one. Ranging through +the six magnitudes of the Greek astronomers, from the brilliant +Sirius to the faintest perceptible points of light, the stars are +scattered in great profusion over the celestial vault. Their number +seems limitless, yet actual count will show that the eye has been +deceived. In a survey of the entire heavens, from pole to pole, +it would not be possible to detect more than from six to seven +thousand stars with the naked eye. From a single viewpoint, even +with the keenest vision, only two or three thousand can be seen. +So many of these are at the limit of visibility that Ptolemy's +"Almagest," a catalogue of all the stars whose places were measured +with the simple instruments of the Greek astronomers, contains +only 1,022 stars. + +Back of Ptolemy, through the speculations of the Greek philosophers, +the mysteries of the Egyptian sun-god, and the observations of the +ancient Chaldeans, the rich and varied traditions of astronomy stretch +far away into a shadowy past. All peoples, in the first stirrings +of their intellectual youth, drawn by the nightly splendor of the +skies and the ceaseless motions of the planets, have set up some +system of the heavens, in which the sense of wonder and the desire +for knowledge were no less concerned than the practical necessities +of life. The measurement of time and the needs of navigation have +always stimulated astronomical research, but the intellectual demand +has been keen from the first. Hipparchus and the Greek astronomers +of the Alexandrian school, shaking off the vagaries of magic and +divination, placed astronomy on a scientific basis, though the +reaction of the Middle Ages caused even such a great astronomer +as Tycho Brahe himself to revert for a time to the practice of +astrology. + +EARLY INSTRUMENTS + +The transparent sky of Egypt, rarely obscured by clouds, greatly +favored Ptolemy's observations. Here was prepared his great star +catalogue, based upon the earlier observations of Hipparchus, and +destined to remain alone in its field for more than twelve centuries, +until Ulugh Bey, Prince of Samarcand, repeated the work of his +Greek predecessor. Throughout this period the stars were looked +upon mainly as points of reference for the observation of planetary +motions, and the instruments of observation underwent little change. +The astrolabe, which consists of a circle divided into degrees, +with a rotating diametral arm for sighting purposes, embodies their +essential principle. In its simple form, the astrolabe was suspended +in a vertical plane, and the stars were observed by bringing the +sights on the movable diameter to bear upon them. Their altitude +was then read off on the circle. Ultimately, the circle of the +astrolabe, mounted with one of its diameters parallel to the earth's +axis, became the armillary sphere, the precursor of our modern +equatorial telescope. Great stone quadrants fixed in the meridian +were also employed from very early times. Out of such furnishings, +little modified by the lapse of centuries, was provided the elaborate +instrumental equipment of Uranibourg, the great observatory built +by Tycho Brahe on the Danish island of Huen in 1576. In this "City +of the Heavens," still dependent solely upon the unaided eye as a +collector of starlight, Tycho made those invaluable observations +that enabled Kepler to deduce the true laws of planetary motion. But +after all these centuries the sidereal world embraced no objects, +barring an occasional comet or temporary star, that lay beyond +the vision of the earliest astronomers. The conceptions of the +stellar universe, except those that ignored the solid ground of +observation, were limited by the small aperture of the human eye. +But the dawn of another age was at hand. + +[Illustration: Fig. 2. The Great Nebula in Orion (Pease). + +Photographed with the 100-inch telescope. This short-exposure photograph +shows only the bright central part of the nebula. A longer exposure +reveals a vast outlying region.] + +The dominance of the sun as the central body of the solar system, +recognized by Aristarchus of Samos nearly three centuries before +the Christian era, but subsequently denied under the authority of +Ptolemy and the teachings of the Church, was reaffirmed by the +Polish monk Copernicus in 1543. Kepler's laws of the motions of the +planets, showing them to revolve in ellipses instead of circles, +removed the last defect of the Copernican system, and left no room for +its rejection. But both the world and the Church clung to tradition, +and some visible demonstration was urgently needed. This was supplied +by Galileo through his invention of the telescope. + +[Illustration: Fig. 3. Model by Ellerman of summit of Mount Wilson, +showing the observatory buildings among the trees and bushes. + +The 60-foot tower on the extreme left, which is at the edge of +a precipitous canon 1,500 feet deep, is the vertical telescope +of the Smithsonian Astrophysical Observatory. Above it are the +"Monastery" and other buildings used as quarters by the astronomers +of the Mount Wilson Observatory while at work on the mountain. (The +offices, computing-rooms, laboratories, and shops are in Pasadena.) +Following the ridge, we come successively to the dome of the 10-inch +photographic telescope, the power-house, laboratory, Snow horizontal +telescope, 60-foot-tower telescope, and 150-foot-tower telescope, +these last three used for the study of the sun. The dome of the +60-inch reflecting telescope is just below the 150-foot tower, +while that of the 100-inch telescope is farther to the right. The +altitude of Mount Wilson is about 5,900 feet.] + +The crystalline lens of the human eye, limited by the iris to a +maximum opening about one-quarter of an inch in diameter, was the +only collector of starlight available to the Greek and Arabian +astronomers. Galileo's telescope, which in 1610 suddenly pushed +out the boundaries of the known stellar universe and brought many +thousands of stars into range, had a lens about 2-1/4 inches in +diameter. The area of this lens, proportional to the square of +its diameter, was about eighty-one times that of the pupil of the +eye. This great increase in the amount of light collected should +bring to view stars down to magnitude 10.5, of which nearly half +a million are known to exist. + +It is not too much to say that Galileo's telescope revolutionized +human thought. Turned to the moon, it revealed mountains, plains, +and valleys, while the sun, previously supposed immaculate in its +perfection, was seen to be blemished with dark spots changing from +day to day. Jupiter, shown to be accompanied by four encircling +satellites, afforded a picture in miniature of the solar system, +and strongly supported the Copernican view of its organization, +which was conclusively demonstrated by Galileo's discovery of the +changing phases of Venus and the variation of its apparent diameter +during its revolution about the sun. Galileo's proof of the Copernican +theory marked the downfall of mediaevalism and established astronomy on +a firm foundation. But while his telescope multiplied a hundredfold +the number of visible stars, more than a century elapsed before +the true possibilities of sidereal astronomy were perceived. + +[Illustration: Fig. 4. The 100-inch Hooker telescope.] + +STRUCTURE OF THE UNIVERSE + +Sir William Herschel was the first astronomer to make a serious +attack upon the problem of the structure of the stellar universe. +In his first memoir on the "Construction of the Heavens," read +before the Royal Society in 1784, he wrote as follows: + +"Hitherto the sidereal heavens have, not inadequately for the purpose +designed, been represented by the concave surface of a sphere in +the centre of which the eye of an observer might be supposed to be +placed.... In future we shall look upon those regions into which we +may now penetrate by means of such large telescopes, as a naturalist +regards a rich extent of ground or chain of mountains containing +strata variously inclined and directed as well as consisting of +very different materials." + +On turning his 18-inch reflecting telescope to a part of the Milky +Way in Orion, he found its whitish appearance to be completely +resolved into small stars, not separately seen with his former +telescopes. "The glorious multitude of stars of all possible sizes +that presented themselves here to my view are truly astonishing; but +as the dazzling brightness of glittering stars may easily mislead +us so far as to estimate their number greater than it really is, +I endeavored to ascertain this point by counting many fields, and +computing from a mean of them, what a certain given portion of +the Milky Way might contain." By this means, applied not only to +the Milky Way but to all parts of the heavens, Herschel determined +the approximate number and distribution of all the stars within +reach of his instrument. + +By comparing many hundred gauges or counts of stars visible in +a field of about one-quarter of the area of the moon, Herschel +found that the average number of stars increased toward the great +circle which most nearly conforms with the course of the Milky Way. +Ninety degrees from this plane, at the pole of the Milky Way, only +four stars, on the average, were seen in the field of the telescope. +In approaching the Milky Way this number increased slowly at first, +and then more and more rapidly, until it rose to an average of +122 stars per field. + +[Illustration: Fig. 5. Erecting the polar axis of the 100-inch +telescope.] + +These observations were made in the northern hemisphere, and +subsequently Sir John Herschel, using his father's telescope at +the Cape of Good Hope, found an almost exactly similar increase +of apparent star density for the southern hemisphere. According to +his estimates, the total number of stars in both hemispheres that +could be seen distinctly enough to be counted in this telescope +would probably be about five and one-half millions. + +The Herschels concluded that "the stars of our firmament, instead +of being scattered in all directions indifferently through space, +form a stratum of which the thickness is small, in comparison with +its length and breadth; and in which the earth occupies a place +somewhere about the middle of its thickness, between the point +where it subdivides into two principal laminae inclined at a small +angle to each other." This view does not differ essentially from our +modern conception of the form of the Galaxy; but as the Herschels +were unable to see stars fainter than the fifteenth magnitude, +it is evident that their conclusions apply only to a restricted +region surrounding the solar system, in the midst of the enormously +extended sidereal universe which modern instruments have brought +within our range. + +MODERN METHODS + +The remarkable progress of modern astronomy is mainly due to two +great instrumental advances: the rise and development of the +photographic telescope, and the application of the spectroscope to +the study of celestial objects. These new and powerful instruments, +supplemented by many accessories which have completely revolutionized +observatory equipment, have not only revealed a vastly greater +number of stars and nebulae: they have also rendered feasible +observations of a type formerly regarded as impossible. The chemical +analysis of a faint star is now so easy that it can be accomplished +in a very short time--as quickly, in fact, as an equally complex +substance can be analyzed in the laboratory. The spectroscope also +measures a star's velocity, the pressure at different levels in +its atmosphere, its approximate temperature, and now, by a new +and ingenious method, its distance from the earth. It determines +the velocity of rotation of the sun and of nebulae, the existence +and periods of orbital revolution of binary stars too close to +be separated by any telescope, the presence of magnetic fields +in sunspots, and the fact that the entire sun, like the earth, is +a magnet. + +[Illustration: Fig. 6. Lowest section of tube of 100-inch telescope, +ready to leave Pasadena for Mount Wilson.] + +Such new possibilities, with many others resulting from the application +of physical methods of the most diverse character, have greatly +enlarged the astronomer's outlook. He may now attack two great +problems: (1) The structure of the universe and the motions of +its constituent bodies, and (2) the evolution of the stars: their +nature, origin, growth, and decline. These two problems are intimately +related and must be studied as one.[*] + +[Footnote *: A third great problem open to the astronomer, the +study of the constitution of matter, is described in Chapter III.] + +If space permitted, it would be interesting to survey the progress +already accomplished by modern methods of astronomical research. +Hundreds of millions of stars have been photographed, and the boundaries +of the stellar universe have been pushed far into space, but have not +been attained. Globular star clusters, containing tens of thousands +of stars, are on so great a scale (according to Shapley) that light, +travelling at the rate of 186,000 miles per second, may take 500 +years to cross one of them, while the most distant of these objects +may be more than 200,000 light-years from the earth. The spiral +nebulae, more than a million in number, are vast whirling masses +in process of development, but we are not yet certain whether they +should be regarded as "island universes" or as subordinate to the +stellar system which includes our minute group of sun and planets, +the great star clouds of the Milky Way, and the distant globular +star clusters. + +[Illustration: Fig. 7. Section of a steel girder for dome covering +the 100-inch telescope, on its way up Mount Wilson.] + +These few particulars may give a slight conception of the scale +of the known universe, but a word must be added regarding some +of its most striking phenomena. The great majority of the stars +whose motions have been determined belong to one or the other of +two great star streams, but the part played by these streams in the +sidereal system as a whole is still obscure. The stars have been +grouped in classes, presumably in the order of their evolutional +development, as they pass from the early state of gaseous masses, of +low density, through the successive stages resulting from loss of +heat by radiation and increased density due to shrinkage. Strangely +enough, their velocities in space show a corresponding change, +increasing as they grow older or perhaps depending upon their mass. + +It is impossible within these limits to do more than to give some +indication of the scope of the new astronomy. Enough has been said, +however, to assist in appreciating the increased opportunity for +investigation, and the nature of the heavy demands made upon the +modern observatory. But before passing on to describe one of the +latest additions to the astronomer's instrumental equipment, a +word should be added regarding the chief classes of telescopes. + +REFRACTORS AND REFLECTORS + +Astronomical telescopes are of two types: refractors and reflectors. +A refracting telescope consists of an object-glass composed of +two or more lenses, mounted at the upper end of a tube, which is +pointed at the celestial object. The light, after passing through +the lenses, is brought to a focus at the lower end of the tube, where +the image is examined visually with an eyepiece, or photographed +upon a sensitive plate. The largest instruments of this type are +the 36-inch Lick telescope and the 40-inch refractor of the Yerkes +Observatory. + +[Illustration: Fig. 8. Erecting the steel building and revolving +dome that cover the Hooker telescope.] + +Reflecting telescopes, which are particularly adapted for photographic +work, though also excellent for visual observations, are very +differently constructed. No lens is used. The telescope tube is +usually built in skeleton form, open at its upper end, and with +a large concave mirror supported at its base. This mirror serves +in place of a lens. Its upper surface is paraboloidal in shape, +as a spherical surface will not unite in a sharp focus the rays +coming from a distant object. The light passes through no glass--a +great advantage, especially for photography, as the absorption +in lenses cuts out much of the blue and violet light, to which +photographic plates are most sensitive. The reflection occurs on +the _upper_ surface of the mirror, which is covered with a coat of +pure silver, renewed several times a year and always kept highly +burnished. Silvered glass is better than metals or other substances +for telescope mirrors, chiefly because of the perfection with which +glass can be ground and polished, and the ease of renewing its +silvered surface when tarnished. + +The great reflectors of Herschel and Lord Rosse, which were provided +with mirrors of speculum metal, were far inferior to much smaller +telescopes of the present day. With these instruments the star images +were watched as they were carried through the field of view by the +earth's rotation, or kept roughly in place by moving the telescope +with ropes or chains. Photographic plates, which reveal invisible +stars and nebulae when exposed for hours in modern instruments, were +not then available. In any case they could not have been used, +in the absence of the perfect mechanism required to keep the star +images accurately fixed in place upon the sensitive film. + +[Illustration: Fig. 9. Building and revolving dome, 100 feet in +diameter, covering the 100-inch Hooker telescope. + +Photographed from the summit of the 150-foot-tower telescope.] + +It would be interesting to trace the long contest for supremacy +between refracting and reflecting telescopes, each of which, at +certain stages in its development, appeared to be unrivalled. In +modern observatories both types are used, each for the purpose for +which it is best adapted. For the photography of nebulae and the +study of the fainter stars, the reflector has special advantages, +illustrated by the work of such instruments as the Crossley and Mills +reflectors of the Lick Observatory; the great 72-inch reflector, +recently brought into effective service at the Dominion Observatory +in Canada; and the 60-inch and 100-inch reflectors of the Mount +Wilson Observatory. + +The unaided eye, with an available area of one-twentieth of a square +inch, permits us to see stars of the sixth magnitude. Herschel's +18-inch reflector, with an area 5,000 times as great, rendered +visible stars of the fifteenth magnitude. The 60-inch reflector, +with an area 57,600 times that of the eye, reveals stars of the +eighteenth magnitude, while to reach stars of about the twentieth +magnitude, photographic exposures of four or five hours suffice +with this instrument. + +Every gain of a magnitude means a great gain in the number of stars +rendered visible. Stars of the second magnitude are 3.4 times as +numerous as those of the first, those of the eighth magnitude are +three times as numerous as those of the seventh, while the sixteenth +magnitude stars are only 1.7 as numerous as those of the fifteenth +magnitude. This steadily decreasing ratio is probably due to an +actual thinning out of the stars toward the boundaries of the stellar +universe, as the most exhaustive tests have failed to give any +evidence of absorption of light in its passage through space. But +in spite of this decrease, the gain of a single additional magnitude +may mean the addition of many millions of stars to the total of those +already shown by the 60-inch reflector. Here is one of the chief +sources of interest in the possibilities of a 100-inch reflecting +telescope. + +100-INCH TELESCOPE + +[Illustration: Fig. 10. One-hundred-inch mirror, just silvered, +rising out of the silvering-room in pier before attachment to lower +end of telescope tube. (Seen above.)] + +In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie +Institution of Washington a sum sufficient to construct a telescope +mirror 100 inches in diameter, and thus large enough to collect +160,000 times the light received by the eye. (Fig. 10.) The casting +and annealing of a suitable glass disk, 101 inches in diameter +and 13 inches thick, weighing four and one-half tons, was a most +difficult operation, finally accomplished by a great French glass +company at their factory in the Forest of St. Gobain. A special +optical laboratory was erected at the Pasadena headquarters of +the Mount Wilson Observatory, and here the long task of grinding, +figuring, and testing the mirror was successfully carried out by +the observatory opticians. This operation, which is one of great +delicacy, required years for its completion. Meanwhile the building, +dome, and mounting for the telescope were designed by members of +the observatory staff, and the working drawings were prepared. An +opportune addition by Mr. Carnegie to the endowment of the Carnegie +Institution of Washington, of which the observatory is a branch, +permitted the necessary appropriations to be made for the completion +and erection of the telescope. Though delayed by the war, during +which the mechanical and optical facilities of the observatory +shops were utilized for military and naval purposes, the telescope +is now in regular use on Mount Wilson. + +The instrument is mounted on a massive pier of reinforced concrete, +33 feet high and 52 feet in diameter at the top. A solid wall extends +south from this pier a distance of 50 feet, on the west side of +which a very powerful spectrograph, for photographing the spectra +of the brightest stars, will be mounted. Within the pier are a +photographic dark room, a room for silvering the large mirror (which +can be lowered into the pier), and the clock-room, where stands +the powerful driving-clock, with which the telescope is caused +to follow the apparent motion of the stars. (Fig. 11.) + +[Illustration: Fig. 11. The driving-clock and worm-gear that cause +the 100-inch Hooker telescope to follow the stars.] + +The telescope mounting is of the English type, in which the telescope +tube is supported by the declination trunnions between the arms of +the polar axis, built in the form of a rectangular yoke carried by +bearings on massive pedestals to the north and south. These bearings +must be aligned exactly parallel to the axis of the earth, and must +support the polar axis so freely that it can be rotated with perfect +precision by the driving-clock, which turns a worm-wheel 17 feet in +diameter, clamped to the lower end of the axis. As this motion +must be sufficiently uniform to counteract exactly the rotation +of the earth on its axis, and thus to maintain the star images +accurately in position in the field of view, the greatest care +had to be taken in the construction of the driving-clock and in +the spacing and cutting of the teeth in the large worm-wheel. Here, +as in the case of all of the more refined parts of the instrument, +the work was done by skilled machinists in the observatory shops in +Pasadena or on Mount Wilson after the assembling of the telescope. +The massive sections of the instrument, some of which weigh as +much as ten tons each, were constructed at Quincy, Mass., where +machinery sufficiently large to build battleships was available. +They were then shipped to California, and transported to the summit +of Mount Wilson over a road built for this purpose by the construction +division of the observatory, which also built the pier on which the +telescope stands, and erected the steel building and dome that +cover it. + +[Illustration: Fig. 12. Large irregular nebula and star cluster +in Sagittarius (Duncan). + +Photographed with the 60-inch telescope.] + +[Illustration: Fig. 13. Faint spiral nebula in the constellation +of the Hunting Dogs (Pease). + +Photographed with the 60-inch telescope.] + +The parts of the telescope which are moved by the driving-clock +weigh about 100 tons, and it was necessary to provide means of +reducing the great friction on the bearings of the polar axis. To +accomplish this, large hollow steel cylinders, floating in mercury +held in cast-iron tanks, were provided at the upper and lower ends +of the polar axis. Almost the entire weight of the instrument is +thus floated in mercury, and in this way the friction is so greatly +reduced that the driving-clock moves the instrument with perfect +ease and smoothness. + +The 100-inch mirror rests at the bottom of the telescope tube on +a special support system, so designed as to prevent any bending of +the glass under its own weight. Electric motors, forty in number, are +provided to move the telescope rapidly or slowly in right ascension +(east or west) and in declination (north or south), for focussing +the mirrors, and for many other purposes. They are also used for +rotating the dome, 100 feet in diameter, under which the telescope +is mounted, and for opening the shutter, 20 feet wide, through +which the observations are made. + +A telescope of this kind can be used in several different ways. +The 100-inch mirror has a focal length of about 42 feet, and in +one of the arrangements of the instrument, the photographic plate +is mounted at the centre of the telescope tube near its upper end, +where it receives directly the image formed by the large mirror. In +another arrangement, a silvered glass mirror, with plane surface, +is supported near the upper end of the tube at an angle of 45 deg., so +as to form the image at the side of the tube, where the photographic +plate can be placed. In this case, the observer stands on a platform, +which is moved up and down by electric motors in front of the opening +in the dome through which the observations are made. + +[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on +(Ritchey). + +Photographed with the 60-inch telescope.] + +Other arrangements of the telescope, for which auxiliary convex +mirrors carried near the upper end of the tube are required, permit +the image to be photographed at the side of the tube near its lower +end, either with or without a spectrograph; or with a very powerful +spectrograph mounted within a constant-temperature chamber south +of the telescope pier. In this last case, the light of a star is +so reflected by auxiliary mirrors that it passes down through a +hole in the south end of the polar axis and brings the star to +a focus on the slit of the fixed spectrograph. + +ATMOSPHERIC LIMITATIONS + +The huge dimensions of such a powerful engine of research as the +Hooker telescope are not in themselves a source of satisfaction to +the astronomer, for they involve a decided increase in the labor +of observation and entail very heavy expense, justifiable only in +case important results, beyond the reach of other instruments, +can be secured. The construction of a telescope of these dimensions +was necessarily an experiment, for it was by no means certain, after +the optical and mechanical difficulties had been overcome, that +even the favorable atmosphere of California would be sufficiently +tranquil to permit sharply defined celestial images to be obtained +with so large an aperture. It is therefore important to learn what +the telescope will actually accomplish under customary observing +conditions. + +Fortunately we are able to measure the performance of the instrument +with certainty. Close beside it on Mount Wilson stands the 60-inch +reflector, of similar type, erected in 1908. The two telescopes can +thus be rigorously compared under identical atmospheric conditions. + +The large mirror of the 100-inch telescope has an area about 2.8 +times that of the 60-inch, and therefore receives nearly three times +as much light from a star. Under atmospheric conditions perfect +enough to allow all of this light to be concentrated in a point, +it should be capable of recording on a photographic plate, with a +given exposure, stars about one magnitude fainter than the faintest +stars within reach of the 60-inch. The increased focal length, +permitting such objects as the moon to be photographed on a larger +scale, should also reveal smaller details of structure and render +possible higher accuracy of measurement. Finally, the greater +theoretical resolving power of the larger aperture, providing it +can be utilized, should permit the separation of the members of +close double stars beyond the range of the smaller instrument. + +CRITICAL TESTS + +The many tests already made indicate that the advantages expected +of the new telescope will be realized in practice. The increased +light-gathering power will mean the addition of many millions of +stars to those already known. Spectroscopic observations now in +regular progress have carried the range of these investigations +far beyond the possibilities of the 60-inch telescope. A great +class of red stars, for example, almost all the members of which +were inaccessible to the 60-inch, are now being made the subject +of special study. And in other fields of research equal advantages +have been gained. + +The increase in the scale of the images over those given by the +60-inch telescope is illustrated by two photographs of the Ring +Nebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion, +photographed with the 100-inch telescope with a comparatively short +exposure, sufficient to bring out the brighter regions, is reproduced +in Fig. 2. It is interesting to compare this picture with the +small-scale image of the same nebula shown in Fig. 1. + +[Illustration: Fig. 15. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease). + +The ring-like formations are the so-called craters, most of them +far larger than anything similar on the earth. That in the lower +left corner with an isolated mountain in the centre is Albategnius, +sixty-four miles in diameter. Peaks in the ring rise to a height +of fifteen thousand feet above the central plain. Note the long +sunset shadows cast by the mountains on the left. The level region +below on the right is an extensive plain, the Mare Nubium.] + +[Illustration: Fig. 16. Photograph of the moon made on September +15, 1919, with the 100-inch Hooker telescope (Pease). + +The mountains above and to the left are the lunar Apennines; those +on the left just below the centre are the Alps. Both ranges include +peaks from fifteen thousand to twenty thousand feet in height. In +the upper right corner is Copernicus, about fifty miles in diameter. +The largest of the conspicuous group of three just below the Apennines +is Archimedes and at the lower end of the Alps is Plato. Note the +long sunset shadows cast by the isolated peaks on the left. The +central portion of the picture is a vast plain, the Mare Imbrium.] + +The sharpness of the images given by the new telescope may be +illustrated by some recent photographs of the moon, obtained with +an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged +region of the moon, containing many ring-like mountains or craters. +Fig. 16 shows the great arc of the lunar Apennines (above) and the +Alps (below), to the left of the broad plain of the Mare Imbrium. +The starlike points along the moon's terminator, which separates +the dark area from the region upon which the sun (on the right) +shines, are the mountain peaks, about to disappear at sunset. The +long shadows cast by the mountains just within the illuminated +area are plainly seen. Some of the peaks of the lunar Apennines +attain a height of 20,000 feet. + +In less powerful telescopes the stars at the centre of the great +globular clusters are so closely crowded together that they cannot +be studied separately with the spectrograph. Moreover, most of +them are much too faint for examination with this instrument. At +the 134-foot focus the 100-inch telescope gives a large-scale image +of such clusters, and permits the spectra of stars as faint as +the fifteenth magnitude to be separately photographed. + +[Illustration: Fig. 17. Hubble's Variable Nebula. One of the few +nebulae known to vary in brightness and form. + +Photographed with the 100-inch telescope (Hubble).] + +CLOSE DOUBLE STARS + +A remarkable use of the 100-inch telescope, which permits its full +theoretical resolving power to be not merely attained but to be +doubled, has been made possible by the first application of Michelson's +interference method to the measurement of very close double stars. +When employing this, the 100-inch mirror is completely covered, +except for two slits. Beams of light from a star, entering by the +slits, unite at the focus of the telescope, where the image is +examined by an eyepiece magnifying about five thousand diameters. +Across the enlarged star image a series of fine, sharp fringes is +seen, even when the atmospheric conditions are poor. If the star is +single the fringes remain visible, whatever the distance between the +slits. But in the case of a star like Capella, previously inferred +to be double from the periodic displacement of the lines in its +spectrum, but with components too close together to be distinguished +separately, the fringes behave differently. As the slits are moved +apart a point is reached where the fringes completely disappear, +only to reappear as the separation is continued. This effect is +obtained when the slits are at right angles to the line joining +the two stars of the pair, found by this method to be 0.0418 of a +second of arc apart (on December 30, 1919). Subsequent measures, +of far greater precision than those obtainable by other methods in +the case of easily separated double stars, show the rapid orbital +motion of the components of the system. This device will be applied +to other close binaries, hitherto beyond the reach of measurement. + +[Illustration: Fig. 18. Ring Nebula in Lyra, photographed with the +60-inch (Ritchey) and 100-inch (Duncan) telescopes. + +Showing the increased scale of the images given by the larger +instrument.] + +Without entering into further details of the tests, it is evident +that the new telescope will afford boundless possibilities for +the study of the stellar universe.[*] The structure and extent of +the galactic system, and the motions of the stars comprising it; +the distribution, distances, and dimensions of the spiral nebulae, +their motions, rotation, and mode of development; the origin of +the stars and the successive stages in their life history: these +are some of the great questions which the new telescope must help +to answer. In such an embarrassment of riches the chief difficulty +is to withstand the temptation toward scattering of effort, and to +form an observing programme directed toward the solution of crucial +problems rather than the accumulation of vast stores of miscellaneous +data. This programme will be supplemented by an extensive study +of the sun, the only star near enough the earth to be examined +in detail, and by a series of laboratory investigations involving +the experimental imitation of solar and stellar conditions, thus +aiding in the interpretation of celestial phenomena. + +[Footnote *: It is not adapted for work on the sun, as the mirrors +would be distorted by its heat. Three other telescopes, especially +designed for solar observations, are in use on Mount Wilson.] + + + + +CHAPTER II + +GIANT STARS + +Our ancestral sun, as pictured by Laplace, originally extended +in a state of luminous vapor beyond the boundaries of the solar +system. Rotating upon its axis, it slowly contracted through loss +of heat by radiation, leaving behind it portions of its mass, which +condensed to form the planets. Still gaseous, though now denser than +water, it continues to pour out the heat on which our existence +depends, as it shrinks imperceptibly toward its ultimate condition +of a cold and darkened globe. + +Laplace's hypothesis has been subjected in recent years to much +criticism, and there is good reason to doubt whether his description +of the mode of evolution of our solar system is correct in every +particular. All critics agree, however, that the sun was once enormously +larger than it now is, and that the planets originally formed part +of its distended mass. + +Even in its present diminished state, the sun is huge beyond easy +conception. Our own earth, though so minute a fragment of the primeval +sun, is nevertheless so large that some parts of its surface have +not yet been explored. Seen beside the sun, by an observer on one +of the planets, the earth would appear as an insignificant speck, +which could be swallowed with ease by the whirling vortex of a +sun-spot. If the sun were hollow, with the earth at its centre, +the moon, though 240,000 miles from us, would have room and to +spare in which to describe its orbit, for the sun is 865,000 miles +in diameter, so that its volume is more than a million times that +of the earth. + +[Illustration: Fig. 19. Gaseous prominence at the sun's limb, 140,000 +miles high (Ellerman). + +Photographed with the spectroheliograph, using the light emitted +by glowing calcium vapor. The comparative size of the earth is +indicated by the white circle.] + +But what of the stars, proved by the spectroscope to be self-luminous, +intensely hot, and formed of the same chemical elements that constitute +the sun and the earth? Are they comparable in size with the sun? Do +they occur in all stages of development, from infancy to old age? +And if such stages can be detected, do they afford indications +of the gradual diminution in volume which Laplace imagined the +sun to experience? + +[Illustration: Fig. 20. The sun, 865,000 miles in diameter, from +a direct photograph showing many sun-spots (Whitney) + +The small black disk in the centre represents the comparative size +of the earth, while the circle surrounding it corresponds in diameter +to the orbit of the moon.] + +STAR IMAGES + +Prior to the application of the powerful new engine of research +described in this article we have had no means of measuring the +diameters of the stars. We have measured their distances and their +motions, determined their chemical composition, and obtained undeniable +evidence of progressive development, but even in the most powerful +telescopes their images are so minute that they appear as points +rather than as disks. In fact, the larger the telescope and the +more perfect the atmospheric conditions at the observer's command, +the smaller do these images appear. On the photographic plate, it is +true, the stars are recorded as measurable disks, but these are due +to the spreading of the light from their bright point-like images, +and their diameters increase as the exposure time is prolonged. +From the images of the brighter stars rays of light project in +straight lines, but these also are instrumental phenomena, due +to diffraction of light by the steel bars that support the small +mirror in the tube of reflecting telescopes. In a word, the stars +are so remote that the largest and most perfect telescopes show +them only as extremely minute needle-points of light, without any +trace of their true disks. + +[Illustration: Fig. 21. Great sun-spot group, August 8, 1917 (Whitney). + +The disk in the corner represents the comparative size of the earth.] + +How, then, may we hope to measure their diameters? By using, as +the man of science must so often do, indirect means when the direct +attack fails. Most of the remarkable progress of astronomy during +the last quarter-century has resulted from the application of new and +ingenious devices borrowed from the physicist. These have multiplied +to such a degree that some of our observatories are literally physical +laboratories, in which the sun and stars are examined by powerful +spectroscopes and other optical instruments that have recently advanced +our knowledge of physics by leaps and bounds. In the present case +we are indebted for our star-measuring device to the distinguished +physicist Professor Albert A. Michelson, who has contributed a long +array of novel apparatus and methods to physics and astronomy. + +THE INTERFEROMETER + +The instrument in question, known as the interferometer, had previously +yielded a remarkable series of results when applied in its various +forms to the solution of fundamental problems. To mention only a +few of those that have helped to establish Michelson's fame, we may +recall that our exact knowledge of the length of the international +metre at Sevres, the world's standard of measurement, was obtained +by him with an interferometer in terms of the invariable length of +light-waves. A different form of interferometer has more recently +enabled him to measure the minute tides within the solid body of the +earth--not the great tides of the ocean, but the slight deformations +of the earth's body, which is as rigid as steel, that are caused by +the varying attractions of the sun and moon. Finally, to mention +only one more case, it was the Michelson-Morley experiment, made +years ago with still another form of interferometer, that yielded +the basic idea from which the theory of relativity was developed +by Lorentz and Einstein. + +[Illustration: Fig. 22. Photograph of the hydrogen atmosphere of +the sun (Ellerman). + +Made with the spectroheliograph, showing the immense vortices, +or whirling storms like tornadoes, that centre in sun-spots. The +comparative size of the earth is shown by the white circle traced +on the largest sun-spot.] + +The history of the method of measuring star diameters is a very +curious one, showing how the most promising opportunities for scientific +progress may lie unused for decades. The fundamental principle +of the device was first suggested by the great French physicist +Fizeau in 1868. In 1874 the theory was developed by the French +astronomer Stephan, who observed interference fringes given by a +large number of stars, and rightly concluded that their angular +diameters must be much smaller than 0.158 of a second of arc, the +smallest measurable with his instrument. In 1890 Michelson, unaware +of the earlier work, published in the _Philosophical Magazine_ a +complete description of an interferometer capable of determining +with surprising accuracy the distance between the components of +double stars so close together that no telescope can separate them. +He also showed how the same principle could be applied to the +measurement of star diameters if a sufficiently large interferometer +could be built for this purpose, and developed the theory much +more completely than Stephan had done. A year later he measured +the diameters of Jupiter's satellites by this means at the Lick +Observatory. But nearly thirty years elapsed before the next step +was taken. Two causes have doubtless contributed to this delay. Both +theory and experiment have demonstrated the extreme sensitiveness +of the "interference fringes," on the observation of which the +method depends, and it was generally supposed by astronomers that +disturbances in the earth's atmosphere would prevent them from +being clearly seen with large telescopes. Furthermore, a very large +interferometer, too large to be carried by any existing telescope, +was required for the star-diameter work, though close double stars +could have been easily studied by this device with several of the +large telescopes of the early nineties. But whatever the reasons, +a powerful method of research lay unused. + +The approaching completion of the 100-inch telescope of the Mount +Wilson Observatory led me to suggest to Professor Michelson, before +the United States entered the war, that the method be thoroughly +tested under the favorable atmospheric conditions of Southern +California. He was at that time at work on a special form of +interferometer, designed to determine whether atmospheric disturbances +could be disregarded in planning large-scale experiments. But the +war intervened, and all of our efforts were concentrated for two +years on the solution of war problems.[*] In 1919, as soon as the +100-inch telescope had been completed and tested, the work was +resumed on Mount Wilson. + +[Footnote *: Professor Michelson's most important contribution during +the war period was a new and very efficient form of range-finder, +adopted for use by the U. S. Navy.] + +A LABORATORY EXPERIMENT + +The principle of the method can be most readily seen by the aid +of an experiment which any one can easily perform for himself with +simple apparatus. Make a narrow slit, a few thousandths of an inch +in width, in a sheet of black paper, and support it vertically +before a brilliant source of light. Observe this from a distance of +40 or 50 feet with a small telescope magnifying about 30 diameters. +The object-glass of the telescope should be covered with an opaque +cap, pierced by two circular holes about one-eighth of an inch in +diameter and half an inch apart. The holes should be on opposite +sides of the centre of the object-glass and equidistant from it, +and the line joining the holes should be horizontal. When this +cap is removed the slit appears as a narrow vertical band with +much fainter bands on both sides of it. With the cap in place, the +central bright band appears to be ruled with narrow vertical lines +or fringes produced by the "interference"[*] of the two pencils of +light coming through different parts of the object-glass from the +distant slit. Cover one of the holes, and the fringes instantly +disappear. Their production requires the joint effect of the two +light-pencils. + +[Footnote *: For an explanation of the phenomena of interference, +see any encyclopaeedia or book on physics.] + +Now suppose the two holes over the object-glass to be in movable +plates, so that their distance apart can be varied. As they are +gradually separated the narrow vertical fringes become less and +less distinct, and finally vanish completely. Measure the distance +between the holes and divide this by the wavelength of light, which +we may call 1/50000 of an inch. The result is the angular width +of the distant slit. Knowing the distance of the slit, we can at +once calculate its linear width. If for the slit we substitute a +minute circular hole, the method of measurement remains the same, +but the angular diameter as calculated above must be multiplied +by 1.22.[*] + +[Footnote *: More complete details may be found in Michelson's Lowell +Lectures on "Light-Waves and Their Uses," University of Chicago +Press, 1907.] + +To measure the diameter of a star we proceed in a similar way, +but, as the angle it subtends is so small, we must use a very large +telescope, for the smaller the angle the farther apart must be the +two holes over the object-glass (or the mirror, in case a reflecting +telescope is employed). In fact, when the holes are moved apart to +the full aperture of the 100-inch Hooker telescope, the interference +fringes are still visible even with the star Betelgeuse, though its +angular diameter is perhaps as great as that of any other star. +Thus, we must build an attachment for the telescope, so arranged +as to permit us to move the openings still farther apart. + +[Illustration: Fig. 23. Diagram showing outline of the 100-inch +Hooker telescope, and path of the two pencils of light from a star +when under observation with the 20-foot Michelson interferometer. + +A photograph of the interferometer is shown in Fig. 24.] + +THE 20-FOOT INSTRUMENT + +The 20-foot interferometer designed by Messrs. Michelson and Pease, +and constructed in the Mount Wilson Observatory instrument-shop, +is shown in the diagram (Fig. 23) and in a photograph of the upper +end of the skeleton tube of the telescope (Fig. 24). The light from +the star is received by two flat mirrors (Ml, M4) which project +beyond the tube and can be moved apart along the supporting arm. +These take the place of the two holes over the object-glass in +our experiment. From these mirrors the light is reflected to a +second pair of flat mirrors (M2, M3), which send it toward the +100-inch concave mirror (M5) at the bottom of the telescope tube. +After this the course of the light is exactly as it would be if +the mirrors M2, M3 were replaced by two holes over the 100-inch +mirror. It is reflected to the convex mirror (M6), then back in +a less rapidly convergent beam toward the large mirror. Before +reaching it the light is caught by the plane mirror (M7) and reflected +through an opening at the side of the telescope tube to the eye-piece +E. Here the fringes are observed with a magnification ranging from +1,500 to 3,000 diameters. + +[Illustration: Fig. 24. Twenty-foot Michelson interferometer for +measuring star diameters, attached to upper end of the skeleton +tube of the 100-inch Hooker telescope. + +The path of the two pencils of light from the star is shown in +Fig. 23. For a photograph of the entire telescope, see Fig. 4.] + +In the practical application of this method to the measurement of +star diameters, the chief problem was whether the atmosphere would +be quiet enough to permit sharp interference fringes to be produced +with light-pencils more than 100 inches apart. After successful +preliminary tests with the 40-inch refracting telescope of the +Yerkes Observatory, Professor Michelson made the first attempt +to see the fringes with the 60-inch and 100-inch reflectors on +Mount Wilson in September, 1919. He was surprised and delighted to +find that the fringes were perfectly sharp and distinct with the +full aperture of both these instruments. Doctor Anderson, of the +observatory staff, then devised a special form of interferometer +for the measurement of close double stars, and applied it with +the 100-inch telescope to the measurement of the orbital motion +of the close components of Capella, with results of extraordinary +accuracy, far beyond anything attainable by previous methods. The +success of this work strongly encouraged the more ambitious project +of measuring the diameter of a star, and the 20-foot interferometer +was built for this purpose. + +The difficult and delicate problem of adjusting the mirrors of +this instrument with the necessary extreme accuracy was solved by +Professor Michelson during his visit to Mount Wilson in the summer +of 1920, and with the assistance of Mr. Pease, of the observatory +staff, interference fringes were observed in the case of certain +stars when the mirrors were as much as 18 feet apart. All was thus +in readiness for a decisive test as soon as a suitable star presented +itself. + +THE GIANT BETELGEUSE + +Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic +for "the giant's shoulder"), the bright red star in the constellation +of Orion (Fig. 25), as the most favorable of all stars for measurement, +and the last-named had given its angular diameter as 0.051 of a +second of arc. This deduction from theory appeared in his recent +presidential address before the British Association for the Advancement +of Science, in which Professor Eddington remarked: "Probably the +greatest need of stellar astronomy at the present day, in order +to make sure that our theoretical deductions are starting on the +right lines, is some means of measuring the apparent angular diameter +of stars." He then referred to the work already in progress on +Mount Wilson, but anticipated "that atmospheric disturbance will +ultimately set the limit to what can be accomplished." + +[Illustration: Fig. 25. The giant Betelgeuse (within the circle), +familiar as the conspicuous red star in the right shoulder of Orion +(Hubble). + +Measures with the interferometer show its angular diameter to be +0.047 of a second of arc, corresponding to a linear diameter of +215,000,000 miles, if the best available determination of its distance +can be relied upon. This determination shows Betelgeuse to be 160 +light-years from the earth. Light travels at the rate of 186,000 +miles per second, and yet spends 160 years on its journey to us +from this star.] + +On December 13, 1920, Mr. Pease successfully measured the diameter +of Betelgeuse with the 20-foot interferometer. As the outer mirrors +were separated the interference fringes gradually became less distinct, +as theory requires, and as Doctor Merrill had previously seen when +observing Betelgeuse with the interferometer used for Capella. At +a separation of 10 feet the fringes disappeared completely, giving +the data required for calculating the diameter of the star. To +test the perfection of the adjustment, the telescope was turned to +other stars, of smaller angular diameter, which showed the fringes +with perfect clearness. Turning back to Betelgeuse, they were seen +beyond doubt to be absent. Assuming the mean wave-length of the +light of this star to be 5750/10000000 of a millimetre, its angular +diameter comes out 0.047 of a second of arc, thus falling between +the values--0.051 and 0.031 of a second--predicted by Eddington and +Russell from slightly different assumptions. Subsequent corrections +and repeated measurement will change Mr. Pease's result somewhat, +but it is almost certainly within 10 or 15 per cent of the truth. +We may therefore conclude that the angular diameter of Betelgeuse +is very nearly the same as that of a ball one inch in diameter, +seen at a distance of seventy miles. + +[Illustration: Fig. 26. Arcturus (within the white circle), known +to the Arabs as the "Lance Bearer," and to the Chinese as the "Great +Horn" or the "Palace of the Emperors" (Hubble). + +Its angular diameter, measured at Mount Wilson by Pease with the +20-foot Michelson interferometer on April 15, 1921, is 0.022 of a +second, in close agreement with Russell's predicted value of 0.019 +of a second. The mean parallax of Arcturus, based upon several +determinations, is 0.095 of a second, corresponding to a distance of +34 light-years. The linear diameter, computed from Pease's measure +and this value of the distance is about 21 million miles.] + +But this represents only the angle subtended by the star's disk. +To learn its linear diameter, we must know its distance. Four +determinations of the parallax, which determines the distance, +have been made. Elkin, with the Yale heliometer, obtained 0.032 +of a second of arc. Schlesinger, from photographs taken with the +30-inch Allegheny refractor, derived 0.016. Adams, by his spectroscopic +method applied with the 60-inch Mount Wilson reflector, obtained +0.012. Lee's recent value, secured photographically with the 40-inch +Yerkes refractor, is 0.022. The heliometer parallax is doubtless +less reliable than the photographic ones, and Doctor Adams states +that the spectral type and luminosity of Betelgeuse make his value +less certain than in the case of most other stars. If we take a +(weighted) mean value of 0.020 of a second, we shall probably not +be far from the truth. This parallax represents the angle subtended +by the radius of the earth's orbit (93,000,000 miles) at the distance +of Betelgeuse. By comparing it with 0.047, the angular diameter of +the star, we see that the linear diameter is about two and one-third +times as great as the distance from the earth to the sun, or +approximately 215,000,000 miles. Thus, if this measure of its distance +is not considerably in error, Betelgeuse would nearly fill the +orbit of Mars. All methods of determining the distances of the +stars are subject to uncertainty, however, and subsequent measures +may reduce this figure very appreciably. But there can be no doubt +that the diameter of Betelgeuse exceeds 100,000,000 miles, and +it is probably much greater. + +The extremely small angle subtended by this enormous disk is explained +by the great distance of the star, which is about 160 light-years. +That is to say, light travelling at the rate of 186,000 miles per +second spends 160 years in crossing the space that lies between +us and Betelgeuse, whose tremendous proportions therefore seem +so minute even in the most powerful telescopes. + +STELLAR EVOLUTION + +This actual measure of the diameter of Betelgeuse supplies a new +and striking test of Russell's and Hertzsprung's theory of dwarf +and giant stars. Just before the war Russell showed that our old +methods of classifying the stars according to their spectra must +be radically changed. Stars in an early stage of their life history +may be regarded as diffuse gaseous masses, enormously larger than +our sun, and at a much lower temperature. Their density must be +very low, and their state that of a perfect gas. These are the +"giants." In the slow process of time they contract through constant +loss of heat by radiation. But, despite this loss, the heat produced +by contraction and from other sources (see p. 82) causes their +temperature to rise, while their color changes from red to bluish +white. The process of shrinkage and rise of temperature goes on so +long as they remain in the state of a perfect gas. But as soon as +contraction has increased the density of the gas beyond a certain +point the cycle reverses and the temperature begins to fall. The +bluish-white light of the star turns yellowish, and we enter the +dwarf stage, of which our own sun is a representative. The density +increases, surpassing that of water in the case of the sun, and +going far beyond this point in later stages. In the lapse of millions +of years a reddish hue appears, finally turning to deep red. The +falling temperature permits the chemical elements, existing in a +gaseous state in the outer atmosphere of the star, to unite into +compounds, which are rendered conspicuous by their characteristic +bands in the spectrum. Finally comes extinction of light, as the +star approaches its ultimate state of a cold and solid globe. + +[Illustration: Fig. 27. The giant star Antares (within the white +circle), notable for its red color in the constellation Scorpio, +and named by the Greeks "A Rival of Mars" (Hubble). + +The distance of Antares, though not very accurately known, is probably +not far from 350 light-years. Its angular diameter of 0.040 of a +second would thus correspond to a linear diameter of about 400 +million miles.] + +We may thus form a new picture of the two branches of the temperature +curve, long since suggested by Lockyer, on very different grounds, as +the outline of stellar life. On the ascending side are the giants, +of vast dimensions and more diffuse than the air we breathe. There +are good reasons for believing that the mass of Betelgeuse cannot +be more than ten times that of the sun, while its volume is at +least a million times as great and may exceed eight million times +the sun's volume. Therefore, its average density must be like that +of an attenuated gas in an electric vacuum tube. Three-quarters +of the naked-eye stars are in the giant stage, which comprises +such familiar objects as Betelgeuse, Antares, and Aldebaran, but +most of them are much denser than these greatly inflated bodies. +The pinnacle is reached in the intensely hot white stars of the +helium class, in whose spectra the lines of this gas are very +conspicuous. The density of these stars is perhaps one-tenth that +of the sun. Sirius, also very hot, is nearly twice as dense. Then +comes the cooling stage, characterized, as already remarked, by +increasing density, and also by increasing chemical complexity +resulting from falling temperature. This life cycle is probably +not followed by all stars, but it may hold true for millions of +them. + +The existence of giant and dwarf stars has been fully proved by +the remarkable work of Adams and his associates on Mount Wilson, +where his method of determining a star's distance and intrinsic +luminosity by spectroscopic observations has already been applied +to 2,000 stars. Discussion of the results leads at once to the +recognition of the two great classes of giants and dwarfs. Now +comes the work of Michelson and Pease to cap the climax, giving us +the actual diameter of a typical giant star, in close agreement with +predictions based upon theory. From this diameter we may conclude that +the density of Betelgeuse is extremely low, in harmony with Russell's +theory, which is further supported by spectroscopic analysis of +the star's light, revealing evidence of the comparatively low +temperature called for by the theory at this early stage of stellar +existence. + +TWO OTHER GIANTS + +The diameter of Arcturus was successfully measured by Mr. Pease +at Mount Wilson on April 15. As the mirrors of the interferometer +were moved apart, the fringes gradually decreased in visibility +until they finally disappeared at a mirror separation of 19.6 feet. +Adopting a mean wave-length of 5600/10000000 of a millimetre for +the light of Arcturus, this gives a value of 0.022 of a second of +arc for the angular diameter of the star. If we use a mean value +of 0.095 of a second for the parallax, the corresponding linear +diameter comes out 21,000,000 miles. The angular diameter, as in +the case of Betelgeuse, is in remarkably close agreement with the +diameter predicted from theory. Antares, the third star measured +by Mr. Pease, is the largest of all. If it is actually a member of +the Scorpius-Centaurus group, as we have strong reason to believe, +it is fully 350 light-years from the earth, and its diameter is +about 400,000,000 miles. + +[Illustration: Fig. 28. Diameters of the Sun, Arcturus, Betelgeuse, +and Antares compared with the orbit of Mars. + +Sun, diameter, 865,000 miles. + +Arcturus, diameter, 21,000,000 miles. + +Betelgeuse, diameter, 215,000,000 miles. + +Antares, diameter, 400,000,000 miles.] + +It now remains to make further measures of Betelgeuse, especially +because its marked changes in brightness suggest possible variations +in diameter. We must also apply the interferometer method to stars +of the various spectral types, in order to afford a sure basis for +future studies of stellar evolution. Unfortunately, only a few +giant stars are certain to fall within the range of our present +instrument. An interferometer of 70-feet aperture would be needed +to measure Sirius accurately, and one of twice this size to deal +with less brilliant white stars. A 100-foot instrument, if feasible +to build, would permit objects representing most of the chief stages +of stellar development to be measured, thus contributing in the +highest degree to the progress of our knowledge of the life history +of the stars. Fortunately, though the mechanical difficulties are +great, the optical problem is insignificant, and the cost of the +entire apparatus, though necessarily high, would be only a small +fraction of that of a telescope of corresponding aperture, if such +could be built. A 100-foot interferometer might be designed in +many different forms, and one of these may ultimately be found +to be within the range of possibility. Meanwhile the 20-foot +interferometer has been improved so materially that it now promises +to yield approximate measures of stars at first supposed to be +beyond its capacity. + +[Illustration: Fig. 29. Aldebaran, the "leader" (of the Pleiades), +was also known to the Arabs as "The Eye of the Bull," "The Heart +of the Bull," and "The Great Camel" (Hubble). + +Like Betelgeuse and Antares, it is notable for its red color, which +accounts for the fact that its image on this photograph is hardly +more conspicuous than the images of stars which are actually much +fainter but contain a larger proportion of blue light, to which +the photographic plates here employed are more sensitive than to +red or yellow. Aldebaran is about 50 light-years from the earth. +Interferometer measures, now in progress on Mount Wilson, indicate +that its angular diameter is about 0.020 of a second.] + +While the theory of dwarf and giant stars and the measurements just +described afford no direct evidence bearing on Laplace's explanation +of the formation of planets, they show that stars exist which are +comparable in diameter with our solar system, and suggest that the +sun must have shrunk from vast dimensions. The mode of formation +of systems like our own, and of other systems numerously illustrated +in the heavens, is one of the most fascinating problems of astronomy. +Much light has been thrown on it by recent investigations, rendered +possible by the development of new and powerful instruments and by +advances in physics of the most fundamental character. All the +evidence confirms the existence of dwarf and giant stars, but much +work must be done before the entire course of stellar evolution +can be explained. + + + + +CHAPTER III + +COSMIC CRUCIBLES + +"Shelter during Raids," marking the entrance to underground passages, +was a sign of common occurrence and sinister suggestion throughout +London during the war. With characteristic ingenuity and craftiness, +ostensibly for purposes of peace but with bomb-carrying capacity +as a prime specification, the Zeppelin had been developed by the +Germans to a point where it seriously threatened both London and +Paris. Searchlights, range-finders, and anti-aircraft guns, surpassed +by the daring ventures of British and French airmen, would have +served but little against the night invader except for its one +fatal defect--the inflammable nature of the hydrogen gas that kept +it aloft. A single explosive bullet served to transform a Zeppelin +into a heap of scorched and twisted metal. This characteristic +of hydrogen caused the failure of the Zeppelin raids. + +Had the war lasted a few months longer, however, the work of American +scientists would have made our counter-attack in the air a formidable +one. At the signing of the armistice hundreds of cylinders of compressed +helium lay at the docks ready for shipment abroad. Extracted from +the natural gas of Texas wells by new and ingenious processes, +this substitute for hydrogen, almost as light and absolutely +uninflammable, produced in quantities of millions of cubic feet, +would have made the dirigibles of the Allies masters of the air. The +special properties of this remarkable gas, previously obtainable only +in minute quantities, would have sufficed to reverse the situation. + +SOLAR HELIUM + +Helium, as its name implies, is of solar origin. In 1868, when +Lockyer first directed his spectroscope to the great flames or +prominences that rise thousands of miles, sometimes hundreds of +thousands, above the surface of the sun, he instantly identified +the characteristic red and blue radiations of hydrogen. In the +yellow, close to the position of the well-known double line of +sodium, but not quite coincident with it, he detected a new line, +of great brilliancy, extending to the highest levels. Its similarity +in this respect with the lines of hydrogen led him to recognize +the existence of a new and very light gas, unknown to terrestrial +chemistry. + +Many years passed before any chemical laboratory on earth was able +to match this product of the great laboratory of the sun. In 1896 +Ramsay at last succeeded in separating helium, recognized by the same +yellow line in its spectrum, in minute quantities from the mineral +uraninite. Once available for study under electrical excitation in +vacuum tubes, helium was found to have many other lines in its +spectrum, which have been identified in the spectra of solar +prominences, gaseous nebulae, and hot stars. Indeed, there is a +stellar class known as helium stars, because of the dominance of +this gas in their atmospheres. + +[Illustration: Fig. 30. Solar prominences, photographed with the +spectroheliograph without an eclipse (Ellerman). + +In these luminous gaseous clouds, which sometimes rise to elevations +exceeding half the sun's diameter, the new gas helium was discovered +by Lockyer in 1868. Helium was not found on the earth until 1896. +Since then it has been shown to be a prominent constituent of nebulae +and hot stars.] + +The chief importance of helium lies in the clue it has afforded to +the constitution of matter and the transmutation of the elements. +Radium and other radioactive substances, such as uranium, spontaneously +emit negatively charged particles of extremely small mass (electrons), +and also positively charged particles of much greater mass, known +as alpha particles. Rutherford and Geiger actually succeeded in +counting the number of alpha particles emitted per second by a +known mass of radium, and showed that these were charged helium +atoms. + +To discuss more at length the extraordinary characteristics of +helium, which plays so large a part in celestial affairs, would +take us too far afield. Let us therefore pass to another case in +which a fundamental discovery, this time in physics, was first +foreshadowed by astronomical observation. + +SUN-SPOTS AS MAGNETS + +No archaeologist, whether Young or Champollion deciphering the Rosetta +Stone, or Rawlinson copying the cuneiform inscription on the cliff +of Behistun, was ever faced by a more fascinating problem than that +which confronts the solar physicist engaged in the interpretation +of the hieroglyphic lines of sun-spot spectra. The colossal whirling +storms that constitute sun-spots, so vast that the earth would make +but a moment's scant mouthful for them, differ materially from +the general light of the sun when examined with the spectroscope. +Observing them visually many years ago, the late Professor Young, +of Princeton, found among their complex features a number of double +lines which he naturally attributed, in harmony with the physical +knowledge of the time, to the effect of "reversal" by superposed +layers of vapors of different density and temperature. What he +actually saw, however, as was proved at the Mount Wilson Observatory +in 1908, was the effect of a powerful magnetic field on radiation, +now known as the Zeeman effect. + +[Illustration: Fig. 31. The 150-foot tower telescope of the Mount +Wilson Observatory. + +An image of the sun about 16 inches in diameter is formed in the +laboratory at the base of the tower. Below this, in a well extending +80 feet into the earth, is the powerful spectroscope with which +the magnetic fields in sun-spots and the general magnetic field +of the sun are studied.] + +Faraday was the first to detect the influence of magnetism on light. +Between the poles of a large electromagnet, powerful for those +days (1845), he placed a block of very dense glass. The plane of +polarization of a beam of light, which passed unaffected through +the glass before the switch was closed, was seen to rotate when the +magnetic field was produced by the flow of the current. A similar +rotation is now familiar in the well-known tests of sugars--laevulose +and dextrose--which rotate plane-polarized light to left and right, +respectively. + +But in this first discovery of a relationship between light and +magnetism Faraday had not taken the more important step that he +coveted--to determine whether the vibration period of a light-emitting +particle is subject to change in a magnetic field. He attempted +this in 1862--the last experiment of his life. A sodium flame was +placed between the poles of a magnet, and the yellow lines were +watched in a spectroscope when the magnet was excited. No change +could be detected, and none was found by subsequent investigators +until Zeeman, of Leiden, with more powerful instruments made his +famous discovery, the twenty-fifth anniversary of which has recently +been celebrated. + +[Illustration: Fig. 32. Pasadena Laboratory of the Mount Wilson +Observatory. + +Showing the large magnet (on the left) and the spectroscopes used +for the study of the effect of magnetism on radiation. A single line +in the spectrum is split by the magnetic field into from three to +twenty-one components, as illustrated in Fig. 34. The corresponding +lines in the spectra of sun-spots are split up in precisely the +same way, thus indicating the presence of powerful magnetic fields +in the sun.] + +His method of procedure was similar to Faraday's, but his magnet and +spectroscope were much more powerful, and a theory due to Lorentz, +predicting the nature of the change to be expected, was available +as a check on his results. When the current was applied the lines +were seen to widen. In a still more powerful magnetic field each +of them split into two components (when the observation was made +along the lines of force), and the light of the components of each +line was found to be circularly polarized in opposite directions. +Strictly in harmony with Lorentz's theory, this splitting and +polarization proved the presence in the luminous vapor of exactly such +negatively charged electrons as had been indicated there previously +by very different experimental methods. + +In 1908 great cyclonic storms, or vortices, were discovered at +the Mount Wilson Observatory centring in sun-spots. Such whirling +masses of hot vapors, inferred from Sir Joseph Thomson's results +to contain electrically charged particles, should give rise to a +magnetic field. This hypothesis at once suggested that the double +lines observed by Young might really represent the Zeeman effect. +The test was made, and all the characteristic phenomena of radiation +in a magnetic field were found. + +Thus a great physical experiment is constantly being performed +for us in the sun. Every large sunspot contains a magnetic field +covering many thousands of square miles, within which the spectrum +lines of iron, manganese, chromium, titanium, vanadium, calcium, +and other metallic vapors are so powerfully affected that their +widening and splitting can be seen with telescopes and spectroscopes +of moderate size. + +THE TOWER TELESCOPE + +Both of these illustrations show how the physicist and chemist, +when adequately armed for astronomical attack, can take advantage +in their studies of the stupendous processes visible in cosmic +crucibles, heated to high temperatures and influenced, as in the +case of sun-spots, by intense magnetic fields. Certain modern +instruments, like the 60-foot and 150-foot tower telescopes on +Mount Wilson, are especially designed for observing the course +of these experiments. The second of these telescopes produces at +a fixed point in a laboratory an image of the sun about 16 inches +in diameter, thus enlarging the sun-spots to such a scale that +the magnetic phenomena of their various parts can be separately +studied. This analysis is accomplished with a spectroscope 80 feet +in length, mounted in a subterranean chamber beneath the tower. The +varied results of such investigations cannot be described here. +Only one of them may be mentioned--the discovery that the entire sun, +rotating on its axis, is a great magnet. Hence we may reasonably +infer that every star, and probably every planet, is also a magnet, +as the earth has been known to be since the days of Gilbert's "De +Magnete." Here lies one of the best clues for the physicist who +seeks the cause of magnetism, and attempts to produce it, as Barnett +has recently succeeded in doing, by rapidly whirling masses of +metal in the laboratory. + +[Illustration: Fig. 33. Sun-spot vortex in the upper hydrogen +atmosphere. (Benioff). + +Photographed with the spectroheliograph. The electric vortex that +causes the magnetic field of the spot lies at a lower level, and +is not shown by such photographs.] + +Perhaps a word of caution should be interpolated at this point. +Solar magnetism in no wise accounts for the sun's gravitational +power. Indeed, its attraction cannot be felt by the most delicate +instruments at the distance of the earth, and would still be unknown +were it not for the influence of magnetism on light. + +Auroras, magnetic storms, and such electric currents as those that +recently deranged several Atlantic cables are due, not to the magnetism +of the sun or its spots, but probably to streams of electrons, shot +out from highly disturbed areas of the solar surface surrounding +great sun-spots, traversing ninety-three million miles of the ether +of space, and penetrating deep into the earth's atmosphere. These +striking phenomena lead us into another chapter of physics, which +limitations of space forbid us to pursue. + +STELLAR CHEMISTRY + +Let us turn again to chemistry, and see where experiments performed +in cosmic laboratories can serve as a guide to the investigator. +A spinning solar tornado, incomparably greater in scale than the +devastating whirlwinds that so often cut narrow paths of destruction +through town and country in the Middle West, gradually gives rise +to a sun-spot. The expansion produced by the centrifugal force at +the centre of the storm cools the intensely hot gases of the solar +atmosphere to a point where chemical union can occur. Titanium +and oxygen, too hot to combine in most regions of the sun, join +to form the vapor of titanium oxide, characterized in the sunspot +spectrum by fluted bands, made up of hundreds of regularly spaced +lines. Similarly magnesium and hydrogen combine as magnesium hydride +and calcium and hydrogen form calcium hydride. None of these compounds, +stable at the high temperatures of sun-spots, has been much studied +in the laboratory. The regions in which they exist, though cooler +than the general atmosphere of the sun, are at temperatures of +several thousand degrees, attained in our laboratories only with +the aid of such devices as powerful electric furnaces. + +[Illustration: Fig. 34. Splitting of spectrum lines by a magnetic +field (Babcock). + +The upper and lower strips show lines in the spectrum of chromium, +observed without a magnetic field. When subjected to the influence +of magnetism, these single lines are split into several components. +Thus the first line on the right is resolved by the field into +three components, one of which (plane polarized) appears in the +second strip, while the other two, which are polarized in a plane +at right angles to that of the middle component, are shown on the +third strip. The next line is split by the magnetic field into +twelve components, four of which appear in the second strip and +eight in the third. The magnetic fields in sun-spots affect these +lines in precisely the same way.] + +It is interesting to follow our line of reasoning to the stars, +which differ widely in temperature at various stages in their +life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely +hot solar vapors are cooled by expansion, giving rise to the compounds +already named. A red star, in Russell's scheme of stellar evolution, +is a cooler sun, vast in volume and far more tenuous than atmospheric +air when in the initial period of the "giant" stage, but compressed +and denser than water in the "dwarf" stage, into which our sun has +already entered as it gradually approaches the last phases of its +existence. Therefore we should find, throughout the entire atmosphere +of such stars, some of the same compounds that are produced within +the comparatively small limits of a sun-spot. This, of course, +on the correct assumption that sun and stars are made of the same +substances. Fowler has already identified the bands of titanium +oxide in such red stars as the giant Betelgeuse, and in others +of its class. It is safe to predict that an interesting chapter +in the chemistry of the future will be based upon the study of +such compounds, both in the laboratory and under the progressive +temperature conditions afforded by the countless stellar "giants" +and "dwarfs" that precede and follow the solar state. + +[Footnote *: See Chapter II.] + +[Illustration: Fig. 35. Electric furnace in the Pasadena laboratory +of the Mount Wilson Observatory. + +With which the chemical phenomena observed in sun-spots and red +stars are experimentally imitated.] + +ASTROPHYSICAL LABORATORIES + +It is precisely in this long sequence of physical and chemical +changes that the astrophysicist and the astrochemist can find the +means of pushing home their attack. It is true, of course, that +the laboratory investigator has a great advantage in his ability +to control his experiments, and to vary their progress at will. +But by judicious use of the transcendental temperatures, far out +ranging those of his furnaces, and extreme conditions, which he +can only partially imitate, afforded by the sun, stars, and nebulae, +he may greatly widen the range of his inquiries. The sequence of +phenomena seen during the growth of a sun-spot, or the observation +of spots of different sizes, and the long series of successive +steps that mark the rise and decay of stellar life, resemble the +changes that the experimenter brings about as he increases and +diminishes the current in the coils of his magnet or raises and +lowers the temperature of his electric furnace, examining from +time to time the spectrum of the glowing vapors, and noting the +changes shown by the varying appearance of their lines. + +[Illustration: Fig. 36. Titanium oxide in red stars. + +The upper spectrum is that of titanium in the flame of the electric +arc, where its combination with oxygen gives rise to the bands of +titanium oxide (Fowler). The lower strip shows the spectrum of +the red star Mira (Omicron Ceti), as drawn by Cortie at Stonyhurst. +The bands of titanium oxide are clearly present in the star.] + +[Illustration: Fig. 37. Titanium oxide in sun-spots. + +The upper strip shows a portion of the spectrum of a sun-spot +(Ellerman); the lower one the corresponding region of the spectrum +of titanium oxide (King). The fluted bands of the oxide spectrum +are easily identified in the spot, where they indicate that titanium +and oxygen, too hot to combine in the solar atmosphere, unite in the +spot because of the cooling produced by expansion in the vortex.] + +Astronomical observations of this character, it should be noted, are +most effective when constantly tested and interpreted by laboratory +experiment. Indeed, a modern astrophysical observatory should be +equipped like a great physical laboratory, provided on the one hand +with telescopes and accessory apparatus of the greatest attainable +power, and on the other with every device known to the investigator +of radiation and the related physical and chemical phenomena. Its +telescopes, especially designed with the aims of the physicist and +chemist in view, bring images of sun, stars, nebulae, and other +heavenly bodies within the reach of powerful spectroscopes, sensitive +bolometers and thermopiles, and the long array of other appliances +available for the measurement and analysis of radiation. Its electric +furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing +and decreasing pressure, varying chemical conditions, and subjecting +luminous gases and vapors to the influence of electric and magnetic +fields, provide the means of imitating celestial phenomena, and of +repeating and interpreting the experiments observed at the telescope. +And the advantage thus derived, as we have seen, is not confined +to the astronomer, who has often been able, by making fundamental +physical and chemical discoveries, to repay his debt to the physicist +and chemist for the apparatus and methods which he owes to them. + +NEWTON AND EINSTEIN + +Take, for another example, the greatest law of physics--Newton's +law of gravitation. Huge balls of lead, as used by Cavendish, produce +by their gravitational effect a minute rotation of a delicately +suspended bar, carrying smaller balls at its extremities. But no +such feeble means sufficed for Newton's purpose. To prove the law +of gravitation he had recourse to the tremendous pull on the moon +of the entire mass of the earth, and then extended his researches +to the mutual attractions of all the bodies of the solar system. +Later Herschel applied this law to the suns which constitute double +stars, and to-day Adams observes from Mount Wilson stars falling +with great velocity toward the centre of the galactic system under +the combined pull of the millions of objects that compose it. Thus +full advantage has been taken of the possibility of utilizing the +great masses of the heavenly bodies for the discovery and application +of a law of physics and its reciprocal use in explaining celestial +motions. + +[Illustration: Fig. 38. The Cavendish experiment. + +Two lead balls, each two inches in diameter, are attached to the +ends of a torsion rod six feet long, which is suspended by a fine +wire. The experiment consists in measuring the rotation of the +suspended system, caused by the gravitational attraction of two +lead spheres, each twelve inches in diameter, acting on the two +small lead balls.] + +Or consider the Einstein theory of relativity, the truth or falsity +of which is no less fundamental to physics. Its inception sprang from +the Michelson-Morley experiment, made in a laboratory in Cleveland, +which showed that motion of the earth through the ether of space could +not be detected. All of the three chief tests of Einstein's general +theory are astronomical--because of the great masses required to +produce the minute effects predicted: the motion of the perihelion +of Mercury, the deflection of the light of a star by the attraction +of the sun, and the shift of the lines of the solar spectrum toward +the red--questions not yet completely answered. + +But it is in the study of the constitution of matter and the evolution +of the elements, the deepest and most critical problem of physics +and chemistry, that the extremes of pressure and temperature in the +heavenly bodies, and the prevalence of other physical conditions not +yet successfully imitated on earth, promise the greatest progress. +It fortunately happens that astrophysical research is now at the +very apex of its development, founded as it is upon many centuries +of astronomical investigation, rejuvenated by the introduction +into the observatory of all the modern devices of the physicist, +and strengthened with instruments of truly extraordinary range +and power. These instruments bring within reach experiments that +are in progress on some minute region of the sun's disk, or in +some star too distant even to be glimpsed with ordinary telescopes. +Indeed, the huge astronomical lenses and mirrors now available +serve for these remote light-sources exactly the purpose of the +lens or mirror employed by the physicist to project upon the slit +of his spectroscope the image of a spark or arc or vacuum tube +within which atoms and molecules are exposed to the influence of +the electric discharge. The physicist has the advantage of complete +control over the experimental conditions, while the astrophysicist +must observe and interpret the experiments performed for him in +remote laboratories. In actual practice, the two classes of work +must be done in the closest conjunction, if adequate utilization +is to be made of either. And this is only natural, for the trend +of recent research has made clear the fact that one of the three +greatest problems of modern astronomy and astrophysics, ranking +with the structure of the universe and the evolution of celestial +bodies, is the constitution of matter. Let us see why this is so. + +TRANSMUTATION OF THE ELEMENTS + +The dream of the alchemist was to transmute one element into another, +with the prime object of producing gold. Such transmutation has been +actually accomplished within the last few years, but the process +is invariably one of disintegration--the more complex elements +being broken up into simpler constituents. Much remains to be done +in this same direction; and here the stars and nebulae, which show +the spectra of the elements under a great variety of conditions, +should help to point the way. The progressive changes in spectra, +from the exclusive indications of the simple elements hydrogen, +helium, nitrogen, possibly carbon, and the terrestrially unknown +gas nebulium in the gaseous nebulae, to the long list of familiar +substances, including several chemical compounds, in the red stars, +may prove to be fundamentally significant when adequately studied +from the standpoint of the investigator of atomic structure. The +existing evidence seems to favor the view, recently expressed by +Saha, that many of these differences are due to varying degrees +of ionization, the outer electrons of the atoms being split off +by high temperature or electrical excitation. It is even possible +that cosmic crucibles, unrivalled by terrestrial ones, may help +materially to reveal the secret of the formation of complex elements +from simpler ones. Physicists now believe that all of the elements are +compounded of hydrogen atoms, bound together by negative electrons. +Thus helium is made up of four hydrogen atoms, yet the atomic weight +of helium (4) is less than four times that of hydrogen (1.008). +The difference may represent the mass of the electrical energy +released when the transmutation occurred. + +[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey). + +The gas "nebulium," not yet found on the earth, is the most +characteristic constituent of irregular nebulae. Nebulium is recognized +by two green lines in its spectrum, which cause the green color of +nebulae of the gaseous type.] + +Eddington has speculated in a most interesting way on this possible +source of stellar heat in his recent presidential address before the +British Association for the Advancement of Science (see _Nature_, +September 2, 1920). He points out that the old contraction hypothesis, +according to which the source of solar and stellar heat was supposed +to reside in the slow condensation of a radiating mass of gas under +the action of gravity, is wholly inadequate to explain the observed +phenomena. If the old view were correct, the earlier history of +a star, from the giant stage of a cool and diaphanous gas to the +period of highest temperature, would be run through within eighty +thousand years, whereas we have the best of evidence that many +thousands of centuries would not suffice. Some other source of energy +is imperatively needed. If 5 per cent of a star's mass consists +originally of hydrogen atoms, which gradually combine in the slow +process of time to form more complex elements, the total heat thus +liberated would more than suffice to account for all demands, and +it would be unnecessary to assume the existence of any other source +of heat. + +[Illustration: Fig. 40. Spiral nebula in Ursa Major (Ritchey). + +Luminous matter, in every variety of physical and chemical state, +is available for study in the most diverse celestial objects, from +the spiral and irregular nebulae through all the types of stars. +Doctor van Maanen's measures of the Mount Wilson photographs indicate +outward motion along the arms of spiral nebulae, while the spectroscope +shows them to be whirling at enormous velocities.] + +COSMIC PRESSURES + +This, it may fairly be said, is very speculative, but the fact +remains that celestial bodies appear to be the only places in which +the complex elements may be in actual process of formation from their +known source--hydrogen. At least we may see what a vast variety +of physical conditions these cosmic crucibles afford. At one end of +the scale we have the excessively tenuous nebulae, the luminosity of +which, mysterious in its origin, resembles the electric glow in our +vacuum tubes. Here we can detect only the lightest and simplest of +the elements. In the giant stars, also extremely tenuous (the density +of Betelgeuse can hardly exceed one-thousandth of an atmosphere) we +observe the spectra of iron, manganese, titanium, calcium, chromium, +magnesium, vanadium, and sodium, in addition to titanium oxide. +The outer part of these bodies, from which light reaches us, must +therefore be at a temperature of only a few thousand degrees, but +vastly higher temperatures must prevail at their centres. In passing +up the temperature curve more and more elements appear, the surface +temperature rises, and the internal temperature may reach millions +of degrees. At the same time the pressure within must also rise, +reaching enormous figures in the last stages of stellar life. Cook +has calculated that the pressure at the centre of the earth is +between 4,000 and 10,000 tons per square inch, and this must be +only a very small fraction of that attained within larger celestial +bodies. Jeans has computed the pressure at the centre of two colliding +stars as they strike and flatten, and finds it may be of the order +of 1,000,000,000 tons per square inch--sufficient, if their diameter +be equal to that of the sun--to vaporize them 100,000 times over. + +Compare these pressures with the highest that can be produced on +earth. If the German gun that bombarded Paris were loaded with a +solid steel projectile of suitable dimensions, a muzzle velocity +of 6,000 feet per second could be reached. Suppose this to be fired +into a tapered hole in a great block of steel. The instantaneous +pressure, according to Cook, would be about 7,000 tons per square +inch, only 1/150000 of that possible through the collision of the +largest stars. + +[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson. + +Michelson is measuring the velocity of light between stations on +Mount Wilson and Mount San Antonio. Astronomical observations afford +the best means, however, of detecting any possible difference between +the velocities of light of different colors. From studies of variable +stars in the cluster Messier 5 Shapley concludes that if there is +any difference between the velocities of blue and yellow light +in free space it cannot exceed two inches in one second, the time +in which light travels 186,000 miles.] + +Finally, we may compare the effects of light pressure on the earth +and stars. Twenty years ago Nichols and Hull succeeded, with the +aid of the most sensitive apparatus, in measuring the minute +displacements produced by the pressure of light. The effect is +so slight, even with the brightest light-sources available, that +great experimental skill is required to measure it. Yet in the +case of some of the larger stars Eddington calculates that one-half +of their mass is supported by radiation pressure, and this against +their enormous gravitational attraction. In fact, if their mass +were as great as ten times that of the sun, the radiation pressure +would so nearly overcome the pull of gravitation that they would +be likely to break up. + +But enough has been said to illustrate the wide variety of experimental +devices that stand at our service in the laboratories of the heavens. +Here the physicist and chemist of the future will more and more +frequently supplement their terrestrial apparatus, and find new +clues to the complex problems which the amazing progress of recent +years has already done so much to solve. + +PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER + +The layman has no difficulty in recognizing the practical value +of researches directed toward the improvement of the incandescent +lamp or the increased efficiency of the telephone. He can see the +results in the greatly decreased cost of electric illumination +and the rapid extension of the range of the human voice. But the +very men who have made these advances, those who have succeeded +beyond all expectation in accomplishing the economic purposes in +view, are most emphatic in their insistence upon the importance +of research of a more fundamental character. Thus Vice-President +J. J. Carty, of the American Telephone and Telegraph Company, who +directs its great Department of Development and Research, and Doctor +W. J. Whitney, Director of the Research Laboratory of the General +Electric Company, have repeatedly expressed their indebtedness +to the investigations of the physicist, made with no thought of +immediate practical return. Faraday, studying the laws of electricity, +discovered the principle which rendered the dynamo possible. Maxwell, +Henry, and Hertz, equally unconcerned with material advantage, +made wireless telegraphy practicable. In fact, all truly great +advances are thus derived from fundamental science, and the future +progress of the world will be largely dependent upon the provision +made for scientific research, especially in the fields of physics +and chemistry, which underlie all branches of engineering. + +The constitution of matter, therefore, instead of appealing as +a subject to research only to the natural philosopher or to the +general student of science, is a question of the greatest practical +concern. Already the by-products of investigations directed toward +its elucidation have been numerous and useful in the highest degree. +Helium has been already cited; X-rays hardly require mention; radium, +which has so materially aided sufferers from cancer, is still better +known. Wireless telephony and transcontinental telephony with wires +were both rendered possible by studies of the nature of the electric +discharge in vacuum tubes. Thus the "practical man," with his distrust +of "pure" science, need not resent investments made for the purpose +of advancing our knowledge of such fundamental subjects as physics +and chemistry. On the contrary, if true to his name, he should +help to multiply them many fold in the interest of economic and +commercial development. + + + + + + +End of the Project Gutenberg EBook of The New Heavens, by George Ellery Hale + +*** END OF THIS PROJECT GUTENBERG EBOOK THE NEW HEAVENS *** + +***** This file should be named 19395.txt or 19395.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/1/9/3/9/19395/ + +Produced by Robert J. 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