[Illustration: Fig. 1. The Constellation of Orion (Hubble). 

Photographed with a small camera lens of 1 inch aperture and 5inches focal length. The three bright stars in the centre of thepicture form the belt of Orion. Just below, in the sword handle, isan irregular white patch about one-eighth of an inch in diameter. This is a small-scale image of the great nebula in Orion, shownon a larger scale in Fig. 2. ]

THE NEW HEAVENS

BY

GEORGE ELLERY HALE

DIRECTOR OF THE MOUNT WILSON OBSERVATORY OF THE CARNEGIE INSTITUTIONOF 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 untechnicallanguage an account of some modern methods of astrophysical research. This book is now out of print, and the rapid progress of science hasleft it completely out of date. As I have found no opportunity toprepare a new edition, or to write another book of similar purpose, I have adopted the simpler expedient of contributing occasionalarticles on recent developments to _Scribner's Magazine_, threeof which are included in the present volume. 

I am chiefly indebted, for the illustrations, to the Mount WilsonObservatory and the present and former members of its staff whosenames appear in the captions. Special thanks are due to Mr. FerdinandEllerman, who made all of the photographs of the observatory buildingsand instruments, and prepared all material for reproduction. Thecut of the original Cavendish apparatus is copied from the_Philosophical Transactions for 1798_ with the kind permissionof the Royal Society, and I am also indebted to the Royal Societyand to Professor Fowler and Father Cortie for the privilege ofreproducing from the _Proceedings_ two illustrations of theirspectroscopic 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 nebulæ 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 tryto count the stars. If your station lies well beyond the glare ofcities, which is often strong enough to conceal all but the brighterobjects, you will find the task a difficult one. Ranging throughthe six magnitudes of the Greek astronomers, from the brilliantSirius to the faintest perceptible points of light, the stars arescattered in great profusion over the celestial vault. Their numberseems limitless, yet actual count will show that the eye has beendeceived. In a survey of the entire heavens, from pole to pole, it would not be possible to detect more than from six to seventhousand stars with the naked eye. From a single viewpoint, evenwith 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 measuredwith the simple instruments of the Greek astronomers, containsonly 1, 022 stars. 

Back of Ptolemy, through the speculations of the Greek philosophers, the mysteries of the Egyptian sun-god, and the observations of theancient Chaldeans, the rich and varied traditions of astronomy stretchfar away into a shadowy past. All peoples, in the first stirringsof their intellectual youth, drawn by the nightly splendor of theskies and the ceaseless motions of the planets, have set up somesystem of the heavens, in which the sense of wonder and the desirefor knowledge were no less concerned than the practical necessitiesof life. The measurement of time and the needs of navigation havealways stimulated astronomical research, but the intellectual demandhas been keen from the first. Hipparchus and the Greek astronomersof the Alexandrian school, shaking off the vagaries of magic anddivination, placed astronomy on a scientific basis, though thereaction of the Middle Ages caused even such a great astronomeras Tycho Brahe himself to revert for a time to the practice ofastrology. 

EARLY INSTRUMENTS

The transparent sky of Egypt, rarely obscured by clouds, greatlyfavored Ptolemy's observations. Here was prepared his great starcatalogue, based upon the earlier observations of Hipparchus, anddestined to remain alone in its field for more than twelve centuries, until Ulugh Bey, Prince of Samarcand, repeated the work of hisGreek predecessor. Throughout this period the stars were lookedupon mainly as points of reference for the observation of planetarymotions, 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 theiressential principle. In its simple form, the astrolabe was suspendedin a vertical plane, and the stars were observed by bringing thesights on the movable diameter to bear upon them. Their altitudewas then read off on the circle. Ultimately, the circle of theastrolabe, mounted with one of its diameters parallel to the earth'saxis, became the armillary sphere, the precursor of our modernequatorial telescope. Great stone quadrants fixed in the meridianwere also employed from very early times. Out of such furnishings, little modified by the lapse of centuries, was provided the elaborateinstrumental equipment of Uranibourg, the great observatory builtby Tycho Brahe on the Danish island of Huen in 1576. In this "Cityof the Heavens, " still dependent solely upon the unaided eye as acollector of starlight, Tycho made those invaluable observationsthat enabled Kepler to deduce the true laws of planetary motion. Butafter all these centuries the sidereal world embraced no objects, barring an occasional comet or temporary star, that lay beyondthe vision of the earliest astronomers. The conceptions of thestellar universe, except those that ignored the solid ground ofobservation, 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 photographshows only the bright central part of the nebula. A longer exposurereveals 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 beforethe Christian era, but subsequently denied under the authority ofPtolemy and the teachings of the Church, was reaffirmed by thePolish monk Copernicus in 1543. Kepler's laws of the motions of theplanets, showing them to revolve in ellipses instead of circles, removed the last defect of the Copernican system, and left no room forits rejection. But both the world and the Church clung to tradition, and some visible demonstration was urgently needed. This was suppliedby 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 ofa precipitous cañon 1, 500 feet deep, is the vertical telescopeof the Smithsonian Astrophysical Observatory. Above it are the"Monastery" and other buildings used as quarters by the astronomersof the Mount Wilson Observatory while at work on the mountain. (Theoffices, computing-rooms, laboratories, and shops are in Pasadena. )Following the ridge, we come successively to the dome of the 10-inchphotographic telescope, the power-house, laboratory, Snow horizontaltelescope, 60-foot-tower telescope, and 150-foot-tower telescope, these last three used for the study of the sun. The dome of the60-inch reflecting telescope is just below the 150-foot tower, while that of the 100-inch telescope is farther to the right. Thealtitude of Mount Wilson is about 5, 900 feet. ]

The crystalline lens of the human eye, limited by the iris to amaximum opening about one-quarter of an inch in diameter, was theonly collector of starlight available to the Greek and Arabianastronomers. Galileo's telescope, which in 1610 suddenly pushedout the boundaries of the known stellar universe and brought manythousands of stars into range, had a lens about 2-1/4 inches indiameter. The area of this lens, proportional to the square ofits diameter, was about eighty-one times that of the pupil of theeye. This great increase in the amount of light collected shouldbring to view stars down to magnitude 10. 5, of which nearly halfa million are known to exist. 

It is not too much to say that Galileo's telescope revolutionizedhuman thought. Turned to the moon, it revealed mountains, plains, and valleys, while the sun, previously supposed immaculate in itsperfection, was seen to be blemished with dark spots changing fromday to day. Jupiter, shown to be accompanied by four encirclingsatellites, 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 thechanging phases of Venus and the variation of its apparent diameterduring its revolution about the sun. Galileo's proof of the Copernicantheory marked the downfall of mediævalism and established astronomy ona firm foundation. But while his telescope multiplied a hundredfoldthe number of visible stars, more than a century elapsed beforethe 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 seriousattack upon the problem of the structure of the stellar universe. In his first memoir on the "Construction of the Heavens, " readbefore the Royal Society in 1784, he wrote as follows:

"Hitherto the sidereal heavens have, not inadequately for the purposedesigned, been represented by the concave surface of a sphere inthe centre of which the eye of an observer might be supposed to beplaced. .. . In future we shall look upon those regions into which wemay now penetrate by means of such large telescopes, as a naturalistregards a rich extent of ground or chain of mountains containingstrata variously inclined and directed as well as consisting ofvery different materials. "

On turning his 18-inch reflecting telescope to a part of the MilkyWay in Orion, he found its whitish appearance to be completelyresolved into small stars, not separately seen with his formertelescopes. "The glorious multitude of stars of all possible sizesthat presented themselves here to my view are truly astonishing; butas the dazzling brightness of glittering stars may easily misleadus so far as to estimate their number greater than it really is, I endeavored to ascertain this point by counting many fields, andcomputing from a mean of them, what a certain given portion ofthe Milky Way might contain. " By this means, applied not only tothe Milky Way but to all parts of the heavens, Herschel determinedthe approximate number and distribution of all the stars withinreach of his instrument. 

By comparing many hundred gauges or counts of stars visible ina field of about one-quarter of the area of the moon, Herschelfound that the average number of stars increased toward the greatcircle which most nearly conforms with the course of the Milky Way. Ninety degrees from this plane, at the pole of the Milky Way, onlyfour 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 of122 stars per field. 

[Illustration: Fig. 5. Erecting the polar axis of the 100-inchtelescope. ]

These observations were made in the northern hemisphere, andsubsequently Sir John Herschel, using his father's telescope atthe Cape of Good Hope, found an almost exactly similar increaseof apparent star density for the southern hemisphere. According tohis estimates, the total number of stars in both hemispheres thatcould be seen distinctly enough to be counted in this telescopewould probably be about five and one-half millions. 

The Herschels concluded that "the stars of our firmament, insteadof being scattered in all directions indifferently through space, form a stratum of which the thickness is small, in comparison withits length and breadth; and in which the earth occupies a placesomewhere about the middle of its thickness, between the pointwhere it subdivides into two principal laminæ inclined at a smallangle to each other. " This view does not differ essentially from ourmodern conception of the form of the Galaxy; but as the Herschelswere unable to see stars fainter than the fifteenth magnitude, it is evident that their conclusions apply only to a restrictedregion surrounding the solar system, in the midst of the enormouslyextended sidereal universe which modern instruments have broughtwithin our range. 

MODERN METHODS

The remarkable progress of modern astronomy is mainly due to twogreat instrumental advances: the rise and development of thephotographic telescope, and the application of the spectroscope tothe study of celestial objects. These new and powerful instruments, supplemented by many accessories which have completely revolutionizedobservatory equipment, have not only revealed a vastly greaternumber of stars and nebulæ: they have also rendered feasibleobservations of a type formerly regarded as impossible. The chemicalanalysis of a faint star is now so easy that it can be accomplishedin a very short time--as quickly, in fact, as an equally complexsubstance can be analyzed in the laboratory. The spectroscope alsomeasures a star's velocity, the pressure at different levels inits atmosphere, its approximate temperature, and now, by a newand ingenious method, its distance from the earth. It determinesthe velocity of rotation of the sun and of nebulæ, the existenceand periods of orbital revolution of binary stars too close tobe separated by any telescope, the presence of magnetic fieldsin sunspots, and the fact that the entire sun, like the earth, isa 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 applicationof physical methods of the most diverse character, have greatlyenlarged the astronomer's outlook. He may now attack two greatproblems: (1) The structure of the universe and the motions ofits constituent bodies, and (2) the evolution of the stars: theirnature, origin, growth, and decline. These two problems are intimatelyrelated and must be studied as one. [*]

[Footnote *: A third great problem open to the astronomer, thestudy of the constitution of matter, is described in Chapter III. ]

If space permitted, it would be interesting to survey the progressalready accomplished by modern methods of astronomical research. Hundreds of millions of stars have been photographed, and the boundariesof the stellar universe have been pushed far into space, but have notbeen attained. Globular star clusters, containing tens of thousandsof stars, are on so great a scale (according to Shapley) that light, travelling at the rate of 186, 000 miles per second, may take 500years to cross one of them, while the most distant of these objectsmay be more than 200, 000 light-years from the earth. The spiralnebulæ, more than a million in number, are vast whirling massesin process of development, but we are not yet certain whether theyshould be regarded as "island universes" or as subordinate to thestellar system which includes our minute group of sun and planets, the great star clouds of the Milky Way, and the distant globularstar clusters. 

[Illustration: Fig. 7. Section of a steel girder for dome coveringthe 100-inch telescope, on its way up Mount Wilson. ]

These few particulars may give a slight conception of the scaleof the known universe, but a word must be added regarding someof its most striking phenomena. The great majority of the starswhose motions have been determined belong to one or the other oftwo great star streams, but the part played by these streams in thesidereal system as a whole is still obscure. The stars have beengrouped in classes, presumably in the order of their evolutionaldevelopment, as they pass from the early state of gaseous masses, oflow density, through the successive stages resulting from loss ofheat by radiation and increased density due to shrinkage. Strangelyenough, 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 someindication of the scope of the new astronomy. Enough has been said, however, to assist in appreciating the increased opportunity forinvestigation, and the nature of the heavy demands made upon themodern observatory. But before passing on to describe one of thelatest additions to the astronomer's instrumental equipment, aword 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 oftwo or more lenses, mounted at the upper end of a tube, which ispointed at the celestial object. The light, after passing throughthe lenses, is brought to a focus at the lower end of the tube, wherethe image is examined visually with an eyepiece, or photographedupon a sensitive plate. The largest instruments of this type arethe 36-inch Lick telescope and the 40-inch refractor of the YerkesObservatory. 

[Illustration: Fig. 8. Erecting the steel building and revolvingdome that cover the Hooker telescope. ]

Reflecting telescopes, which are particularly adapted for photographicwork, though also excellent for visual observations, are verydifferently constructed. No lens is used. The telescope tube isusually built in skeleton form, open at its upper end, and witha large concave mirror supported at its base. This mirror servesin place of a lens. Its upper surface is paraboloidal in shape, as a spherical surface will not unite in a sharp focus the rayscoming from a distant object. The light passes through no glass--agreat advantage, especially for photography, as the absorptionin lenses cuts out much of the blue and violet light, to whichphotographic plates are most sensitive. The reflection occurs onthe _upper_ surface of the mirror, which is covered with a coat ofpure silver, renewed several times a year and always kept highlyburnished. Silvered glass is better than metals or other substancesfor telescope mirrors, chiefly because of the perfection with whichglass can be ground and polished, and the ease of renewing itssilvered surface when tarnished. 

The great reflectors of Herschel and Lord Rosse, which were providedwith mirrors of speculum metal, were far inferior to much smallertelescopes of the present day. With these instruments the star imageswere watched as they were carried through the field of view by theearth's rotation, or kept roughly in place by moving the telescopewith ropes or chains. Photographic plates, which reveal invisiblestars and nebulæ when exposed for hours in modern instruments, werenot then available. In any case they could not have been used, in the absence of the perfect mechanism required to keep the starimages accurately fixed in place upon the sensitive film. 

[Illustration: Fig. 9. Building and revolving dome, 100 feet indiameter, 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 supremacybetween refracting and reflecting telescopes, each of which, atcertain stages in its development, appeared to be unrivalled. Inmodern observatories both types are used, each for the purpose forwhich it is best adapted. For the photography of nebulæ and thestudy of the fainter stars, the reflector has special advantages, illustrated by the work of such instruments as the Crossley and Millsreflectors of the Lick Observatory; the great 72-inch reflector, recently brought into effective service at the Dominion Observatoryin Canada; and the 60-inch and 100-inch reflectors of the MountWilson Observatory. 

The unaided eye, with an available area of one-twentieth of a squareinch, permits us to see stars of the sixth magnitude. Herschel's18-inch reflector, with an area 5, 000 times as great, renderedvisible stars of the fifteenth magnitude. The 60-inch reflector, with an area 57, 600 times that of the eye, reveals stars of theeighteenth magnitude, while to reach stars of about the twentiethmagnitude, photographic exposures of four or five hours sufficewith this instrument. 

Every gain of a magnitude means a great gain in the number of starsrendered visible. Stars of the second magnitude are 3. 4 times asnumerous as those of the first, those of the eighth magnitude arethree times as numerous as those of the seventh, while the sixteenthmagnitude stars are only 1. 7 as numerous as those of the fifteenthmagnitude. This steadily decreasing ratio is probably due to anactual thinning out of the stars toward the boundaries of the stellaruniverse, as the most exhaustive tests have failed to give anyevidence of absorption of light in its passage through space. Butin spite of this decrease, the gain of a single additional magnitudemay mean the addition of many millions of stars to the total of thosealready shown by the 60-inch reflector. Here is one of the chiefsources of interest in the possibilities of a 100-inch reflectingtelescope. 

100-INCH TELESCOPE

[Illustration: Fig. 10. One-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lowerend of telescope tube. (Seen above. )]

In 1906 the late John D. Hooker, of Los Angeles, gave the CarnegieInstitution of Washington a sum sufficient to construct a telescopemirror 100 inches in diameter, and thus large enough to collect160, 000 times the light received by the eye. (Fig. 10. ) The castingand annealing of a suitable glass disk, 101 inches in diameterand 13 inches thick, weighing four and one-half tons, was a mostdifficult operation, finally accomplished by a great French glasscompany at their factory in the Forest of St. Gobain. A specialoptical laboratory was erected at the Pasadena headquarters ofthe Mount Wilson Observatory, and here the long task of grinding, figuring, and testing the mirror was successfully carried out bythe observatory opticians. This operation, which is one of greatdelicacy, required years for its completion. Meanwhile the building, dome, and mounting for the telescope were designed by members ofthe observatory staff, and the working drawings were prepared. Anopportune addition by Mr. Carnegie to the endowment of the CarnegieInstitution of Washington, of which the observatory is a branch, permitted the necessary appropriations to be made for the completionand erection of the telescope. Though delayed by the war, duringwhich the mechanical and optical facilities of the observatoryshops were utilized for military and naval purposes, the telescopeis 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 extendssouth from this pier a distance of 50 feet, on the west side ofwhich a very powerful spectrograph, for photographing the spectraof the brightest stars, will be mounted. Within the pier are aphotographic dark room, a room for silvering the large mirror (whichcan be lowered into the pier), and the clock-room, where standsthe powerful driving-clock, with which the telescope is causedto follow the apparent motion of the stars. (Fig. 11. )

[Illustration: Fig. 11. The driving-clock and worm-gear that causethe 100-inch Hooker telescope to follow the stars. ]

The telescope mounting is of the English type, in which the telescopetube is supported by the declination trunnions between the arms ofthe polar axis, built in the form of a rectangular yoke carried bybearings on massive pedestals to the north and south. These bearingsmust be aligned exactly parallel to the axis of the earth, and mustsupport the polar axis so freely that it can be rotated with perfectprecision by the driving-clock, which turns a worm-wheel 17 feet indiameter, clamped to the lower end of the axis. As this motionmust be sufficiently uniform to counteract exactly the rotationof the earth on its axis, and thus to maintain the star imagesaccurately in position in the field of view, the greatest carehad to be taken in the construction of the driving-clock and inthe 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 inPasadena or on Mount Wilson after the assembling of the telescope. The massive sections of the instrument, some of which weigh asmuch as ten tons each, were constructed at Quincy, Mass. , wheremachinery sufficiently large to build battleships was available. They were then shipped to California, and transported to the summitof Mount Wilson over a road built for this purpose by the constructiondivision of the observatory, which also built the pier on which thetelescope stands, and erected the steel building and dome thatcover it. 

[Illustration: Fig. 12. Large irregular nebula and star clusterin Sagittarius (Duncan). 

Photographed with the 60-inch telescope. ]

[Illustration: Fig. 13. Faint spiral nebula in the constellationof the Hunting Dogs (Pease). 

Photographed with the 60-inch telescope. ]

The parts of the telescope which are moved by the driving-clockweigh about 100 tons, and it was necessary to provide means ofreducing the great friction on the bearings of the polar axis. Toaccomplish this, large hollow steel cylinders, floating in mercuryheld in cast-iron tanks, were provided at the upper and lower endsof the polar axis. Almost the entire weight of the instrument isthus floated in mercury, and in this way the friction is so greatlyreduced that the driving-clock moves the instrument with perfectease and smoothness. 

The 100-inch mirror rests at the bottom of the telescope tube ona special support system, so designed as to prevent any bending ofthe glass under its own weight. Electric motors, forty in number, areprovided to move the telescope rapidly or slowly in right ascension(east or west) and in declination (north or south), for focussingthe mirrors, and for many other purposes. They are also used forrotating the dome, 100 feet in diameter, under which the telescopeis mounted, and for opening the shutter, 20 feet wide, throughwhich 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 inone of the arrangements of the instrument, the photographic plateis mounted at the centre of the telescope tube near its upper end, where it receives directly the image formed by the large mirror. Inanother arrangement, a silvered glass mirror, with plane surface, is supported near the upper end of the tube at an angle of 45°, soas to form the image at the side of the tube, where the photographicplate 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 openingin 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 convexmirrors carried near the upper end of the tube are required, permitthe image to be photographed at the side of the tube near its lowerend, either with or without a spectrograph; or with a very powerfulspectrograph mounted within a constant-temperature chamber southof the telescope pier. In this last case, the light of a star isso reflected by auxiliary mirrors that it passes down through ahole in the south end of the polar axis and brings the star toa focus on the slit of the fixed spectrograph. 

ATMOSPHERIC LIMITATIONS

The huge dimensions of such a powerful engine of research as theHooker telescope are not in themselves a source of satisfaction tothe astronomer, for they involve a decided increase in the laborof observation and entail very heavy expense, justifiable only incase important results, beyond the reach of other instruments, can be secured. The construction of a telescope of these dimensionswas necessarily an experiment, for it was by no means certain, afterthe optical and mechanical difficulties had been overcome, thateven the favorable atmosphere of California would be sufficientlytranquil to permit sharply defined celestial images to be obtainedwith so large an aperture. It is therefore important to learn whatthe telescope will actually accomplish under customary observingconditions. 

Fortunately we are able to measure the performance of the instrumentwith certainty. Close beside it on Mount Wilson stands the 60-inchreflector, of similar type, erected in 1908. The two telescopes canthus be rigorously compared under identical atmospheric conditions. 

The large mirror of the 100-inch telescope has an area about 2. 8times that of the 60-inch, and therefore receives nearly three timesas much light from a star. Under atmospheric conditions perfectenough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with agiven exposure, stars about one magnitude fainter than the fainteststars within reach of the 60-inch. The increased focal length, permitting such objects as the moon to be photographed on a largerscale, should also reveal smaller details of structure and renderpossible higher accuracy of measurement. Finally, the greatertheoretical resolving power of the larger aperture, providing itcan be utilized, should permit the separation of the members ofclose double stars beyond the range of the smaller instrument. 

CRITICAL TESTS

The many tests already made indicate that the advantages expectedof the new telescope will be realized in practice. The increasedlight-gathering power will mean the addition of many millions ofstars to those already known. Spectroscopic observations now inregular progress have carried the range of these investigationsfar beyond the possibilities of the 60-inch telescope. A greatclass of red stars, for example, almost all the members of whichwere inaccessible to the 60-inch, are now being made the subjectof special study. And in other fields of research equal advantageshave been gained. 

The increase in the scale of the images over those given by the60-inch telescope is illustrated by two photographs of the RingNebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion, photographed with the 100-inch telescope with a comparatively shortexposure, sufficient to bring out the brighter regions, is reproducedin Fig. 2. It is interesting to compare this picture with thesmall-scale image of the same nebula shown in Fig. 1. 

[Illustration: Fig. 15. Photograph of the moon made on September15, 1919, with the 100-inch Hooker telescope (Pease). 

The ring-like formations are the so-called craters, most of themfar larger than anything similar on the earth. That in the lowerleft corner with an isolated mountain in the centre is Albategnius, sixty-four miles in diameter. Peaks in the ring rise to a heightof fifteen thousand feet above the central plain. Note the longsunset shadows cast by the mountains on the left. The level regionbelow on the right is an extensive plain, the Mare Nubium. ]

[Illustration: Fig. 16. Photograph of the moon made on September15, 1919, with the 100-inch Hooker telescope (Pease). 

The mountains above and to the left are the lunar Apennines; thoseon the left just below the centre are the Alps. Both ranges includepeaks from fifteen thousand to twenty thousand feet in height. Inthe upper right corner is Copernicus, about fifty miles in diameter. The largest of the conspicuous group of three just below the Apenninesis Archimedes and at the lower end of the Alps is Plato. Note thelong sunset shadows cast by the isolated peaks on the left. Thecentral portion of the picture is a vast plain, the Mare Imbrium. ]

The sharpness of the images given by the new telescope may beillustrated by some recent photographs of the moon, obtained withan equivalent focal length of 134 feet. In Fig. 15 is shown a ruggedregion of the moon, containing many ring-like mountains or craters. Fig. 16 shows the great arc of the lunar Apennines (above) and theAlps (below), to the left of the broad plain of the Mare Imbrium. The starlike points along the moon's terminator, which separatesthe dark area from the region upon which the sun (on the right)shines, are the mountain peaks, about to disappear at sunset. Thelong shadows cast by the mountains just within the illuminatedarea are plainly seen. Some of the peaks of the lunar Apenninesattain a height of 20, 000 feet. 

In less powerful telescopes the stars at the centre of the greatglobular clusters are so closely crowded together that they cannotbe studied separately with the spectrograph. Moreover, most ofthem are much too faint for examination with this instrument. Atthe 134-foot focus the 100-inch telescope gives a large-scale imageof such clusters, and permits the spectra of stars as faint asthe fifteenth magnitude to be separately photographed. 

[Illustration: Fig. 17. Hubble's Variable Nebula. One of the fewnebulæ 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 fulltheoretical resolving power to be not merely attained but to bedoubled, has been made possible by the first application of Michelson'sinterference 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 theslits, unite at the focus of the telescope, where the image isexamined by an eyepiece magnifying about five thousand diameters. Across the enlarged star image a series of fine, sharp fringes isseen, even when the atmospheric conditions are poor. If the star issingle the fringes remain visible, whatever the distance between theslits. But in the case of a star like Capella, previously inferredto be double from the periodic displacement of the lines in itsspectrum, but with components too close together to be distinguishedseparately, the fringes behave differently. As the slits are movedapart a point is reached where the fringes completely disappear, only to reappear as the separation is continued. This effect isobtained when the slits are at right angles to the line joiningthe two stars of the pair, found by this method to be 0. 0418 of asecond of arc apart (on December 30, 1919). Subsequent measures, of far greater precision than those obtainable by other methods inthe case of easily separated double stars, show the rapid orbitalmotion of the components of the system. This device will be appliedto other close binaries, hitherto beyond the reach of measurement. 

[Illustration: Fig. 18. Ring Nebula in Lyra, photographed with the60-inch (Ritchey) and 100-inch (Duncan) telescopes. 

Showing the increased scale of the images given by the largerinstrument. ]

Without entering into further details of the tests, it is evidentthat the new telescope will afford boundless possibilities forthe study of the stellar universe. [*] The structure and extent ofthe 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 ofthe stars and the successive stages in their life history: theseare some of the great questions which the new telescope must helpto answer. In such an embarrassment of riches the chief difficultyis to withstand the temptation toward scattering of effort, and toform an observing programme directed toward the solution of crucialproblems rather than the accumulation of vast stores of miscellaneousdata. This programme will be supplemented by an extensive studyof the sun, the only star near enough the earth to be examinedin detail, and by a series of laboratory investigations involvingthe experimental imitation of solar and stellar conditions, thusaiding in the interpretation of celestial phenomena. 

[Footnote *: It is not adapted for work on the sun, as the mirrorswould be distorted by its heat. Three other telescopes, especiallydesigned for solar observations, are in use on Mount Wilson. ]

CHAPTER II

GIANT STARS

Our ancestral sun, as pictured by Laplace, originally extendedin a state of luminous vapor beyond the boundaries of the solarsystem. Rotating upon its axis, it slowly contracted through lossof heat by radiation, leaving behind it portions of its mass, whichcondensed to form the planets. Still gaseous, though now denser thanwater, it continues to pour out the heat on which our existencedepends, as it shrinks imperceptibly toward its ultimate conditionof a cold and darkened globe. 

Laplace's hypothesis has been subjected in recent years to muchcriticism, and there is good reason to doubt whether his descriptionof the mode of evolution of our solar system is correct in everyparticular. All critics agree, however, that the sun was once enormouslylarger than it now is, and that the planets originally formed partof its distended mass. 

Even in its present diminished state, the sun is huge beyond easyconception. Our own earth, though so minute a fragment of the primevalsun, is nevertheless so large that some parts of its surface havenot yet been explored. Seen beside the sun, by an observer on oneof the planets, the earth would appear as an insignificant speck, which could be swallowed with ease by the whirling vortex of asun-spot. If the sun were hollow, with the earth at its centre, the moon, though 240, 000 miles from us, would have room and tospare in which to describe its orbit, for the sun is 865, 000 milesin diameter, so that its volume is more than a million times thatof the earth. 

[Illustration: Fig. 19. Gaseous prominence at the sun's limb, 140, 000miles high (Ellerman). 

Photographed with the spectroheliograph, using the light emittedby glowing calcium vapor. The comparative size of the earth isindicated 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 constitutethe sun and the earth? Are they comparable in size with the sun? Dothey occur in all stages of development, from infancy to old age?And if such stages can be detected, do they afford indicationsof the gradual diminution in volume which Laplace imagined thesun to experience?

[Illustration: Fig. 20. The sun, 865, 000 miles in diameter, froma direct photograph showing many sun-spots (Whitney)

The small black disk in the centre represents the comparative sizeof the earth, while the circle surrounding it corresponds in diameterto the orbit of the moon. ]

STAR IMAGES

Prior to the application of the powerful new engine of researchdescribed in this article we have had no means of measuring thediameters of the stars. We have measured their distances and theirmotions, determined their chemical composition, and obtained undeniableevidence of progressive development, but even in the most powerfultelescopes their images are so minute that they appear as pointsrather than as disks. In fact, the larger the telescope and themore perfect the atmospheric conditions at the observer's command, the smaller do these images appear. On the photographic plate, it istrue, the stars are recorded as measurable disks, but these are dueto 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 instraight lines, but these also are instrumental phenomena, dueto diffraction of light by the steel bars that support the smallmirror in the tube of reflecting telescopes. In a word, the starsare so remote that the largest and most perfect telescopes showthem only as extremely minute needle-points of light, without anytrace 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, asthe man of science must so often do, indirect means when the directattack fails. Most of the remarkable progress of astronomy duringthe last quarter-century has resulted from the application of new andingenious devices borrowed from the physicist. These have multipliedto such a degree that some of our observatories are literally physicallaboratories, in which the sun and stars are examined by powerfulspectroscopes and other optical instruments that have recently advancedour knowledge of physics by leaps and bounds. In the present casewe are indebted for our star-measuring device to the distinguishedphysicist Professor Albert A. Michelson, who has contributed a longarray of novel apparatus and methods to physics and astronomy. 

THE INTERFEROMETER

The instrument in question, known as the interferometer, had previouslyyielded a remarkable series of results when applied in its variousforms to the solution of fundamental problems. To mention only afew of those that have helped to establish Michelson's fame, we mayrecall that our exact knowledge of the length of the internationalmetre at Sevres, the world's standard of measurement, was obtainedby him with an interferometer in terms of the invariable length oflight-waves. A different form of interferometer has more recentlyenabled him to measure the minute tides within the solid body of theearth--not the great tides of the ocean, but the slight deformationsof the earth's body, which is as rigid as steel, that are caused bythe varying attractions of the sun and moon. Finally, to mentiononly one more case, it was the Michelson-Morley experiment, madeyears ago with still another form of interferometer, that yieldedthe basic idea from which the theory of relativity was developedby Lorentz and Einstein. 

[Illustration: Fig. 22. Photograph of the hydrogen atmosphere ofthe sun (Ellerman). 

Made with the spectroheliograph, showing the immense vortices, or whirling storms like tornadoes, that centre in sun-spots. Thecomparative size of the earth is shown by the white circle tracedon the largest sun-spot. ]

The history of the method of measuring star diameters is a verycurious one, showing how the most promising opportunities for scientificprogress may lie unused for decades. The fundamental principleof the device was first suggested by the great French physicistFizeau in 1868. In 1874 the theory was developed by the Frenchastronomer Stéphan, who observed interference fringes given by alarge number of stars, and rightly concluded that their angulardiameters must be much smaller than 0. 158 of a second of arc, thesmallest measurable with his instrument. In 1890 Michelson, unawareof the earlier work, published in the _Philosophical Magazine_ acomplete description of an interferometer capable of determiningwith surprising accuracy the distance between the components ofdouble stars so close together that no telescope can separate them. He also showed how the same principle could be applied to themeasurement of star diameters if a sufficiently large interferometercould be built for this purpose, and developed the theory muchmore completely than Stéphan had done. A year later he measuredthe diameters of Jupiter's satellites by this means at the LickObservatory. But nearly thirty years elapsed before the next stepwas taken. Two causes have doubtless contributed to this delay. Boththeory and experiment have demonstrated the extreme sensitivenessof the "interference fringes, " on the observation of which themethod depends, and it was generally supposed by astronomers thatdisturbances in the earth's atmosphere would prevent them frombeing clearly seen with large telescopes. Furthermore, a very largeinterferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double starscould have been easily studied by this device with several of thelarge 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 MountWilson Observatory led me to suggest to Professor Michelson, beforethe United States entered the war, that the method be thoroughlytested under the favorable atmospheric conditions of SouthernCalifornia. He was at that time at work on a special form ofinterferometer, designed to determine whether atmospheric disturbancescould be disregarded in planning large-scale experiments. But thewar intervened, and all of our efforts were concentrated for twoyears on the solution of war problems. [*] In 1919, as soon as the100-inch telescope had been completed and tested, the work wasresumed on Mount Wilson. 

[Footnote *: Professor Michelson's most important contribution duringthe 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 aidof an experiment which any one can easily perform for himself withsimple apparatus. Make a narrow slit, a few thousandths of an inchin width, in a sheet of black paper, and support it verticallybefore a brilliant source of light. Observe this from a distance of40 or 50 feet with a small telescope magnifying about 30 diameters. The object-glass of the telescope should be covered with an opaquecap, pierced by two circular holes about one-eighth of an inch indiameter and half an inch apart. The holes should be on oppositesides of the centre of the object-glass and equidistant from it, and the line joining the holes should be horizontal. When thiscap is removed the slit appears as a narrow vertical band withmuch fainter bands on both sides of it. With the cap in place, thecentral bright band appears to be ruled with narrow vertical linesor fringes produced by the "interference"[*] of the two pencils oflight coming through different parts of the object-glass from thedistant slit. Cover one of the holes, and the fringes instantlydisappear. Their production requires the joint effect of the twolight-pencils. 

[Footnote *: For an explanation of the phenomena of interference, see any encyclopæedia or book on physics. ]

Now suppose the two holes over the object-glass to be in movableplates, so that their distance apart can be varied. As they aregradually separated the narrow vertical fringes become less andless distinct, and finally vanish completely. Measure the distancebetween the holes and divide this by the wavelength of light, whichwe may call 1/50000 of an inch. The result is the angular widthof the distant slit. Knowing the distance of the slit, we can atonce calculate its linear width. If for the slit we substitute aminute circular hole, the method of measurement remains the same, but the angular diameter as calculated above must be multipliedby 1. 22. [*]

[Footnote *: More complete details may be found in Michelson's LowellLectures on "Light-Waves and Their Uses, " University of ChicagoPress, 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 largetelescope, for the smaller the angle the farther apart must be thetwo holes over the object-glass (or the mirror, in case a reflectingtelescope is employed). In fact, when the holes are moved apart tothe full aperture of the 100-inch Hooker telescope, the interferencefringes are still visible even with the star Betelgeuse, though itsangular diameter is perhaps as great as that of any other star. Thus, we must build an attachment for the telescope, so arrangedas to permit us to move the openings still farther apart. 

[Illustration: Fig. 23. Diagram showing outline of the 100-inchHooker telescope, and path of the two pencils of light from a starwhen 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 upperend of the skeleton tube of the telescope (Fig. 24). The light fromthe star is received by two flat mirrors (Ml, M4) which projectbeyond the tube and can be moved apart along the supporting arm. These take the place of the two holes over the object-glass inour experiment. From these mirrors the light is reflected to asecond pair of flat mirrors (M2, M3), which send it toward the100-inch concave mirror (M5) at the bottom of the telescope tube. After this the course of the light is exactly as it would be ifthe mirrors M2, M3 were replaced by two holes over the 100-inchmirror. It is reflected to the convex mirror (M6), then back ina less rapidly convergent beam toward the large mirror. Beforereaching it the light is caught by the plane mirror (M7) and reflectedthrough an opening at the side of the telescope tube to the eye-pieceE. Here the fringes are observed with a magnification ranging from1, 500 to 3, 000 diameters. 

[Illustration: Fig. 24. Twenty-foot Michelson interferometer formeasuring star diameters, attached to upper end of the skeletontube of the 100-inch Hooker telescope. 

The path of the two pencils of light from the star is shown inFig. 23. For a photograph of the entire telescope, see Fig. 4. ]

In the practical application of this method to the measurement ofstar diameters, the chief problem was whether the atmosphere wouldbe quiet enough to permit sharp interference fringes to be producedwith light-pencils more than 100 inches apart. After successfulpreliminary tests with the 40-inch refracting telescope of theYerkes Observatory, Professor Michelson made the first attemptto see the fringes with the 60-inch and 100-inch reflectors onMount Wilson in September, 1919. He was surprised and delighted tofind that the fringes were perfectly sharp and distinct with thefull aperture of both these instruments. Doctor Anderson, of theobservatory staff, then devised a special form of interferometerfor the measurement of close double stars, and applied it withthe 100-inch telescope to the measurement of the orbital motionof the close components of Capella, with results of extraordinaryaccuracy, far beyond anything attainable by previous methods. Thesuccess of this work strongly encouraged the more ambitious projectof measuring the diameter of a star, and the 20-foot interferometerwas built for this purpose. 

The difficult and delicate problem of adjusting the mirrors ofthis instrument with the necessary extreme accuracy was solved byProfessor Michelson during his visit to Mount Wilson in the summerof 1920, and with the assistance of Mr. Pease, of the observatorystaff, interference fringes were observed in the case of certainstars when the mirrors were as much as 18 feet apart. All was thusin readiness for a decisive test as soon as a suitable star presenteditself. 

THE GIANT BETELGEUSE

Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabicfor "the giant's shoulder"), the bright red star in the constellationof 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 asecond of arc. This deduction from theory appeared in his recentpresidential address before the British Association for the Advancementof Science, in which Professor Eddington remarked: "Probably thegreatest need of stellar astronomy at the present day, in orderto make sure that our theoretical deductions are starting on theright lines, is some means of measuring the apparent angular diameterof stars. " He then referred to the work already in progress onMount Wilson, but anticipated "that atmospheric disturbance willultimately 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 be0. 047 of a second of arc, corresponding to a linear diameter of215, 000, 000 miles, if the best available determination of its distancecan be relied upon. This determination shows Betelgeuse to be 160light-years from the earth. Light travels at the rate of 186, 000miles per second, and yet spends 160 years on its journey to usfrom this star. ]

On December 13, 1920, Mr. Pease successfully measured the diameterof Betelgeuse with the 20-foot interferometer. As the outer mirrorswere separated the interference fringes gradually became less distinct, as theory requires, and as Doctor Merrill had previously seen whenobserving Betelgeuse with the interferometer used for Capella. Ata separation of 10 feet the fringes disappeared completely, givingthe data required for calculating the diameter of the star. Totest the perfection of the adjustment, the telescope was turned toother stars, of smaller angular diameter, which showed the fringeswith perfect clearness. Turning back to Betelgeuse, they were seenbeyond doubt to be absent. Assuming the mean wave-length of thelight of this star to be 5750/10000000 of a millimetre, its angulardiameter comes out 0. 047 of a second of arc, thus falling betweenthe values--0. 051 and 0. 031 of a second--predicted by Eddington andRussell from slightly different assumptions. Subsequent correctionsand 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 Betelgeuseis 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), knownto the Arabs as the "Lance Bearer, " and to the Chinese as the "GreatHorn" or the "Palace of the Emperors" (Hubble). 

Its angular diameter, measured at Mount Wilson by Pease with the20-foot Michelson interferometer on April 15, 1921, is 0. 022 of asecond, in close agreement with Russell's predicted value of 0. 019of a second. The mean parallax of Arcturus, based upon severaldeterminations, is 0. 095 of a second, corresponding to a distance of34 light-years. The linear diameter, computed from Pease's measureand 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. Fourdeterminations of the parallax, which determines the distance, have been made. Elkin, with the Yale heliometer, obtained 0. 032of a second of arc. Schlesinger, from photographs taken with the30-inch Allegheny refractor, derived 0. 016. Adams, by his spectroscopicmethod applied with the 60-inch Mount Wilson reflector, obtained0. 012. Lee's recent value, secured photographically with the 40-inchYerkes refractor, is 0. 022. The heliometer parallax is doubtlessless reliable than the photographic ones, and Doctor Adams statesthat the spectral type and luminosity of Betelgeuse make his valueless 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 notbe far from the truth. This parallax represents the angle subtendedby the radius of the earth's orbit (93, 000, 000 miles) at the distanceof Betelgeuse. By comparing it with 0. 047, the angular diameter ofthe star, we see that the linear diameter is about two and one-thirdtimes as great as the distance from the earth to the sun, orapproximately 215, 000, 000 miles. Thus, if this measure of its distanceis not considerably in error, Betelgeuse would nearly fill theorbit of Mars. All methods of determining the distances of thestars are subject to uncertainty, however, and subsequent measuresmay reduce this figure very appreciably. But there can be no doubtthat the diameter of Betelgeuse exceeds 100, 000, 000 miles, andit is probably much greater. 

The extremely small angle subtended by this enormous disk is explainedby 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 persecond spends 160 years in crossing the space that lies betweenus and Betelgeuse, whose tremendous proportions therefore seemso minute even in the most powerful telescopes. 

STELLAR EVOLUTION

This actual measure of the diameter of Betelgeuse supplies a newand striking test of Russell's and Hertzsprung's theory of dwarfand giant stars. Just before the war Russell showed that our oldmethods of classifying the stars according to their spectra mustbe radically changed. Stars in an early stage of their life historymay be regarded as diffuse gaseous masses, enormously larger thanour sun, and at a much lower temperature. Their density must bevery low, and their state that of a perfect gas. These are the"giants. " In the slow process of time they contract through constantloss of heat by radiation. But, despite this loss, the heat producedby contraction and from other sources (see p. 82) causes theirtemperature to rise, while their color changes from red to bluishwhite. The process of shrinkage and rise of temperature goes on solong as they remain in the state of a perfect gas. But as soon ascontraction has increased the density of the gas beyond a certainpoint the cycle reverses and the temperature begins to fall. Thebluish-white light of the star turns yellowish, and we enter thedwarf stage, of which our own sun is a representative. The densityincreases, surpassing that of water in the case of the sun, andgoing far beyond this point in later stages. In the lapse of millionsof years a reddish hue appears, finally turning to deep red. Thefalling temperature permits the chemical elements, existing in agaseous state in the outer atmosphere of the star, to unite intocompounds, which are rendered conspicuous by their characteristicbands in the spectrum. Finally comes extinction of light, as thestar approaches its ultimate state of a cold and solid globe. 

[Illustration: Fig. 27. The giant star Antares (within the whitecircle), 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 probablynot far from 350 light-years. Its angular diameter of 0. 040 of asecond would thus correspond to a linear diameter of about 400million miles. ]

We may thus form a new picture of the two branches of the temperaturecurve, long since suggested by Lockyer, on very different grounds, asthe outline of stellar life. On the ascending side are the giants, of vast dimensions and more diffuse than the air we breathe. Thereare good reasons for believing that the mass of Betelgeuse cannotbe more than ten times that of the sun, while its volume is atleast a million times as great and may exceed eight million timesthe sun's volume. Therefore, its average density must be like thatof an attenuated gas in an electric vacuum tube. Three-quartersof the naked-eye stars are in the giant stage, which comprisessuch familiar objects as Betelgeuse, Antares, and Aldebaran, butmost of them are much denser than these greatly inflated bodies. The pinnacle is reached in the intensely hot white stars of thehelium class, in whose spectra the lines of this gas are veryconspicuous. The density of these stars is perhaps one-tenth thatof the sun. Sirius, also very hot, is nearly twice as dense. Thencomes the cooling stage, characterized, as already remarked, byincreasing density, and also by increasing chemical complexityresulting from falling temperature. This life cycle is probablynot followed by all stars, but it may hold true for millions ofthem. 

The existence of giant and dwarf stars has been fully proved bythe remarkable work of Adams and his associates on Mount Wilson, where his method of determining a star's distance and intrinsicluminosity by spectroscopic observations has already been appliedto 2, 000 stars. Discussion of the results leads at once to therecognition of the two great classes of giants and dwarfs. Nowcomes the work of Michelson and Pease to cap the climax, giving usthe actual diameter of a typical giant star, in close agreement withpredictions based upon theory. From this diameter we may conclude thatthe density of Betelgeuse is extremely low, in harmony with Russell'stheory, which is further supported by spectroscopic analysis ofthe star's light, revealing evidence of the comparatively lowtemperature called for by the theory at this early stage of stellarexistence. 

TWO OTHER GIANTS

The diameter of Arcturus was successfully measured by Mr. Peaseat Mount Wilson on April 15. As the mirrors of the interferometerwere moved apart, the fringes gradually decreased in visibilityuntil they finally disappeared at a mirror separation of 19. 6 feet. Adopting a mean wave-length of 5600/10000000 of a millimetre forthe light of Arcturus, this gives a value of 0. 022 of a second ofarc for the angular diameter of the star. If we use a mean valueof 0. 095 of a second for the parallax, the corresponding lineardiameter comes out 21, 000, 000 miles. The angular diameter, as inthe case of Betelgeuse, is in remarkably close agreement with thediameter predicted from theory. Antares, the third star measuredby Mr. Pease, is the largest of all. If it is actually a member ofthe Scorpius-Centaurus group, as we have strong reason to believe, it is fully 350 light-years from the earth, and its diameter isabout 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, especiallybecause its marked changes in brightness suggest possible variationsin diameter. We must also apply the interferometer method to starsof the various spectral types, in order to afford a sure basis forfuture studies of stellar evolution. Unfortunately, only a fewgiant stars are certain to fall within the range of our presentinstrument. An interferometer of 70-feet aperture would be neededto measure Sirius accurately, and one of twice this size to dealwith less brilliant white stars. A 100-foot instrument, if feasibleto build, would permit objects representing most of the chief stagesof stellar development to be measured, thus contributing in thehighest degree to the progress of our knowledge of the life historyof the stars. Fortunately, though the mechanical difficulties aregreat, the optical problem is insignificant, and the cost of theentire apparatus, though necessarily high, would be only a smallfraction of that of a telescope of corresponding aperture, if suchcould be built. A 100-foot interferometer might be designed inmany different forms, and one of these may ultimately be foundto be within the range of possibility. Meanwhile the 20-footinterferometer has been improved so materially that it now promisesto yield approximate measures of stars at first supposed to bebeyond its capacity. 

[Illustration: Fig. 29. Aldebaran, the "leader" (of the Pleiades), was also known to the Arabs as "The Eye of the Bull, " "The Heartof the Bull, " and "The Great Camel" (Hubble). 

Like Betelgeuse and Antares, it is notable for its red color, whichaccounts for the fact that its image on this photograph is hardlymore conspicuous than the images of stars which are actually muchfainter but contain a larger proportion of blue light, to whichthe photographic plates here employed are more sensitive than tored or yellow. Aldebaran is about 50 light-years from the earth. Interferometer measures, now in progress on Mount Wilson, indicatethat its angular diameter is about 0. 020 of a second. ]

While the theory of dwarf and giant stars and the measurements justdescribed afford no direct evidence bearing on Laplace's explanationof the formation of planets, they show that stars exist which arecomparable in diameter with our solar system, and suggest that thesun must have shrunk from vast dimensions. The mode of formationof systems like our own, and of other systems numerously illustratedin the heavens, is one of the most fascinating problems of astronomy. Much light has been thrown on it by recent investigations, renderedpossible by the development of new and powerful instruments and byadvances in physics of the most fundamental character. All theevidence confirms the existence of dwarf and giant stars, but muchwork must be done before the entire course of stellar evolutioncan be explained. 

CHAPTER III

COSMIC CRUCIBLES

"Shelter during Raids, " marking the entrance to underground passages, was a sign of common occurrence and sinister suggestion throughoutLondon during the war. With characteristic ingenuity and craftiness, ostensibly for purposes of peace but with bomb-carrying capacityas a prime specification, the Zeppelin had been developed by theGermans to a point where it seriously threatened both London andParis. Searchlights, range-finders, and anti-aircraft guns, surpassedby the daring ventures of British and French airmen, would haveserved but little against the night invader except for its onefatal defect--the inflammable nature of the hydrogen gas that keptit aloft. A single explosive bullet served to transform a Zeppelininto a heap of scorched and twisted metal. This characteristicof hydrogen caused the failure of the Zeppelin raids. 

Had the war lasted a few months longer, however, the work of Americanscientists would have made our counter-attack in the air a formidableone. At the signing of the armistice hundreds of cylinders of compressedhelium lay at the docks ready for shipment abroad. Extracted fromthe natural gas of Texas wells by new and ingenious processes, this substitute for hydrogen, almost as light and absolutelyuninflammable, produced in quantities of millions of cubic feet, would have made the dirigibles of the Allies masters of the air. Thespecial properties of this remarkable gas, previously obtainable onlyin minute quantities, would have sufficed to reverse the situation. 

SOLAR HELIUM

Helium, as its name implies, is of solar origin. In 1868, whenLockyer first directed his spectroscope to the great flames orprominences that rise thousands of miles, sometimes hundreds ofthousands, above the surface of the sun, he instantly identifiedthe characteristic red and blue radiations of hydrogen. In theyellow, close to the position of the well-known double line ofsodium, but not quite coincident with it, he detected a new line, of great brilliancy, extending to the highest levels. Its similarityin this respect with the lines of hydrogen led him to recognizethe existence of a new and very light gas, unknown to terrestrialchemistry. 

Many years passed before any chemical laboratory on earth was ableto match this product of the great laboratory of the sun. In 1896Ramsay at last succeeded in separating helium, recognized by the sameyellow line in its spectrum, in minute quantities from the mineraluraninite. Once available for study under electrical excitation invacuum tubes, helium was found to have many other lines in itsspectrum, which have been identified in the spectra of solarprominences, gaseous nebulæ, and hot stars. Indeed, there is astellar class known as helium stars, because of the dominance ofthis gas in their atmospheres. 

[Illustration: Fig. 30. Solar prominences, photographed with thespectroheliograph without an eclipse (Ellerman). 

In these luminous gaseous clouds, which sometimes rise to elevationsexceeding half the sun's diameter, the new gas helium was discoveredby Lockyer in 1868. Helium was not found on the earth until 1896. Since then it has been shown to be a prominent constituent of nebulæand hot stars. ]

The chief importance of helium lies in the clue it has afforded tothe constitution of matter and the transmutation of the elements. Radium and other radioactive substances, such as uranium, spontaneouslyemit negatively charged particles of extremely small mass (electrons), and also positively charged particles of much greater mass, knownas alpha particles. Rutherford and Geiger actually succeeded incounting the number of alpha particles emitted per second by aknown mass of radium, and showed that these were charged heliumatoms. 

To discuss more at length the extraordinary characteristics ofhelium, which plays so large a part in celestial affairs, wouldtake us too far afield. Let us therefore pass to another case inwhich a fundamental discovery, this time in physics, was firstforeshadowed by astronomical observation. 

SUN-SPOTS AS MAGNETS

No archæologist, whether Young or Champollion deciphering the RosettaStone, or Rawlinson copying the cuneiform inscription on the cliffof Behistun, was ever faced by a more fascinating problem than thatwhich confronts the solar physicist engaged in the interpretationof the hieroglyphic lines of sun-spot spectra. The colossal whirlingstorms that constitute sun-spots, so vast that the earth would makebut a moment's scant mouthful for them, differ materially fromthe 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 doublelines which he naturally attributed, in harmony with the physicalknowledge of the time, to the effect of "reversal" by superposedlayers of vapors of different density and temperature. What heactually saw, however, as was proved at the Mount Wilson Observatoryin 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 MountWilson Observatory. 

An image of the sun about 16 inches in diameter is formed in thelaboratory at the base of the tower. Below this, in a well extending80 feet into the earth, is the powerful spectroscope with whichthe magnetic fields in sun-spots and the general magnetic fieldof 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 thosedays (1845), he placed a block of very dense glass. The plane ofpolarization of a beam of light, which passed unaffected throughthe glass before the switch was closed, was seen to rotate when themagnetic field was produced by the flow of the current. A similarrotation is now familiar in the well-known tests of sugars--lævuloseand dextrose--which rotate plane-polarized light to left and right, respectively. 

But in this first discovery of a relationship between light andmagnetism Faraday had not taken the more important step that hecoveted--to determine whether the vibration period of a light-emittingparticle is subject to change in a magnetic field. He attemptedthis in 1862--the last experiment of his life. A sodium flame wasplaced between the poles of a magnet, and the yellow lines werewatched in a spectroscope when the magnet was excited. No changecould be detected, and none was found by subsequent investigatorsuntil Zeeman, of Leiden, with more powerful instruments made hisfamous discovery, the twenty-fifth anniversary of which has recentlybeen celebrated. 

[Illustration: Fig. 32. Pasadena Laboratory of the Mount WilsonObservatory. 

Showing the large magnet (on the left) and the spectroscopes usedfor the study of the effect of magnetism on radiation. A single linein the spectrum is split by the magnetic field into from three totwenty-one components, as illustrated in Fig. 34. The correspondinglines in the spectra of sun-spots are split up in precisely thesame way, thus indicating the presence of powerful magnetic fieldsin the sun. ]

His method of procedure was similar to Faraday's, but his magnet andspectroscope were much more powerful, and a theory due to Lorentz, predicting the nature of the change to be expected, was availableas a check on his results. When the current was applied the lineswere seen to widen. In a still more powerful magnetic field eachof them split into two components (when the observation was madealong the lines of force), and the light of the components of eachline was found to be circularly polarized in opposite directions. Strictly in harmony with Lorentz's theory, this splitting andpolarization proved the presence in the luminous vapor of exactly suchnegatively charged electrons as had been indicated there previouslyby very different experimental methods. 

In 1908 great cyclonic storms, or vortices, were discovered atthe Mount Wilson Observatory centring in sun-spots. Such whirlingmasses of hot vapors, inferred from Sir Joseph Thomson's resultsto contain electrically charged particles, should give rise to amagnetic field. This hypothesis at once suggested that the doublelines observed by Young might really represent the Zeeman effect. The test was made, and all the characteristic phenomena of radiationin a magnetic field were found. 

Thus a great physical experiment is constantly being performedfor us in the sun. Every large sunspot contains a magnetic fieldcovering many thousands of square miles, within which the spectrumlines of iron, manganese, chromium, titanium, vanadium, calcium, and other metallic vapors are so powerfully affected that theirwidening and splitting can be seen with telescopes and spectroscopesof moderate size. 

THE TOWER TELESCOPE

Both of these illustrations show how the physicist and chemist, when adequately armed for astronomical attack, can take advantagein their studies of the stupendous processes visible in cosmiccrucibles, heated to high temperatures and influenced, as in thecase of sun-spots, by intense magnetic fields. Certain moderninstruments, like the 60-foot and 150-foot tower telescopes onMount Wilson, are especially designed for observing the courseof these experiments. The second of these telescopes produces ata fixed point in a laboratory an image of the sun about 16 inchesin diameter, thus enlarging the sun-spots to such a scale thatthe magnetic phenomena of their various parts can be separatelystudied. This analysis is accomplished with a spectroscope 80 feetin length, mounted in a subterranean chamber beneath the tower. Thevaried 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 reasonablyinfer 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 "DeMagnete. " Here lies one of the best clues for the physicist whoseeks the cause of magnetism, and attempts to produce it, as Barnetthas recently succeeded in doing, by rapidly whirling masses ofmetal in the laboratory. 

[Illustration: Fig. 33. Sun-spot vortex in the upper hydrogenatmosphere. (Benioff). 

Photographed with the spectroheliograph. The electric vortex thatcauses the magnetic field of the spot lies at a lower level, andis 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 gravitationalpower. Indeed, its attraction cannot be felt by the most delicateinstruments at the distance of the earth, and would still be unknownwere it not for the influence of magnetism on light. 

Auroras, magnetic storms, and such electric currents as those thatrecently deranged several Atlantic cables are due, not to the magnetismof the sun or its spots, but probably to streams of electrons, shotout from highly disturbed areas of the solar surface surroundinggreat sun-spots, traversing ninety-three million miles of the etherof space, and penetrating deep into the earth's atmosphere. Thesestriking phenomena lead us into another chapter of physics, whichlimitations of space forbid us to pursue. 

STELLAR CHEMISTRY

Let us turn again to chemistry, and see where experiments performedin cosmic laboratories can serve as a guide to the investigator. A spinning solar tornado, incomparably greater in scale than thedevastating whirlwinds that so often cut narrow paths of destructionthrough town and country in the Middle West, gradually gives riseto a sun-spot. The expansion produced by the centrifugal force atthe centre of the storm cools the intensely hot gases of the solaratmosphere to a point where chemical union can occur. Titaniumand oxygen, too hot to combine in most regions of the sun, jointo form the vapor of titanium oxide, characterized in the sunspotspectrum by fluted bands, made up of hundreds of regularly spacedlines. Similarly magnesium and hydrogen combine as magnesium hydrideand calcium and hydrogen form calcium hydride. None of these compounds, stable at the high temperatures of sun-spots, has been much studiedin the laboratory. The regions in which they exist, though coolerthan the general atmosphere of the sun, are at temperatures ofseveral thousand degrees, attained in our laboratories only withthe aid of such devices as powerful electric furnaces. 

[Illustration: Fig. 34. Splitting of spectrum lines by a magneticfield (Babcock). 

The upper and lower strips show lines in the spectrum of chromium, observed without a magnetic field. When subjected to the influenceof magnetism, these single lines are split into several components. Thus the first line on the right is resolved by the field intothree components, one of which (plane polarized) appears in thesecond strip, while the other two, which are polarized in a planeat right angles to that of the middle component, are shown on thethird strip. The next line is split by the magnetic field intotwelve components, four of which appear in the second strip andeight in the third. The magnetic fields in sun-spots affect theselines 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 theirlife-cycle. [*] A sun-spot is a solar tornado, wherein the intenselyhot solar vapors are cooled by expansion, giving rise to the compoundsalready named. A red star, in Russell's scheme of stellar evolution, is a cooler sun, vast in volume and far more tenuous than atmosphericair when in the initial period of the "giant" stage, but compressedand denser than water in the "dwarf" stage, into which our sun hasalready entered as it gradually approaches the last phases of itsexistence. Therefore we should find, throughout the entire atmosphereof such stars, some of the same compounds that are produced withinthe comparatively small limits of a sun-spot. This, of course, on the correct assumption that sun and stars are made of the samesubstances. Fowler has already identified the bands of titaniumoxide in such red stars as the giant Betelgeuse, and in othersof its class. It is safe to predict that an interesting chapterin the chemistry of the future will be based upon the study ofsuch compounds, both in the laboratory and under the progressivetemperature 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 laboratoryof the Mount Wilson Observatory. 

With which the chemical phenomena observed in sun-spots and redstars are experimentally imitated. ]

ASTROPHYSICAL LABORATORIES

It is precisely in this long sequence of physical and chemicalchanges that the astrophysicist and the astrochemist can find themeans of pushing home their attack. It is true, of course, thatthe laboratory investigator has a great advantage in his abilityto control his experiments, and to vary their progress at will. But by judicious use of the transcendental temperatures, far outranging those of his furnaces, and extreme conditions, which hecan only partially imitate, afforded by the sun, stars, and nebulæ, he may greatly widen the range of his inquiries. The sequence ofphenomena seen during the growth of a sun-spot, or the observationof spots of different sizes, and the long series of successivesteps that mark the rise and decay of stellar life, resemble thechanges that the experimenter brings about as he increases anddiminishes the current in the coils of his magnet or raises andlowers the temperature of his electric furnace, examining fromtime to time the spectrum of the glowing vapors, and noting thechanges 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 electricarc, where its combination with oxygen gives rise to the bands oftitanium oxide (Fowler). The lower strip shows the spectrum ofthe 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 spectrumof titanium oxide (King). The fluted bands of the oxide spectrumare easily identified in the spot, where they indicate that titaniumand oxygen, too hot to combine in the solar atmosphere, unite in thespot because of the cooling produced by expansion in the vortex. ]

Astronomical observations of this character, it should be noted, aremost effective when constantly tested and interpreted by laboratoryexperiment. Indeed, a modern astrophysical observatory should beequipped like a great physical laboratory, provided on the one handwith telescopes and accessory apparatus of the greatest attainablepower, and on the other with every device known to the investigatorof radiation and the related physical and chemical phenomena. Itstelescopes, especially designed with the aims of the physicist andchemist in view, bring images of sun, stars, nebulæ, and otherheavenly bodies within the reach of powerful spectroscopes, sensitivebolometers and thermopiles, and the long array of other appliancesavailable for the measurement and analysis of radiation. Its electricfurnaces, arcs, sparks, and vacuum tubes, its apparatus for increasingand decreasing pressure, varying chemical conditions, and subjectingluminous gases and vapors to the influence of electric and magneticfields, provide the means of imitating celestial phenomena, and ofrepeating and interpreting the experiments observed at the telescope. And the advantage thus derived, as we have seen, is not confinedto the astronomer, who has often been able, by making fundamentalphysical and chemical discoveries, to repay his debt to the physicistand chemist for the apparatus and methods which he owes to them. 

NEWTON AND EINSTEIN

Take, for another example, the greatest law of physics--Newton'slaw of gravitation. Huge balls of lead, as used by Cavendish, produceby their gravitational effect a minute rotation of a delicatelysuspended bar, carrying smaller balls at its extremities. But nosuch feeble means sufficed for Newton's purpose. To prove the lawof gravitation he had recourse to the tremendous pull on the moonof the entire mass of the earth, and then extended his researchesto the mutual attractions of all the bodies of the solar system. Later Herschel applied this law to the suns which constitute doublestars, and to-day Adams observes from Mount Wilson stars fallingwith great velocity toward the centre of the galactic system underthe combined pull of the millions of objects that compose it. Thusfull advantage has been taken of the possibility of utilizing thegreat masses of the heavenly bodies for the discovery and applicationof a law of physics and its reciprocal use in explaining celestialmotions. 

[Illustration: Fig. 38. The Cavendish experiment. 

Two lead balls, each two inches in diameter, are attached to theends of a torsion rod six feet long, which is suspended by a finewire. The experiment consists in measuring the rotation of thesuspended system, caused by the gravitational attraction of twolead spheres, each twelve inches in diameter, acting on the twosmall lead balls. ]

Or consider the Einstein theory of relativity, the truth or falsityof which is no less fundamental to physics. Its inception sprang fromthe Michelson-Morley experiment, made in a laboratory in Cleveland, which showed that motion of the earth through the ether of space couldnot be detected. All of the three chief tests of Einstein's generaltheory are astronomical--because of the great masses required toproduce the minute effects predicted: the motion of the perihelionof Mercury, the deflection of the light of a star by the attractionof the sun, and the shift of the lines of the solar spectrum towardthe red--questions not yet completely answered. 

But it is in the study of the constitution of matter and the evolutionof the elements, the deepest and most critical problem of physicsand chemistry, that the extremes of pressure and temperature in theheavenly bodies, and the prevalence of other physical conditions notyet successfully imitated on earth, promise the greatest progress. It fortunately happens that astrophysical research is now at thevery apex of its development, founded as it is upon many centuriesof astronomical investigation, rejuvenated by the introductioninto the observatory of all the modern devices of the physicist, and strengthened with instruments of truly extraordinary rangeand power. These instruments bring within reach experiments thatare in progress on some minute region of the sun's disk, or insome star too distant even to be glimpsed with ordinary telescopes. Indeed, the huge astronomical lenses and mirrors now availableserve for these remote light-sources exactly the purpose of thelens or mirror employed by the physicist to project upon the slitof his spectroscope the image of a spark or arc or vacuum tubewithin which atoms and molecules are exposed to the influence ofthe electric discharge. The physicist has the advantage of completecontrol over the experimental conditions, while the astrophysicistmust observe and interpret the experiments performed for him inremote laboratories. In actual practice, the two classes of workmust be done in the closest conjunction, if adequate utilizationis to be made of either. And this is only natural, for the trendof recent research has made clear the fact that one of the threegreatest problems of modern astronomy and astrophysics, rankingwith the structure of the universe and the evolution of celestialbodies, 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 beenactually accomplished within the last few years, but the processis invariably one of disintegration--the more complex elementsbeing broken up into simpler constituents. Much remains to be donein this same direction; and here the stars and nebulæ, which showthe 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 unknowngas nebulium in the gaseous nebulæ, to the long list of familiarsubstances, including several chemical compounds, in the red stars, may prove to be fundamentally significant when adequately studiedfrom the standpoint of the investigator of atomic structure. Theexisting evidence seems to favor the view, recently expressed bySaha, that many of these differences are due to varying degreesof ionization, the outer electrons of the atoms being split offby high temperature or electrical excitation. It is even possiblethat cosmic crucibles, unrivalled by terrestrial ones, may helpmaterially to reveal the secret of the formation of complex elementsfrom simpler ones. Physicists now believe that all of the elements arecompounded of hydrogen atoms, bound together by negative electrons. Thus helium is made up of four hydrogen atoms, yet the atomic weightof helium (4) is less than four times that of hydrogen (1. 008). The difference may represent the mass of the electrical energyreleased when the transmutation occurred. 

[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey). 

The gas "nebulium, " not yet found on the earth, is the mostcharacteristic constituent of irregular nebulæ. Nebulium is recognizedby two green lines in its spectrum, which cause the green color ofnebulæ of the gaseous type. ]

Eddington has speculated in a most interesting way on this possiblesource of stellar heat in his recent presidential address before theBritish 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 supposedto reside in the slow condensation of a radiating mass of gas underthe action of gravity, is wholly inadequate to explain the observedphenomena. If the old view were correct, the earlier history ofa star, from the giant stage of a cool and diaphanous gas to theperiod of highest temperature, would be run through within eightythousand years, whereas we have the best of evidence that manythousands of centuries would not suffice. Some other source of energyis imperatively needed. If 5 per cent of a star's mass consistsoriginally of hydrogen atoms, which gradually combine in the slowprocess of time to form more complex elements, the total heat thusliberated would more than suffice to account for all demands, andit would be unnecessary to assume the existence of any other sourceof 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, fromthe spiral and irregular nebulæ through all the types of stars. Doctor van Maanen's measures of the Mount Wilson photographs indicateoutward motion along the arms of spiral nebulæ, while the spectroscopeshows them to be whirling at enormous velocities. ]

COSMIC PRESSURES

This, it may fairly be said, is very speculative, but the factremains that celestial bodies appear to be the only places in whichthe complex elements may be in actual process of formation from theirknown source--hydrogen. At least we may see what a vast varietyof physical conditions these cosmic crucibles afford. At one end ofthe scale we have the excessively tenuous nebulæ, the luminosity ofwhich, mysterious in its origin, resembles the electric glow in ourvacuum tubes. Here we can detect only the lightest and simplest ofthe elements. In the giant stars, also extremely tenuous (the densityof Betelgeuse can hardly exceed one-thousandth of an atmosphere) weobserve 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, musttherefore be at a temperature of only a few thousand degrees, butvastly higher temperatures must prevail at their centres. In passingup the temperature curve more and more elements appear, the surfacetemperature rises, and the internal temperature may reach millionsof degrees. At the same time the pressure within must also rise, reaching enormous figures in the last stages of stellar life. Cookhas calculated that the pressure at the centre of the earth isbetween 4, 000 and 10, 000 tons per square inch, and this must beonly a very small fraction of that attained within larger celestialbodies. Jeans has computed the pressure at the centre of two collidingstars as they strike and flatten, and finds it may be of the orderof 1, 000, 000, 000 tons per square inch--sufficient, if their diameterbe equal to that of the sun--to vaporize them 100, 000 times over. 

Compare these pressures with the highest that can be produced onearth. If the German gun that bombarded Paris were loaded with asolid steel projectile of suitable dimensions, a muzzle velocityof 6, 000 feet per second could be reached. Suppose this to be firedinto a tapered hole in a great block of steel. The instantaneouspressure, according to Cook, would be about 7, 000 tons per squareinch, only 1/150000 of that possible through the collision of thelargest stars. 

[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson. 

Michelson is measuring the velocity of light between stations onMount Wilson and Mount San Antonio. Astronomical observations affordthe best means, however, of detecting any possible difference betweenthe velocities of light of different colors. From studies of variablestars in the cluster Messier 5 Shapley concludes that if there isany difference between the velocities of blue and yellow lightin free space it cannot exceed two inches in one second, the timein which light travels 186, 000 miles. ]

Finally, we may compare the effects of light pressure on the earthand stars. Twenty years ago Nichols and Hull succeeded, with theaid of the most sensitive apparatus, in measuring the minutedisplacements produced by the pressure of light. The effect isso slight, even with the brightest light-sources available, thatgreat experimental skill is required to measure it. Yet in thecase of some of the larger stars Eddington calculates that one-halfof their mass is supported by radiation pressure, and this againsttheir enormous gravitational attraction. In fact, if their masswere as great as ten times that of the sun, the radiation pressurewould so nearly overcome the pull of gravitation that they wouldbe likely to break up. 

But enough has been said to illustrate the wide variety of experimentaldevices that stand at our service in the laboratories of the heavens. Here the physicist and chemist of the future will more and morefrequently supplement their terrestrial apparatus, and find newclues to the complex problems which the amazing progress of recentyears 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 valueof researches directed toward the improvement of the incandescentlamp or the increased efficiency of the telephone. He can see theresults in the greatly decreased cost of electric illuminationand the rapid extension of the range of the human voice. But thevery men who have made these advances, those who have succeededbeyond all expectation in accomplishing the economic purposes inview, are most emphatic in their insistence upon the importanceof research of a more fundamental character. Thus Vice-PresidentJ. J. Carty, of the American Telephone and Telegraph Company, whodirects its great Department of Development and Research, and DoctorW. J. Whitney, Director of the Research Laboratory of the GeneralElectric Company, have repeatedly expressed their indebtednessto the investigations of the physicist, made with no thought ofimmediate 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 greatadvances are thus derived from fundamental science, and the futureprogress of the world will be largely dependent upon the provisionmade for scientific research, especially in the fields of physicsand chemistry, which underlie all branches of engineering. 

The constitution of matter, therefore, instead of appealing asa subject to research only to the natural philosopher or to thegeneral student of science, is a question of the greatest practicalconcern. Already the by-products of investigations directed towardits 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 betterknown. Wireless telephony and transcontinental telephony with wireswere both rendered possible by studies of the nature of the electricdischarge in vacuum tubes. Thus the "practical man, " with his distrustof "pure" science, need not resent investments made for the purposeof advancing our knowledge of such fundamental subjects as physicsand chemistry. On the contrary, if true to his name, he shouldhelp to multiply them many fold in the interest of economic andcommercial development.