The New Heavens.
by George Ellery Hale.
PREFACE
Fourteen years ago, in a book entitled "The Study of Stellar Evolution"
(University of Chicago Press, 1908), I attempted to give in untechnical language an account of some modern methods of astrophysical research.
This book is now out of print, and the rapid progress of science has left it completely out of date. As I have found no opportunity to prepare a new edition, or to write another book of similar purpose, I have adopted the simpler expedient of contributing occasional articles on recent developments to _Scribner's Magazine_, three of which are included in the present volume.
I am chiefly indebted, for the illustrations, to the Mount Wilson Observatory and the present and former members of its staff whose names appear in the captions. Special thanks are due to Mr. Ferdinand Ellerman, who made all of the photographs of the observatory buildings and instruments, and prepared all material for reproduction. The cut of the original Cavendish apparatus is copied from the _Philosophical Transactions for 1798_ with the kind permission of the Royal Society, and I am also indebted to the Royal Society and to Professor Fowler and Father Cortie for the privilege of reproducing from the _Proceedings_ two illustrations of their spectroscopic results.
G. E. H.
January, 1922.
CHAPTER I
THE NEW HEAVENS
Go out under the open sky, on a clear and moon-less night, and try to count the stars. If your station lies well beyond the glare of cities, which is often strong enough to conceal all but the brighter objects, you will find the task a difficult one. Ranging through the six magnitudes of the Greek astronomers, from the brilliant Sirius to the faintest perceptible points of light, the stars are scattered in great profusion over the celestial vault. Their number seems limitless, yet actual count will show that the eye has been deceived. In a survey of the entire heavens, from pole to pole, it would not be possible to detect more than from six to seven thousand stars with the naked eye. From a single viewpoint, even with the keenest vision, only two or three thousand can be seen.
So many of these are at the limit of visibility that Ptolemy's "Almagest," a catalogue of all the stars whose places were measured with the simple instruments of the Greek astronomers, contains only 1,022 stars.
Back of Ptolemy, through the speculations of the Greek philosophers, the mysteries of the Egyptian sun-god, and the observations of the ancient Chaldeans, the rich and varied traditions of astronomy stretch far away into a shadowy past. All peoples, in the first stirrings of their intellectual youth, drawn by the nightly splendor of the skies and the ceaseless motions of the planets, have set up some system of the heavens, in which the sense of wonder and the desire for knowledge were no less concerned than the practical necessities of life. The measurement of time and the needs of navigation have always stimulated astronomical research, but the intellectual demand has been keen from the first. Hipparchus and the Greek astronomers of the Alexandrian school, shaking off the vagaries of magic and divination, placed astronomy on a scientific basis, though the reaction of the Middle Ages caused even such a great astronomer as Tycho Brahe himself to revert for a time to the practice of astrology.
EARLY INSTRUMENTS
The transparent sky of Egypt, rarely obscured by clouds, greatly favored Ptolemy's observations. Here was prepared his great star catalogue, based upon the earlier observations of Hipparchus, and destined to remain alone in its field for more than twelve centuries, until Ulugh Bey, Prince of Samarcand, repeated the work of his Greek predecessor. Throughout this period the stars were looked upon mainly as points of reference for the observation of planetary motions, and the instruments of observation underwent little change.
The astrolabe, which consists of a circle divided into degrees, with a rotating diametral arm for sighting purposes, embodies their essential principle. In its simple form, the astrolabe was suspended in a vertical plane, and the stars were observed by bringing the sights on the movable diameter to bear upon them. Their altitude was then read off on the circle. Ultimately, the circle of the astrolabe, mounted with one of its diameters parallel to the earth's axis, became the armillary sphere, the precursor of our modern equatorial telescope. Great stone quadrants fixed in the meridian were also employed from very early times. Out of such furnishings, little modified by the lapse of centuries, was provided the elaborate instrumental equipment of Uranibourg, the great observatory built by Tycho Brahe on the Danish island of Huen in 1576. In this "City of the Heavens," still dependent solely upon the unaided eye as a collector of starlight, Tycho made those invaluable observations that enabled Kepler to deduce the true laws of planetary motion. But after all these centuries the sidereal world embraced no objects, barring an occasional comet or temporary star, that lay beyond the vision of the earliest astronomers. The conceptions of the stellar universe, except those that ignored the solid ground of observation, were limited by the small aperture of the human eye.
But the dawn of another age was at hand.
[Illustration: Fig. 2. The Great Nebula in Orion (Pease).
Photographed with the 100-inch telescope. This short-exposure photograph shows only the bright central part of the nebula. A longer exposure reveals a vast outlying region.]
The dominance of the sun as the central body of the solar system, recognized by Aristarchus of Samos nearly three centuries before the Christian era, but subsequently denied under the authority of Ptolemy and the teachings of the Church, was reaffirmed by the Polish monk Copernicus in 1543. Kepler's laws of the motions of the planets, showing them to revolve in ellipses instead of circles, removed the last defect of the Copernican system, and left no room for its rejection. But both the world and the Church clung to tradition, and some visible demonstration was urgently needed. This was supplied by Galileo through his invention of the telescope.
[Illustration: Fig. 3. Model by Ellerman of summit of Mount Wilson, showing the observatory buildings among the trees and bushes.
The 60-foot tower on the extreme left, which is at the edge of a precipitous canon 1,500 feet deep, is the vertical telescope of the Smithsonian Astrophysical Observatory. Above it are the "Monastery" and other buildings used as quarters by the astronomers of the Mount Wilson Observatory while at work on the mountain. (The offices, computing-rooms, laboratories, and shops are in Pasadena.) Following the ridge, we come successively to the dome of the 10-inch photographic telescope, the power-house, laboratory, Snow horizontal telescope, 60-foot-tower telescope, and 150-foot-tower telescope, these last three used for the study of the sun. The dome of the 60-inch reflecting telescope is just below the 150-foot tower, while that of the 100-inch telescope is farther to the right. The altitude of Mount Wilson is about 5,900 feet.]
The crystalline lens of the human eye, limited by the iris to a maximum opening about one-quarter of an inch in diameter, was the only collector of starlight available to the Greek and Arabian astronomers. Galileo's telescope, which in 1610 suddenly pushed out the boundaries of the known stellar universe and brought many thousands of stars into range, had a lens about 2-1/4 inches in diameter. The area of this lens, proportional to the square of its diameter, was about eighty-one times that of the pupil of the eye. This great increase in the amount of light collected should bring to view stars down to magnitude 10.5, of which nearly half a million are known to exist.
It is not too much to say that Galileo's telescope revolutionized human thought. Turned to the moon, it revealed mountains, plains, and valleys, while the sun, previously supposed immaculate in its perfection, was seen to be blemished with dark spots changing from day to day. Jupiter, shown to be accompanied by four encircling satellites, afforded a picture in miniature of the solar system, and strongly supported the Copernican view of its organization, which was conclusively demonstrated by Galileo's discovery of the changing phases of Venus and the variation of its apparent diameter during its revolution about the sun. Galileo's proof of the Copernican theory marked the downfall of mediaevalism and established astronomy on a firm foundation. But while his telescope multiplied a hundredfold the number of visible stars, more than a century elapsed before the true possibilities of sidereal astronomy were perceived.
[Illustration: Fig. 4. The 100-inch Hooker telescope.]
STRUCTURE OF THE UNIVERSE
Sir William Herschel was the first astronomer to make a serious attack upon the problem of the structure of the stellar universe.
In his first memoir on the "Construction of the Heavens," read before the Royal Society in 1784, he wrote as follows:
"Hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere in the centre of which the eye of an observer might be supposed to be placed.... In future we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed as well as consisting of very different materials."
On turning his 18-inch reflecting telescope to a part of the Milky Way in Orion, he found its whitish appearance to be completely resolved into small stars, not separately seen with his former telescopes. "The glorious multitude of stars of all possible sizes that presented themselves here to my view are truly astonishing; but as the dazzling brightness of glittering stars may easily mislead us so far as to estimate their number greater than it really is, I endeavored to ascertain this point by counting many fields, and computing from a mean of them, what a certain given portion of the Milky Way might contain." By this means, applied not only to the Milky Way but to all parts of the heavens, Herschel determined the approximate number and distribution of all the stars within reach of his instrument.
By comparing many hundred gauges or counts of stars visible in a field of about one-quarter of the area of the moon, Herschel found that the average number of stars increased toward the great circle which most nearly conforms with the course of the Milky Way.
Ninety degrees from this plane, at the pole of the Milky Way, only four stars, on the average, were seen in the field of the telescope.
In approaching the Milky Way this number increased slowly at first, and then more and more rapidly, until it rose to an average of 122 stars per field.
[Illustration: Fig. 5. Erecting the polar axis of the 100-inch telescope.]
These observations were made in the northern hemisphere, and subsequently Sir John Herschel, using his father's telescope at the Cape of Good Hope, found an almost exactly similar increase of apparent star density for the southern hemisphere. According to his estimates, the total number of stars in both hemispheres that could be seen distinctly enough to be counted in this telescope would probably be about five and one-half millions.
The Herschels concluded that "the stars of our firmament, instead of being scattered in all directions indifferently through space, form a stratum of which the thickness is small, in comparison with its length and breadth; and in which the earth occupies a place somewhere about the middle of its thickness, between the point where it subdivides into two principal laminae inclined at a small angle to each other." This view does not differ essentially from our modern conception of the form of the Galaxy; but as the Herschels were unable to see stars fainter than the fifteenth magnitude, it is evident that their conclusions apply only to a restricted region surrounding the solar system, in the midst of the enormously extended sidereal universe which modern instruments have brought within our range.
MODERN METHODS
The remarkable progress of modern astronomy is mainly due to two great instrumental advances: the rise and development of the photographic telescope, and the application of the spectroscope to the study of celestial objects. These new and powerful instruments, supplemented by many accessories which have completely revolutionized observatory equipment, have not only revealed a vastly greater number of stars and nebulae: they have also rendered feasible observations of a type formerly regarded as impossible. The chemical analysis of a faint star is now so easy that it can be accomplished in a very short time--as quickly, in fact, as an equally complex substance can be analyzed in the laboratory. The spectroscope also measures a star's velocity, the pressure at different levels in its atmosphere, its approximate temperature, and now, by a new and ingenious method, its distance from the earth. It determines the velocity of rotation of the sun and of nebulae, the existence and periods of orbital revolution of binary stars too close to be separated by any telescope, the presence of magnetic fields in sunspots, and the fact that the entire sun, like the earth, is a magnet.
[Illustration: Fig. 6. Lowest section of tube of 100-inch telescope, ready to leave Pasadena for Mount Wilson.]
Such new possibilities, with many others resulting from the application of physical methods of the most diverse character, have greatly enlarged the astronomer's outlook. He may now attack two great problems: (1) The structure of the universe and the motions of its constituent bodies, and (2) the evolution of the stars: their nature, origin, growth, and decline. These two problems are intimately related and must be studied as one.[*]
[Footnote *: A third great problem open to the astronomer, the study of the constitution of matter, is described in Chapter III.]
If space permitted, it would be interesting to survey the progress already accomplished by modern methods of astronomical research.
Hundreds of millions of stars have been photographed, and the boundaries of the stellar universe have been pushed far into space, but have not been attained. Globular star clusters, containing tens of thousands of stars, are on so great a scale (according to Shapley) that light, travelling at the rate of 186,000 miles per second, may take 500 years to cross one of them, while the most distant of these objects may be more than 200,000 light-years from the earth. The spiral nebulae, more than a million in number, are vast whirling masses in process of development, but we are not yet certain whether they should be regarded as "island universes" or as subordinate to the stellar system which includes our minute group of sun and planets, the great star clouds of the Milky Way, and the distant globular star clusters.
[Illustration: Fig. 7. Section of a steel girder for dome covering the 100-inch telescope, on its way up Mount Wilson.]
These few particulars may give a slight conception of the scale of the known universe, but a word must be added regarding some of its most striking phenomena. The great majority of the stars whose motions have been determined belong to one or the other of two great star streams, but the part played by these streams in the sidereal system as a whole is still obscure. The stars have been grouped in classes, presumably in the order of their evolutional development, as they pass from the early state of gaseous masses, of low density, through the successive stages resulting from loss of heat by radiation and increased density due to shrinkage. Strangely enough, their velocities in space show a corresponding change, increasing as they grow older or perhaps depending upon their mass.
It is impossible within these limits to do more than to give some indication of the scope of the new astronomy. Enough has been said, however, to assist in appreciating the increased opportunity for investigation, and the nature of the heavy demands made upon the modern observatory. But before passing on to describe one of the latest additions to the astronomer's instrumental equipment, a word should be added regarding the chief classes of telescopes.
REFRACTORS AND REFLECTORS
Astronomical telescopes are of two types: refractors and reflectors.
A refracting telescope consists of an object-glass composed of two or more lenses, mounted at the upper end of a tube, which is pointed at the celestial object. The light, after passing through the lenses, is brought to a focus at the lower end of the tube, where the image is examined visually with an eyepiece, or photographed upon a sensitive plate. The largest instruments of this type are the 36-inch Lick telescope and the 40-inch refractor of the Yerkes Observatory.
[Illustration: Fig. 8. Erecting the steel building and revolving dome that cover the Hooker telescope.]
Reflecting telescopes, which are particularly adapted for photographic work, though also excellent for visual observations, are very differently constructed. No lens is used. The telescope tube is usually built in skeleton form, open at its upper end, and with a large concave mirror supported at its base. This mirror serves in place of a lens. Its upper surface is paraboloidal in shape, as a spherical surface will not unite in a sharp focus the rays coming from a distant object. The light passes through no glass--a great advantage, especially for photography, as the absorption in lenses cuts out much of the blue and violet light, to which photographic plates are most sensitive. The reflection occurs on the _upper_ surface of the mirror, which is covered with a coat of pure silver, renewed several times a year and always kept highly burnished. Silvered glass is better than metals or other substances for telescope mirrors, chiefly because of the perfection with which glass can be ground and polished, and the ease of renewing its silvered surface when tarnished.
The great reflectors of Herschel and Lord Rosse, which were provided with mirrors of speculum metal, were far inferior to much smaller telescopes of the present day. With these instruments the star images were watched as they were carried through the field of view by the earth's rotation, or kept roughly in place by moving the telescope with ropes or chains. Photographic plates, which reveal invisible stars and nebulae when exposed for hours in modern instruments, were not then available. In any case they could not have been used, in the absence of the perfect mechanism required to keep the star images accurately fixed in place upon the sensitive film.
[Illustration: Fig. 9. Building and revolving dome, 100 feet in diameter, covering the 100-inch Hooker telescope.