Popular Science Monthly/Volume 60/February 1902/Stellar Evolution in the Light of Recent Research
GREAT NEBULA IN ANDROMEDA.
Photographed with the two-foot reflecting Telescope of the Yerkes Observatory (Ritchey).
THE
POPULAR SCIENCE
MONTHLY
FEBRUARY, 1902.
| STELLAR EVOLUTION IN THE LIGHT OF RECENT RESEARCH.[1] |
By Professor GEORGE E. HALE,
DIRECTOR OF THE YERKES OBSERVATORY, UNIVERSITY OF CHICAGO.
MANY attempts have been made to sum up the work of the nineteenth century, and to define its principal lines of progress. In estimates of the relative importance of the books published during this period there has been some divergence of view, but regarding one of them no element of doubt seems to have entered the minds of the critics. By unanimous consent Darwin's 'Origin of Species' is accorded a commanding position among the works which have influenced the intellectual life of the century. It would be difficult to overestimate the effect which the doctrine of evolution has wrought. The principle of orderly and harmonious development which it embodies has found application, not only in explaining the wide diversity of organic species, but in unifying the events of history, in elucidating the origin of language, and in throwing light on difficult questions in every department of human knowledge. The idea of evolution may indeed be traced back through the writings of many centuries. The early philosophers, though not possessed of the immense collection of recorded phenomena by which modern men of science may test their theories, were constantly occupied with great problems demanding the widest generalization. In attempting to account for the earth and its inhabitants they made the first steps in the direction which Darwin subsequently pursued.
It would be interesting to recall the strange traditions in which primitive peoples have recorded their vague imaginings of the origin of things. But the absence of even an attempt at careful reasoning renders such tales of no value for our present purpose. The Greek philosophers were not oblivious to the value of observation as a check on speculation regarding the solar system, but the instruments then available were too crude to give accurate positions of the heavenly bodies. Even Copernicus, though he established the sun at the center of our system, and thus paved the way for the nebular hypothesis, retained the epicycles of the Greeks. Kepler, basing his investigations upon the observations of Tycho Brahe, proved that the planets move in ellipses with the sun at the focus, and removed all vestige of doubt as to the general plan of the solar system. The harmony which characterizes the motions of the planets and a knowledge of the effect of gravitation led Kant to formulate an explanation of the origin of the solar system, which subsequently found more perfect expression in the nebular hypothesis of Laplace.
In this hypothesis Laplace seeks to account for the formation of the sun and planets through the contraction of a vast nebulous cloud, which once filled the entire solar system, extending to the orbit of Neptune. This mass, which he considered to be fiery hot, was supposed to be in rotation. As it cooled, through radiation into space, it contracted toward the center. The result of this contraction was to increase the velocity of rotation, and when through increasing velocity the centrifugal force at the periphery counterbalanced the attraction of the central mass, a ring was thrown off. Further contraction resulted in the formation of other rings, in each of which the matter collected about its densest part, and thus produced a planet. Before they had time to cool these planets in turn threw off rings, which, with the single exception of Saturn's ring system, condensed into satellites.
This celebrated hypothesis, though unsupported by mathematical proof, has occupied a dominant position since the time of its publication more than a century ago. It has been subjected to much criticism, but most of the objections raised by Faye and others have been met by modifications of the hypothesis. Of late it has encountered fresh attacks on the part of Chamberlin and Moulton, and it now seems doubtful whether it will be possible to overcome their criticisms, which are based on dynamical considerations. It may prove to be sufficient, however, to forsake the lenticular mass of vapor predicated by Laplace in favor of the spiral form which Keeler has shown to characterize so many nebulæ.
The nebular hypothesis seeks to account for a system like our own, wherein a central sun is surrounded by planets and satellites, originally self-luminous, but ultimately cooled to the point where they are luminous only through reflected light. The stars are so distant from us that any planets which may attend them are beyond the reach of the most powerful telescopes. In some of the planetary and spiral nebulæ, such as the Great Nebula in Andromeda (Fig. 1), we perhaps
Star-trails photographed with two and one-half inch Portrait Lens (Ritchey).
observe the earlier stages of the process of condensation, but no distinct evidence of progressive change has yet been gathered from telescopic observation. In seeking for evidence of stellar evolution, on a plan comprehensive enough to include a place for every star in the heavens. we may begin with visual and photographic observations with the telescope. Such remarkable photographs as that of the Andromeda nebula seem to bring us into the very presence of a greater system, more nearly comparable in size with the Milky Way than with the solar system, in the actual process of formation. But on account of the long periods of time, which must elapse before changes in this distant mass may become sufficiently great to be appreciable, and for many other reasons, we could not hope to base a complete scheme of stellar evolution on such photographs alone. Our observational methods must also include the means of solving physical, chemical and gravitational problems as they present themselves, not close at hand in the laboratory, but at inconceivably distant regions of space. For this reason it would have been impossible prior to the invention of the spectroscope to arrange the stars according to any clearly defined system of development. The principal advances which have been made in the study of stellar evolution are therefore confined to the period which has elapsed since the middle of the nineteenth century.
Thus the investigation of stellar evolution has been contemporaneous with the investigation of organic evolution. Indeed, the epoch-making discovery of the chemical composition of the sun by Kirchhoff and Bunsen was made in the year of the publication of the 'Origin of Species.' Before this discovery the meaning of spectral lines had been as obscure as the meaning of Egyptian hieroglyphs prior to the discovery of the Rosetta stone. After it the chemical analysis of a star became hardly less difficult than the analysis of an unknown substance in the laboratory. Furthermore, it soon became apparent that the light of a star, as decomposed by a prism, was competent to define the star's position in a general scheme of development, in which every advance, from the unformed nebulous cloud on through the highest degree of stellar brilliancy to such a final stage as is typified by the moon, can be defined with but little danger of error. Before we proceed to consider some of the evidences of stellar evolution, let us examine some of the instruments and methods without which the discoveries to be subsequently described would have been impossible.
I shall confine my remarks on modern astrophysical instruments to those at present employed at the Yerkes Observatory, partly because nearly all the celestial photographs reproduced in the figures were taken with these instruments and partly because of the convenience of illustrating them. But before describing the great telescope which forms the principal apparatus of the observatory, I wish to point out that many of the most important results of astronomy, results which could not be obtained with a powerful telescope for the very reason of its great power—have been derived from the use of an ordinary camera, with just such a lens as is found in the possession of thousands of amateur photographers. If we take an ordinary camera and point it on a clear night toward the north pole, it will be found after an exposure of one or two hours that the stars which lie near the pole have drawn arcs of circles upon the plate (Fig. 2). This is due to the fact that the earth is rotating upon its axis at such a rate as to cause every star in the sky to appear to travel through a complete circle once in twenty-four hours. The nearer the star to the pole the smaller does this circle become. As we move away from the pole we find the curvature of the star trails growing less and less, until at the equator they appear as straight lines.
Just such photographs as these are frequently employed in astronomical investigations; e. g., for the purpose of recording variations in a star's brightness, which would be shown on the plate by changes in the brightness of the trail. But for most purposes it is desirable to have photographs of stars in which they are represented as points of light rather than as lines. To obtain such photographs it is necessary to mount the camera in such a way that it can be turned about an axis parallel to the earth's axis once in twenty-four hours. A camera so mounted becomes an equatorial photographic telescope, differing in no important respect save in the construction of its lens from an instrument like the 40-inch Yerkes telescope.
But the scale of the photographs obtained with such a camera differs in marked degree from that of the photographs furnished by the telescope. Here, for example, is a region of the Milky Way photographed by Professor Barnard with one of the old-fashioned lenses formerly employed in portrait galleries (Fig. 3). Such a picture as this is of the greatest service in all studies of the structure of the Milky Way, for it brings before us at a single glance an immense region of the sky, thus permitting us to trace the general features which are common to this area. You will notice in the midst of this star cloud a little cluster of stars, here so densely packed together that no details of the cluster can be distinguished. If our investigations required us to single out some individual star in the cluster, perhaps for the purpose of analyzing its light, it is evident that the portrait lens would prove inadequate for our purpose. It is in such a case as this that an instrument like the 40-inch telescope comes into play. The camera with which this photograph was taken has a lens six inches in diameter, of thirty-one inches focal length. The great telescope has a lens forty inches in diameter, of sixty-four feet focal length. Thus the scale of the photographs made with the telescope is about twenty-five times that of the photographs made with the portrait lens. The portrait lens covers a large area of the sky on a very small scale, while the field of the telescope is limited to a small region, which is depicted on a large scale. Let us see the difference between the two instruments as illustrated by the photographs themselves (compare Fig. 3 with Fig. 4). The small cluster, which in reality contains several thousands of stars, is resolved by Mr. Ritchey's photograph taken with the large telescope into all its constituent parts, stars less than one second of arc apart being clearly separated on this great scale.
Star Cluster Messier 11 and the surrounding milky way.
Small scale photograph taken with portrait lens (Barnard). (The cluster, here about one-sixteenth of an inch in diameter, lies just above the middle of the picture.)
Having seen this illustration of the superior power of the large telescope you may perhaps be interested to become more closely acquainted with the instrument itself (Fig. 5). The great weight of the 40-inch lens, amounting with its cell to half a ton, requires that the tube which supports it, here taking the place of the camera box of the previous instrument, shall be of immense rigidity and strength. This tube, 64 feet in length, is supported at its middle point by the declination axis, which in its turn is carried by the polar axis, adjusted to accurate parallelism with the axis of the earth. By means of driving mechanism in the upper section of the iron column the whole instrument
Star Cluster Messier 11.
Large scale photograph taken with the forty-inch Yerkes telescope (Ritchey).
is turned about this polar axis at such a rate that it would complete one revolution in twenty-four hours. Although the moving parts weigh over twenty tons the telescope can be directed to any part of the sky by hand, but this operation is much facilitated by the use of electric motors provided for the purpose. When once directed toward the object to be observed it will frequently happen that the lower end of the telescope is far out of reach above the observer's head. For this reason the
The forty-inch telescope of the Yerkes Observatory.
entire floor of the observing room, 75 feet in diameter, is constructed like an electric elevator, which by throwing a switch can be made to rise or fall through a distance of twenty-three feet. Thus the lower end of the telescope is rendered accessible even for objects near the horizon. In order that the observing slit may be directed to any part of the sky the dome, 90 feet in diameter, is mounted on wheels and can be turned to any desired position by means of an electric motor controlled from the rising-floor.
The telescope is used for a great variety of purposes in conjunction with appropriate instruments, which are attached to the lower end of the tube near the point where the image is formed. I have already shown a photograph of a star cluster taken with this telescope, but without describing the process of making it. As a matter of fact the object-glass of the 40-inch telescope was designed for visual observations, and its maker, the late Alvan G. Clark, had no idea that it would ever be employed for photography. Without dwelling upon the distinguishing features of visual and photographic lenses I may say that the former is so designed by the optician as to unite into an image those rays of light, particularly the yellow and the green, to which the eye is most sensitive. With the only varieties of optical glass which can be obtained in large pieces it is impossible to unite in a single clearly defined image all of the red, the yellow, the green, the blue, and the violet rays which reach us from a star. Therefore when the optician decides to produce an image most suitable for eye observations he deliberately discards the blue and violet rays, simply because they are less important to the eye than the yellow and green rays. For this reason the image of a star produced by a large refracting telescope is surrounded by a blue halo containing the rays discarded by the optician. These very rays, however, are the ones to which the ordinary photographic plate is most sensitive; hence in a photographic telescope the blue and violet rays are united, while the yellow and green rays are discarded.
The 40-inch telescope is of the first type, constructed primarily for visual observations. In order to adapt it for photography Mr. G. W. Ritchey of the observatory staff simply places before the (isochromatic) plate a thin screen of yellow glass, which cuts out the blue rays, but allows the yellow and green rays to pass. As isochromatic plates are sensitive to yellow and green light there is no difficulty in securing an image with the rays which the object-glass unites into a perfect image. During the entire time of the exposure a star which lies just outside the region to be photographed is observed through an eye-piece magnifying 1,000 diameters. This eye-piece is attached to the frame which carries the photographic plate, and is susceptible of motion in two directions at right angles to each other. In the center of the eye-piece are two very fine cross-hairs of spider web illuminated by a small incandescent lamp. If the observer notices that through some slight irregularity in the motion of the telescope, or through some change of refraction in the earth's atmosphere, the star image is moving away from the point of intersection of the cross-hairs, he instantly brings it back by means of one or both of the screws. As the plate moves with the eye-piece it is evident that this method furnishes a means of keeping the star images exactly at the same point on the plate throughout the entire exposure. With such apparatus data are gathered for the study of stellar development.
Great Nebula in Orion.
Photographed with the forty-inch Yerkes telescope (Ritchey).
It is easier to trace the successive steps in the development of a star after it has been formed than it is to account for its origin. But all the evidence that has been accumulated up to the present time tends to show that stars are condensed out of the cloudlike masses which we know as nebulæ. Less than half a century has passed since the true nature of a gaseous nebula was determined. In his extensive observations of astronomical phenomena Sir William Herschel examined a great number of star clusters similar to that shown in Fig. 4. His telescope was a large one, but it can safely be said that he never saw a cluster so well as this object can be perceived through the aid of photography. He found in studying object after object in all parts of the heavens that many clusters could be resolved into their constituent stars. In some of these clusters the stars are widely separated by a powerful instrument, as they appear in this photograph. In others, either on account of their greater distance or because the stars are less widely spaced, the central regions are no longer clearly resolvable as separate objects. It is thus quite possible to imagine a cluster in which the stars are so closely grouped that no telescope, however powerful, could separately distinguish them.
Now as a matter of fact we find in all parts of the heavens luminous objects which can not be separated into stars. Some of these are of definite outline and are perfectly symmetrical in form, in many cases with a brilliant star-like nucleus at their center. These are known as the planetary nebula. Other nebulæ, like the great nebula in Orion (Fig. 6), are diffuse and irregular and extend over great regions of the sky. It was long a question whether such objects were capable of resolution into stars with a sufficiently powerful telescope. Herschel rightly concluded that an important distinction can be drawn between a nebula and a star cluster, though his son did not admit this distinction.
It was only after Huggins had applied the spectroscope to an analysis of the light of a nebula that it could be said without danger of contradiction that the phenomenon is not one produced by the crowding together of separate stars, but is due to the presence of a mass of incandescent gas. Sir William Huggins' account of his first spectroscopic examination of a nebula is recorded in the first volume of the 'Publications of the Tulse Hill Observatory':
"On the evening of August 29, 1864, I directed the spectroscope for the first time to a planetary nebula in Draco. I looked into the spectroscope, No spectrum such as I had expected! A single bright line only! At first I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of its faces. This thought was scarcely more than momentary; then the true interpretation flashed upon me. The light of the nebula was monochromatic and so, unlike any other light I had as yet subjected to prismatic examination, could not be extended out to form a complete spectrum. After passing through the two prisms it remained concentrated into a single bright line, having a width corresponding to the width of the slit, and occupying in the instrument a position at that part of the spectrum to which its light belongs in refrangibility. A little closer looking showed two other bright lines on the side towards the blue, all three lines being separated by intervals relatively dark. The riddle of the nebulæ was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas."
With this advance a new era of progress began. The power of the spectroscope to distinguish between a glowing gas and a mass of partially condensed vapors like a star established it at once in its place as the chief instrument of the student of stellar evolution. It became apparent that the unformed nebula might furnish the stuff from which stars are made. Observations tending to this conclusion were not long in presenting themselves. In the heart of the Orion nebula are four small stars which constitute the well-known Trapezium. Situated as they are in the midst of this far-reaching mass of gas, it is not hard to picture them as centers of condensation, toward which the play of gravitational forces tends to concentrate the gases of the nebula. It might therefore be expected that stars in this early stage of growth should show through the spectroscopic analysis of their light some evidence of relationship with the surrounding nebula. Now this is precisely what the spectroscope has demonstrated. Not only these stars, but many other stars in the constellation of Orion, are shown by the spectroscope to contain the same gases which constitute the nebula. For this and other reasons they are considered to represent one of the earliest stages of stellar growth.
It may be many years before the exact nature of the process by which a star is formed from a nebulous mass is clearly understood. Shortly before his death the late Professor Keeler made a most important discovery in the course of his photographic work with the Crossley reflector of the Lick Observatory. Spiral nebulæ have long been known, but it was not supposed that they were sufficiently numerous to be regarded as type objects. The great spiral nebula illustrated in Fig. 7 from one of Mr. Ritchey's recent reflector photographs has long been regarded as one of the most remarkable objects in the heavens, and the possible significance of its form had by no means been overlooked. But few astronomers were prepared for Professor Keeler's announcement that the majority of nebulæ are of the spiral form and that many thousands of these objects are within the reach of such an instrument as the Crossley reflector. It does not seem improbable that this spiral form may prove to represent the original condensing mass more truly than the lenticular form from which Laplace imagined the solar system to be evolved.
Enough has already been said to indicate how large a part the methods of spectroscopy must play in a study of the life history of stars. In spite of the common opinion that the spectroscope is an intricate instrument and that the principles of spectroscopy are obscure and difficult of comprehension, it is a fact that the processes used in this field of investigation can be easily understood by any one who will devote a very small amount of time to the subject. As you doubtless know, the essential feature of a star spectroscope is the prism or train of prisms by which the star light is divided into its constituent parts. After passing through the prisms the light of the star is spread out into a long band, which shows all the colors of the rainbow, beginning
Spiral Nebula in Canes Venatici.
Photographed with the two-foot reflecting telescope of the Yerkes Observatory (Ritchey).
with red at one end and passing through orange, yellow, green and blue, to violet at the other. This band is crossed by lines, and the problem of the spectroscopist is to interpret the meaning of these lines. If the lines are dark he knows that the light of the star after originating in an interior incandescent body has passed through a mass of cooler vapors, and that during its transmission some of the light has suffered absorption. If, on the other hand, the lines are bright, he knows that the region where they are produced is hotter than that lying below. Thus a single glance at the spectrum of a star is sufficient to give important information regarding the physical condition of its atmosphere.
But the spectral lines are able to tell a far more complete story of stellar conditions. If their exact position in the spectrum can be measured it becomes possible to determine the chemical composition of the star's atmosphere. And here the spectroscopist may be said to have the advantage of the archeologist, in that the key to stellar hieroglyphs is a master key, capable of interpreting not merely the language of a single people or a single age, but of laying bare the secrets of the most distant portions of the universe and applying with equal force to the primitive and to the most highly developed forms of celestial phenomena. If we take a piece of iron wire and turn it into vapor in the intense heat of an electric arc lamp we find that the light which the glowing iron vapor emits, when spread out into a spectrum by a prism, consists of a series of lines characteristically spaced and always occupying the same relative positions. In the same way every other element when transformed into vapor by a sufficiently intense heat emits characteristic radiations, consisting of groups of lines occupying definite positions in the spectrum. It is thus easy to see how the presence of iron vapor can be detected in the atmosphere of Sirius or in that of the sun. In the spectrum of each of these stars we find a group of lines occupying the same relative positions as the lines furnished by the iron vapor in an electric arc. Hydrogen gives an even more characteristic group of lines, which grow closer and closer together as we pass from the red end of the spectrum toward the violet. This group occurs in the spectra of thousands of stars and serves as an important guide in determining a star's place in a general scheme of stellar evolution.
The practical means of carrying out this method of research may be illustrated by a reference to the stellar spectroscope employed with the 40-inch Yerkes telescope. The spectroscope is rigidly attached to the lower end of the telescope tube. The image of a star formed by the 40-inch lens passes into the spectroscope through a slit about one one-thousandth of an inch wide. After analysis by a train of three prisms an image of the resulting spectrum is formed by a suitable lens upon a photographic plate. In making the photograph it is only necessary to keep the image of a star exactly on the slit throughout the exposure, which may occupy from one minute to several hours, the duration depending upon the brightness of the star.
We have seen that a single glance at the spectrum of a star is sufficient to give us important information as to the structure of its atmosphere, while a study of the position of the lines tells what chemical elements are present. We might go on to consider how the width and sharpness of the lines, together with shifts in their position toward the red end of the spectrum, furnish the means of estimating the density of the vapors and the pressure to which they are subjected. The relative intensities of certain lines also serve as a clue to the temperature. Thus in the spectrum of magnesium there is a pair of lines, one of which is the stronger at the temperature of the electric spark, while the other is the stronger at the lower temperature of the electric arc. In the spectra of certain stars the greater intensity of the first line indicates that the temperature is high and approximates that of the electric spark, while in other stars the relative intensities are reversed, indicating that the temperature is lower and corresponds more closely with that of the electric arc. In addition to all this, certain easily measurable changes in the position of the spectral lines are known from Doppler's principle to indicate motion of the star in the direction of the earth. Thus if the lines are shifted toward the red with reference to their normal position, and if we have evidence that the shift is not due to pressure, we may conclude that the distance between the earth and the star is increasing, while if the lines are shifted toward the violet we conclude that the distance between the earth and the star is decreasing. As the earth's motion is known, the velocity of the star in the line of sight can therefore be accurately determined.
After this glance at the methods employed by the spectroscopist, we may return to a further consideration of the stages of stellar evolution. We have seen that the long continued action of gravity tends to produce condensation of a cosmical cloud. The constellation of Orion contains many examples of stars in this early stage of development. As the mass condenses its temperature rises, and corresponding with this rise in temperature and in the density of the vapors which constitute the star we find characteristic changes in the spectrum and also in the star's color. Such a brilliant white or bluish-white star as Sirius or Vega may be taken as representative of the next stage of stellar development. Here the broad bands of hydrogen, which constitute a beautiful series expressible by a simple mathematical formula, serve as the chief mark of distinction. The conditions are not yet ripe for the marked development of metallic lines, though doubtless the numerous elements which constitute the sun and which for the most part are familiar to us on the earth, are present in such stars, though they are not revealed through a study of the spectrum. It is true that evidence exists of the presence of iron and a few other substances, but the lines are thin and few in number and would be overlooked in a casual examination of the spectrum. The period for their greatest development has not yet arrived. The light gas hydrogen, reaching far above the white-hot mass of condensed vapors which constitutes the nucleus of the star, is at this stage the predominant element, at least so far as we may judge from a study of the light radiation.
An interesting question has arisen regarding the period in a star's life at which the highest temperature is attained. The apparently paradoxical statement of Lane's law that the temperature of a cooling mass of incandescent vapors, instead of falling, actually increases until a certain stage has passed, applies in the present instance. We indeed know that a condensing nebula losing heat by radiation into space will continue to rise in temperature for thousands and even millions of years. A question which has received some discussion of late is with regard to the precise period at which the maximum temperature occurs. Shall we seek it in white stars like Sirius or in yellow stars like the sun, which represents the next well-defined stage of stellar evolution? With an instrument of extraordinary delicacy Professor Nichols has recently measured at the Yerkes Observatory the amount of heat which we receive from Vega and Arcturus. The distance of these stars is so inconceivably great that the quantity of heat which they send to the surface of the earth has hitherto been too small to be detected by the most sensitive instruments. Professor Nichols' radiometer, which in combination with a large concave mirror renders it easy to measure the heat radiated from a man's face 2,000 feet away, proved adequate for the task. He found that Arcturus sends us about as much heat as we should get from a candle six miles away if there were no intervening atmosphere to reduce the candle's intensity. Vega, which to the eye is precisely equal to Arcturus in brightness, was found to send us only half as much heat. If the absorbing atmospheres of Arcturus and Vega were similar in character, it would follow from Professor Nichols' results that Vega, though it sends us less heat, is really the hotter of the two stars. For we know from laboratory experiments that the proportion of long (heat) waves to short (light) waves is greater in the radiation of the cooler of two bodies heated to incandescence. In this case the fact that Arcturus sends the greater amount of heat would be ascribed rather to greater size than to lesser distance, as there is good reason to believe that it is farther from us than Vega.
But unfortunately the dissimilarity of the atmospheres of the two stars renders it uncertain whether such conclusions can safely be drawn. This is particularly true in view of the fact that Sir William Huggins concludes from his spectroscopic studies that the highest stage of stellar temperature is reached in stars like Vega, while stars like Arcturus and the Sun have passed the stage of highest temperature and are already well advanced in their decline.
While some uncertainty must therefore prevail until further investigations have been completed regarding the exact stage at which the highest stellar temperatures are attained, there can be little doubt as to the path which is followed when through the long continued action of gravitation a young star like Vega develops into a star like the Sun. We are fortunate in possessing examples of a great number of intermediate stages in this orderly progress (Fig. 8). As condensation continues, and as the vapors which constitute the star continue to crowd upon each other, the stellar nucleus becomes denser and denser and the vast atmosphere of hydrogen gradually gives place to a much shallower atmosphere, in which hydrogen is still conspicuous, though it no longer predominates in a very striking manner over the other elements. The spectral lines of such elements as iron, magnesium, sodium and calcium,
Characteristic spectra of (a) white, (b) yellow, and (c) red stars (Huggins).
rise into prominence as the hydrogen lines fade. Meanwhile the light of the star undergoes a change of color, completely losing its bluish cast and assuming a distinctly yellow hue. There can be little if any doubt that our own sun once passed through the successive stages which are represented by the spectra shown in Fig. 8. The time which has elapsed since it acquired its present size and density as the result of the condensation of the great nebula in which the earth and the other planets also had their origin, covers many millions of years. It is fortunate for the study of stellar evolution that the stages through which the sun once passed are all exemplified in existing stars, which for unknown reasons began their stellar life at widely different times.
It will be profitable to consider for a moment some of the remarkable phenomena which are presented to us by the sun, not only because of their intrinsic interest, but also because it is perfectly safe to assume that similar phenomena, sometimes on a much greater scale, would be presented by other stars, were they not at so great a distance from the earth as to reduce them to mere points of light, even in the most powerful telescope. The sun has a diameter of 860,000 miles and, as its distance from the earth is only 93,000,000 miles, an extremely small fraction of the distance of the other stars, it is possible to observe and to study in detail its extraordinary phenomena, which are incomparably more violent than anything observed on the earth. When we speak of the sun we speak collectively of a great number of phenomena, some of which extend for millions of miles from the sun's visible disk. Chief of these is the corona, a vast filmy atmosphere so rare that it offers little or no resistance to the passage of a comet, as it sweeps around the sun under the action of gravitation and returns into the space from which it came. The polar streamers of the corona (Fig. 9) suggest the
Photographed by Yerkes Observatory Eclipse Expedition, May 28, 1900 (Barnard and Ritchey).
action of magnetic forces and offer material for long continued study of this, the most mysterious of all the solar appendages. At the base of the corona, rising out of a sea of flame which completely encircles the sun, are the prominences, some of which occasionally attain a height of nearly 400,000 miles. Like the corona, the prominences are hidden by the brilliant illumination of our own atmosphere, and are visible to the naked eye only when the direct light of the sun's disk is cut off by the interposition of the moon at a total eclipse. But methods have been devised by which they can be observed or photographed on any clear day through the agency of a modified form of spectroscope. The prominences are constantly changing in form, sometimes slowly, as in the case of this group (Fig. 10), a photograph of which, taken at the eclipse of May 28, 1900, by the Astronomer Royal of England in Spain, is shown for comparison with the photograph taken about two hours earlier by the Yerkes Observatory party in North Carolina. Here the
Cloud-like Prominences photographed at the Eclipse of May 28, 1900. a, by Yerkes Observatory Party at Wadesboro, N. C. b, by Astronomer Royal of England at Ovar, Portugal, two hours later. (The bright cross on the right of this picture is due to a defect in the original photograph.)
change in the form of the mass of gas which constitutes the prominence, is comparatively small, but that violent forces are sometimes at work may be illustrated by photographs of an eruptive prominence taken at the Kenwood Observatory in 1895 (Fig. 11). At the moment when the first photograph was made the prominence had attained a height of 160,000 miles and was rising rapidly. Eighteen minutes later another picture was taken; during the interval the prominence had been going upward at the rate of six thousand miles a minute, and when the exposure was made it had reached an elevation of 280,000 miles. When looked for a few minutes later it had completely disappeared.
The constitution of the chromosphere, the sea of flame some 10,000 miles deep from which the prominences arise, increases in complexity as the surface of the solar disk is approached. In its upper part only the vapor of calcium and the light gases, hydrogen and helium, are found. But in proceeding downward the vapors of magnesium, sodium, iron, chromium, and last of all, carbon, are successively encountered. At this part of the solar atmosphere the dark lines of the solar spectrum take their rise through the effect of absorption.
Time does not permit a detailed description of the phenomena of the sun's disk. When photographed with an instrument which excludes from the sensitive plate all light except that which is characteristic of the vapor of calcium, its surface is found to be dotted over with extensive luminous regions. Associated with these are the sun-spots, the minute study of which has revealed some strikingly beautiful phenomena, which have been most successfully drawn by Langley. The surface of the sun in the regions devoid of spots is shown by the photographs of Janssen to consist of brilliant granules separated by darker spaces. Much might be said of the peculiar law of rotation of the sun, which causes a point near the equator to complete an axial rotation in much less time than a point nearer the poles. Much might also be said of the periodicity of sun-spots, which at times are
 | ||
| a | Fig. 11. | b |
| Eruptive prominence photographed in full sunlight at the Kenwood Observatory, Chicago, March 25, 1895. a, at 10h. 40m. (height, 162.000 miles), b, at 10h. 58m height, 281,000 miles). (Figs. 10 and 11 are reproduced on the same scale.) | ||
very numerous and again, as at present, are absent from the sun's disk for weeks together. But enough has already been told to indicate some of the chief characteristics of this central star of the solar system, which has thousands of counterparts among other stars of the same spectral class.
We are now approaching the last chapters in the life history of a star. After the solar stage has passed the color changes from yellow to orange, and subsequently to red, as the temperature falls. The spectral lines of hydrogen become fainter and fainter and finally disappear completely. The lines of the metallic elements, on the contrary, become more and more complex and the changes in their relative intensities are those which are characteristic of lower temperatures. But curiously enough. there are two well-defined classes of these older stars, which until recently were not known to have any points in common except their red color. These are the stars of Secchi's third and fourth types. In general appearance their spectra are wholly unlike, particularly on account of the absence from third class spectra of the broad dark bands due to the absorption of carbon vapor, the most characteristic feature of the fourth type. But in spite of this apparent dissimilarity, photographs recently taken with the 40-inch Yerkes telescope show that in certain regions of the spectrum stars of the two types are practically identical and are thus probably more closely related than formerly appeared to be the
Spectra of four red stars (Hale and Ellerman). Showing how the dark band due to carbon increases in intensity as the star cools.
case. The measurements and reductions of a long series of photographs of fourth type spectra now in progress at the Yerkes Observatory should soon permit us to form an opinion of the nature of these interesting stars.
In both the third and fourth types it is easy to trace the successive stages of development. In stars of the fourth type the signs of increasing age are particularly striking. The carbon vapor which produces the broad dark bands becomes denser and denser, until it is not difficult to imagine that through the further increase of such absorption the light of the star might be completely extinguished (Fig. 12).
The phenomena of the red stars indicate that this final stage is close at hand, and curiously enough, in further testimony of the remarkable power of the spectroscope, the total extinction of a star's light does not always prove sufficient to place that star beyond the reach of this instrument. It is true that a spectroscope cannot reveal the chemical composition of a solid body which is devoid of intrinsic light, but such a body may form a system with another object which is still luminous,
Lunar Crater Theophilus and surrounding region. Photographed with the forty-inch Yerkes telescope (Ritchey).
and its gravitational power may cause the luminous body to move in an orbit. As we have already seen, the spectroscope is capable of revealing the motions of such a body. From a knowledge of these motions and the time in which the revolution is effected it is possible to determine the mass and dimensions of the system, and in some special cases like that of Algol, the diameter and density of the invisible component of the pair.
We must look to the solar system for examples of stars in the last stage of development. Each of the planets may in fact be regarded as an object of this kind. The bare and rocky surface of the moon affords a desolate picture of what may result from this long continued process of condensation. The volcanic region which is shown to excellent advantage in a photograph recently taken with the Yerkes telescope, (Fig. 13) gives no evidence of the existence of life; in fact, the spectroscope indicates that if there is any air on the moon it is much too rare to support life as we know it.
Fortunately, the moon is not the only example of a worn-out star. The earth, which probably has many counterparts in the universe, is another example of a less desolate kind. Here, though the process of condensation which is the chief cause of celestial phenomena has ceased, the problem of evolution has not ended. In fact, though the cosmical problems which we have considered in their barest elements will not be completely solved for centuries, it may be truly said that the questions raised by the countless living organisms in a single drop of ditch water are still more complex, and will require a still longer time for their solution.
- ↑ Revised from an address delivered on June 5, 1901, before the Minnesota Chapter of the Honorary Scientific Society of Sigma XI, University of Minnesota.