Chapter 3

In speaking of Professor Comstock’s extraordinary theory of the Milky Way, the fact was mentioned that, broadly speaking, the nebulæ are less numerous in the galactic belt than in the comparatively open spaces on either side of it, but that they are, nevertheless, abundant in the broader half of the Milky Way which he designates as the front of the gigantic “plough” supposed to be forcing its way through the enveloping chaos. In and around the Sagittarius region the intermingling of nebulæ and galactic star clouds and clusters is particularly remarkable. That there is a causal connection no thoughtful person can doubt. We are unable to get away from the evidence that a nebula is like a seed-ground from which stars spring forth; or we may say that nebulæ resemble clouds in whose bosom raindrops are forming. The wonderful aspect of the admixtures of nebulæ and star-clusters in Sagittarius has been described in Chapter 1. We now come to a still more extraordinary phenomenon of this kind—the Pleiades nebulæ.

The group of the Pleiades, although lying outside the main course of the Galaxy, is connected with it by a faint loop, and is the scene of the most remarkable association of stars and nebulous matter known in the visible universe. The naked eye is unaware of the existence of nebulæ in the Pleiades, or, at the best, merely suspects that there is something of the kind there; and even the most powerful telescopes are far from revealing the full wonder of the spectacle; but in photographs which have been exposed for many hours consecutively, in order to accumulate the impression of the actinic rays, the revelation is stunning. The principle stars are seen surrounded by, and, as it were, _drowned in,_ dense nebulous clouds of an unparalleled kind. The forms assumed by these clouds seem at first sight inexplicable. They look like fleeces, or perhaps more like splashes and daubs of luminous paint dashed carelessly from a brush. But closer inspection shows that they are, to a large extent, _woven_ out of innumerable threads of filmy texture, and there are many indications of spiral tendencies. Each of the bright stars of the group—Alcyone, Merope, Maia, Electra, Taygeta, Atlas—is the focus of a dense fog (totally invisible, remember, alike to the naked eye and to the telescope), and these particular stars are veiled from sight behind the strange mists. Running in all directions across the relatively open spaces are nebulous wisps and streaks of the most curious forms. On some of the nebular lines, which are either straight throughout, or if they change direction do so at an angle, little stars are strung like beads. In one case seven or eight stars are thus aligned, and, as if to emphasize their dependence upon the chain which connects them, when it makes a slight bend the file of stars turns the same way. Many other star rows in the group suggest by their arrangement that they, too, were once strung upon similar threads which have now disappeared, leaving the stars spaced along their ancient tracks. We seem forced to the conclusion that there was a time when the Pleiades were embedded in a vast nebula resembling that of Orion, and that the cloud has now become so rare by gradual condensation into stars that the merest trace of it remains, and this would probably have escaped detection but for the remarkable actinic power of the radiant matter of which it consists. The richness of many of these faint nebulous masses in ultra-violet radiations, which are those that specifically affect the photographic plate, is the cause of the marvelous revelatory power of celestial photography. So the veritable unseen universe, as distinguished from the “unseen universe” of metaphysical speculation, is shown to us.

A different kind of association between stars and nebulæ is shown in some surprising photographic objects in the constellation Cygnus, where long, wispy nebulæ, billions of miles in length, some of them looking like tresses streaming in a breeze, lie amid fields of stars which seem related to them. But the relation is of a most singular kind, for notwithstanding the delicate structure of the long nebulæ they appear to act as barriers, causing the stars to heap themselves on one side. The stars are two, three, or four times as numerous on one side of the nebulæ as on the other. These nebulæ, as far as appearance goes, might be likened to rail fences, or thin hedges, against which the wind is driving drifts of powdery snow, which, while scattered plentifully all around, tends to bank itself on the leeward side of the obstruction. The imagination is at a loss to account for these extraordinary phenomena; yet there they are, faithfully giving us their images whenever the photographic plate is exposed to their radiations.

Thus the more we see of the universe with improved methods of observation, and the more we invent aids to human senses, each enabling us to penetrate a little deeper into the unseen, the greater becomes the mystery. The telescope carried us far, photography is carrying us still farther; but what as yet unimagined instrument will take us to the bottom, the top, and the end? And then, what hitherto untried power of thought will enable us to comprehend the meaning of it all?

III Stellar Migrations

To the untrained eye the stars and the planets are not distinguishable. It is customary to call them all alike “stars.” But since the planets more or less rapidly change their places in the sky, in consequence of their revolution about the sun, while the stars proper seem to remain always in the same relative positions, the latter are spoken of as “fixed stars.” In the beginnings of astronomy it was not known that the “fixed stars” had any motion independent of their apparent annual revolution with the whole sky about the earth as a seeming center. Now, however, we know that the term “fixed stars” is paradoxical, for there is not a single really fixed object in the whole celestial sphere. The apparent fixity in the positions of the stars is due to their immense distance, combined with the shortness of the time during which we are able to observe them. It is like viewing the plume of smoke issuing from a steamer, hull down, at sea: if one does not continue to watch it for a long time it appears to be motionless, although in reality it may be traveling at great speed across the line of sight. Even the planets seem fixed in position if one watches them for a single night only, and the more distant ones do not sensibly change their places, except after many nights of observation. Neptune, for instance, moves but little more than two degrees in the course of an entire year, and in a month its change of place is only about one-third of the diameter of the full moon.

Yet, fixed as they seem, the stars are actually moving with a speed in comparison with which, in some cases, the planets might almost be said to stand fast in their tracks. Jupiter’s speed in his orbit is about eight miles per second, Neptune’s is less than three and one-half miles, and the earth’s is about eighteen and one-half miles; while there are “fixed stars” which move two hundred or three hundred miles per second. They do not all, however, move with so great a velocity, for some appear to travel no faster than the planets. But in all cases, notwithstanding their real speed, long-continued and exceedingly careful observations are required to demonstrate that they are moving at all. No more overwhelming impression of the frightful depths of space in which the stars are buried can be obtained than by reflecting upon the fact that a star whose actual motion across the line of sight amounts to two hundred miles per second does not change its apparent place in the sky, in the course of a thousand years, sufficiently to be noticed by the casual observer of the heavens!

There is one vast difference between the motions of the stars and those of the planets to which attention should be at once called: the planets, being under the control of a central force emanating from their immediate master, the sun, all move in the same direction and in orbits concentric about the sun; the stars, on the other hand, move in every conceivable direction and have no apparent center of motion, for all efforts to discover such a center have failed. At one time, when theology had finally to accept the facts of science, a grandiose conception arose in some pious minds, according to which the Throne of God was situated at the exact center of His Creation, and, seated there, He watched the magnificent spectacle of the starry systems obediently revolving around Him. Astronomical discoveries and speculations seemed for a time to afford some warrant for this view, which was, moreover, an acceptable substitute for the abandoned geocentric theory in minds that could only conceive of God as a superhuman artificer, constantly admiring his own work. No longer ago than the middle of the nineteenth century a German astronomer, Maedler, believed that he had actually found the location of the center about which the stellar universe revolved. He placed it in the group of the Pleiades, and upon his authority an extraordinary imaginative picture was sometimes drawn of the star Alcyone, the brightest of the Pleiades, as the very seat of the Almighty. This idea even seemed to gain a kind of traditional support from the mystic significance, without known historical origin, which has for many ages, and among widely separated peoples, been attached to the remarkable group of which Alcyone is the chief. But since Maedler’s time it has been demonstrated that the Pleiades cannot be the center of revolution of the universe, and, as already remarked, all attempts to find or fix such a center have proved abortive. Yet so powerful was the hold that the theory took upon the popular imagination, that even today astronomers are often asked if Alcyone is not the probable site of “Jerusalem the Golden.”

If there were a discoverable center of predominant gravitative power, to which the motions of all the stars could be referred, those motions would appear less mysterious, and we should then be able to conclude that the universe was, as a whole, a prototype of the subsidiary systems of which it is composed. We should look simply to the law of gravitation for an explanation, and, naturally, the center would be placed within the opening enclosed by the Milky Way. If it were there the Milky Way itself should exhibit signs of revolution about it, like a wheel turning upon its hub. No theory of the star motions as a whole could stand which failed to take account of the Milky Way as the basis of all. But the very form of that divided wreath of stars forbids the assumption of its revolution about a center. Even if it could be conceived as a wheel having no material center it would not have the form which it actually presents. As was shown in Chapter 2, there is abundant evidence of motion in the Milky Way; but it is not motion of the system as a whole, but motion affecting its separate parts. Instead of all moving one way, the galactic stars, as far as their movements can be inferred, are governed by local influences and conditions. They appear to travel crosswise and in contrary directions, and perhaps they eddy around foci where great numbers have assembled; but of a universal revolution involving the entire mass we have no evidence.

Most of our knowledge of star motions, called “proper motions,” relates to individual stars and to a few groups which happen to be so near that the effects of their movements are measurable. In some cases the motion is so rapid (not in appearance, but in reality) that the chief difficulty is to imagine how it can have been imparted, and what will eventually become of the “runaways.” Without a collision, or a series of very close approaches to great gravitational centers, a star traveling through space at the rate of two hundred or three hundred miles per second could not be arrested or turned into an orbit which would keep it forever flying within the limits of the visible universe. A famous example of these speeding stars is “1830 Groombridge,” a star of only the sixth magnitude, and consequently just visible to the naked eye, whose motion across the line of sight is so rapid that it moves upon the face of the sky a distance equal to the apparent diameter of the moon every 280 years. The distance of this star is at least 200,000,000,000,000 miles, and may be two or three times greater, so that its actual speed cannot be less than two hundred, and may be as much as four hundred, miles per second. It could be turned into a new course by a close approach to a great sun, but it could only be stopped by collision, head-on, with a body of enormous mass. Barring such accidents it must, as far as we can see, keep on until it has traversed our stellar system, whence in may escape and pass out into space beyond, to join, perhaps, one of those other universes of which we have spoken. Arcturus, one of the greatest suns in the universe, is also a runaway, whose speed of flight has been estimated all the way from fifty to two hundred miles per second. Arcturus, we have every reason to believe, possesses hundreds of times the mass of our sun—think, then, of the prodigious momentum that its motion implies! Sirius moves more moderately, its motion across the line of sight amounting to only ten miles per second, but it is at the same time approaching the sun at about the same speed, its actual velocity in space being the resultant of the two displacements.

What has been said about the motion of Sirius brings us to another aspect of this subject. The fact is, that in every case of stellar motion the displacement that we observe represents only a part of the actual movement of the star concerned. There are stars whose motion carries them straight toward or straight away from the earth, and such stars, of course, show no cross motion. But the vast majority are traveling in paths inclined from a perpendicular to our line of sight. Taken as a whole, the stars may be said to be flying about like the molecules in a mass of gas. The discovery of the radial component in the movements of the stars is due to the spectroscope. If a star is approaching, its spectral lines are shifted toward the violet end of the spectrum by an amount depending upon the velocity of approach; if it is receding, the lines are correspondingly shifted toward the red end. Spectroscopic observation, then, combined with micrometric measurements of the cross motion, enables us to detect the real movement of the star in space. Sometimes it happens that a star’s radial movement is periodically reversed; first it approaches, and then it recedes. This indicates that it is revolving around a near-by companion, which is often invisible, and superposed upon this motion is that of the two stars concerned, which together may be approaching or receding or traveling across the line of sight. Thus the complications involved in the stellar motions are often exceedingly great and puzzling.

Yet another source of complication exists in the movement of our own star, the sun. There is no more difficult problem in astronomy than that of disentangling the effects of the solar motion from those of the motions of the other stars. But the problem, difficult as it is, has been solved, and upon its solution depends our knowledge of the speed and direction of the movement of the solar system through space, for of course the sun carries its planets with it. One element of the solution is found in the fact that, as a result of perspective, the stars toward which we are going appear to move apart toward all points of the compass, while those behind appear to close up together. Then the spectroscopic principle already mentioned is invoked for studying the shift of the lines, which is toward the violet in the stars ahead of us and toward the red in those that we are leaving behind. Of course the effects of the independent motions of the stars must be carefully excluded. The result of the studies devoted to this subject is to show that we are traveling at a speed of twelve to fifteen miles per second in a northerly direction, toward the border of the constellations Hercules and Lyra. A curious fact is that the more recent estimates show that the direction is not very much out of a straight line drawn from the sun to the star Vega, one of the most magnificent suns in the heavens. But it should not be inferred from this that Vega is drawing us on; it is too distant for its gravitation to have such an effect.

Many unaccustomed thoughts are suggested by this mighty voyage of the solar system. Whence have we come, and whither do we go? Every year of our lives we advance at least 375,000,000 miles. Since the traditional time of Adam the sun has led his planets through the wastes of space no less than 225,000,000,000 miles, or more than 2400 times the distance that separates him from the earth. Go back in imagination to the geologic ages, and try to comprehend the distance over which the earth has flown. Where was our little planet when it emerged out of the clouds of chaos? Where was the sun when his “thunder march” began? What strange constellations shone down upon our globe when its masters of life were the monstrous beasts of the “Age of Reptiles”? A million years is not much of a span of time in geologic reckoning, yet a million years ago the earth was farther from its present place in space than any of the stars with a measurable parallax are now. It was more than seven times as far as Sirius, nearly fourteen times as far as Alpha Centauri, three times as far as Vega, and twice as far as Arcturus. But some geologists demand two hundred, three hundred, even one thousand million years to enable them to account for the evolutionary development of the earth and its inhabitants. In a thousand million years the earth would have traveled farther than from the remotest conceivable depths of the Milky Way!

Other curious reflections arise when we think of the form of the earth’s track as it follows the lead of the sun, in a journey which has neither known beginning nor conceivable end. There are probably many minds which have found a kind of consolation in the thought that every year the globe returns to the same place, on the same side of the sun. This idea may have an occult connection with our traditional regard for anniversaries. When that period of the year returns at which any great event in our lives has occurred we have the feeling that the earth, in its annual round, has, in a manner, brought us back to the scene of that event. We think of the earth’s orbit as a well-worn path which we traverse many times in the course of a lifetime. It seems familiar to us, and we grow to have a sort of attachment to it. The sun we are accustomed to regard as a fixed center in space, like the mill or pump around which the harnessed patient mule makes his endless circuits. But the real fact is that the earth never returns to the place in space where it has once quitted. In consequence of the motion of the sun carrying the earth and the other planets along, the track pursued by our globe is a vast spiral in space continually developing and never returning upon its course. It is probable that the tracks of the sun and the others stars are also irregular, and possibly spiral, although, as far as can be at present determined, they appear to be practically straight. Every star, wherever it may be situated, is attracted by its fellow-stars from many sides at once, and although the force is minimized by distance, yet in the course of many ages its effects must become manifest.

Looked at from another side, is there not something immensely stimulating and pleasing to the imagination in the idea of so stupendous a journey, which makes all of us the greatest of travelers? In the course of a long life a man is transported through space thirty thousand million miles; Halley’s Comet does not travel one-quarter as far in making one of its immense circuits. And there are adventures on this voyage of which we are just beginning to learn to take account. Space is full of strange things, and the earth must encounter some of them as it advances through the unknown. Many singular speculations have been indulged in by astronomers concerning the possible effects upon the earth of the varying state of the space that it traverses. Even the alternation of hot and glacial periods has sometimes been ascribed to this source. When tropical life flourished around the poles, as the remains in the rocks assure us, the needed high temperature may, it has been thought, have been derived from the presence of the earth in a warm region of space. Then, too, there is a certain interest for us in the thought of what our familiar planet has passed through. We cannot but admire it for its long journeying as we admire the traveler who comes to us from remote and unexplored lands, or as we gaze with a glow of interest upon the first locomotive that has crossed a continent, or a ship that has visited the Arctic or Antarctic regions. If we may trust the indications of the present course, the earth, piloted by the sun, has come from the Milky Way in the far south and may eventually rejoin that mighty band of stars in the far north.

While the stars in general appear to travel independently of one another, except when they are combined in binary or trinary systems, there are notable exceptions to this rule. In some quarters of the sky we behold veritable migrations of entire groups of stars whose members are too widely separated to show any indications of revolution about a common center of gravity. This leads us back again to the wonderful group of the Pleiades. All of the principle stars composing that group are traveling in virtually parallel lines. Whatever force set them going evidently acted upon all alike. This might be explained by the assumption that when the original projective force acted upon them they were more closely united than they are at present, and that in drifting apart they have not lost the impulse of the primal motion. Or it may be supposed that they are carried along by some current in space, although it would be exceedingly difficult, in the present state of our knowledge, to explain the nature of such a current. Yet the theory of a current has been proposed. As to an attractive center around which they might revolve, none has been found. Another instance of similar “star-drift” is furnished by five of the seven stars constituting the figure of the “Great Dipper.” In this case the stars concerned are separated very widely, the two extreme ones by not less than fifteen degrees, so that the idea of a common motion would never have been suggested by their aspect in the sky; and the case becomes the more remarkable from the fact that among and between them there are other stars, some of the same magnitude, which do not share their motion, but are traveling in other directions. Still other examples of the same phenomenon are found in other parts of the sky. Of course, in the case of compact star-clusters, it is assumed that all the members share a like motion of translation through space, and the same is probably true of dense star-swarms and star-clouds.