Part 3

From the Milky Way as a whole we pass to the vast clouds, swarms, and clusters of stars of which it is made up. It may be, as some astronomers hold, that most of the galactic stars are much smaller than the sun, so that their faintness is not due entirely to the effect of distance. Still, their intrinsic brilliance attests their solar character, and considering their remoteness, which has been estimated at not less than ten thousand to twenty thousand light-years (a light-year is equal to nearly six thousand thousand million miles) their actual masses cannot be extremely small. The minutest of them are entitled to be regarded as real suns, and they vary enormously in magnitude. The effects of their attractions upon one another can only be inferred from their clustering, because their relative movements are not apparent on account of the brevity of the observations that we can make. But imagine a being for whom a million years would be but as a flitting moment; to him the Milky Way would appear in a state of ceaseless agitation—swirling with “a fury of whirlpool motion.”

The cloud-like aspect of large parts of the Galaxy must always have attracted attention, even from naked-eye observers, but the true star-clouds were first satisfactorily represented in Barnard’s photographs. The resemblance to actual clouds is often startling. Some are close-packed and dense, like cumuli; some are wispy or mottled, like cirri. The rifts and modulations, as well as the general outlines, are the same as those of clouds of vapor or dust, and one notices also the characteristic thinning out at the edges. But we must beware of supposing that the component suns are thickly crowded as the particles forming an ordinary cloud. They look, indeed, as if they were matted together, because of the irradiation of light, but in reality millions and billions of miles separate each star from its neighbors. Nevertheless they form real assemblages, whose members are far more closely related to one another than is our sun to the stars around him, and if we were in the Milky Way the aspect of the nocturnal sky would be marvelously different from its present appearance.

Stellar clouds are characteristic of the Galaxy and are not found beyond its borders, except in the “Magellanic Clouds” of the southern hemisphere, which resemble detached portions of the Milky Way. These singular objects form as striking a peculiarity of the austral heavens as does the great “Coal-sack” described in Chapter 1. But it is their isolation that makes them so remarkable, for their composition is essentially galactic, and if they were included within its boundaries they would not appear more wonderful than many other parts of the Milky Way. Placed where they are, they look like masses fallen from the great stellar arch. They are full of nebulæ and star-clusters, and show striking evidences of spiral movement.

Star-swarms, which are also characteristic features of the Galaxy, differ from star-clouds very much in the way that their name would imply—i.e., their component stars are so arranged, even when they are countless in number, that the idea of an exceedingly numerous assemblage rather than that of a cloud is impressed on the observer’s mind. In a star-swarm the separate members are distinguishable because they are either larger or nearer than the stars composing a “cloud.” A splendid example of a true star-swarm is furnished by Chi Persei, in that part of the Milky Way which runs between the constellations Perseus and Cassiopeia. This swarm is much coarser than many others, and can be seen by the naked eye. In a small telescope it appears double, as if the suns composing it had divided into two parties which keep on their way side by side, with some commingling of their members where the skirts of the two companies come in contact.

Smaller than either star-clouds or star-swarms, and differing from both in their organization, are star-clusters. These, unlike the others, are found outside as well as inside the Milky Way, although they are more numerous inside its boundaries than elsewhere. The term star-cluster is sometimes applied, though improperly, to assemblages which are rather groups, such, for instance, as the Pleiades. In their most characteristic aspect star-clusters are of a globular shape—globes of suns! A famous example of a globular star-cluster, but one not included in the Milky Way, is the “Great Cluster in Hercules.” This is barely visible to the naked eye, but a small telescope shows its character, and in a large one it presents a marvelous spectacle. Photographs of such clusters are, perhaps, less effective than those of star-clouds, because the central condensation of stars in them is so great that their light becomes blended in an indistinguishable blur. The beautiful effect of the incessant play of infinitesimal rays over the apparently compact surface of the cluster, as if it were a globe of the finest frosted silver shining in an electric beam, is also lost in a photograph. Still, even to the eye looking directly at the cluster through a powerful telescope, the central part of the wonderful congregation seems almost a solid mass in which the stars are packed like the ice crystals in a snowball.

The same question rises to the lips of every observer: How can they possibly have been brought into such a situation? The marvel does not grow less when we know that, instead of being closely compacted, the stars of the cluster are probably separated by millions of miles; for we know that their distances apart are slight as compared with their remoteness from the Earth. Sir William Herschel estimated their number to be about fourteen thousand, but in fact they are uncountable. If we could view them from a point just within the edge of the assemblage, they would offer the appearance of a hollow hemisphere emblazoned with stars of astonishing brilliancy; the near-by ones unparalleled in splendor by any celestial object known to us, while the more distant ones would resemble ordinary stars. An inhabitant of the cluster would not know, except by a process of ratiocination, that he was dwelling in a globular assemblage of suns; only from a point far outside would their spherical arrangement become evident to the eye. Imagine fourteen-thousand fire-balloons with an approach to regularity in a spherical space—say, ten miles in diameter; there would be an average of less than thirty in every cubic mile, and it would be necessary to go to a considerable distance in order to see them as a globular aggregation; yet from a point sufficiently far away they would blend into a glowing ball.

Photographs show even better than the best telescopic views that the great cluster is surrounded with a multitude of dispersed stars, suggestively arrayed in more or less curving lines, which radiate from the principle mass, with which their connection is manifest. These stars, situated outside the central sphere, look somewhat like vagrant bees buzzing round a dense swarm where the queen bee is sitting. Yet while there is so much to suggest the operation of central forces, bringing and keeping the members of the cluster together, the attentive observer is also impressed with the idea that the whole wonderful phenomenon may be the result of explosion. As soon as this thought seizes the mind, confirmation of it seems to be found in the appearance of the outlying stars, which could be as readily explained by the supposition that they have been blown apart as that they have flocked together toward a center. The probable fact that the stars constituting the cluster are very much smaller than our sun might be regarded as favoring the hypothesis of an explosion. Of their real size we know nothing, but, on the basis of an uncertain estimate of their parallax, it has been calculated that they may average forty-five thousand miles in diameter—something more than half the diameter of the planet Jupiter. Assuming the same mean density, fourteen thousand such stars might have been formed by the explosion of a body about twice the size of the sun. This recalls the theory of Olbers, which has never been altogether abandoned or disproved, that the Asteroids were formed by the explosion of a planet circulating between the orbits of Mars and Jupiter. The Asteroids, whatever their manner of origin, form a ring around the sun; but, of course, the explosion of a great independent body, not originally revolving about a superior center of gravitational force, would not result in the formation of a ring of small bodies, but rather of a dispersed mass of them. But back of any speculation of this kind lies the problem, at present insoluble: How could the explosion be produced? (See the question of explosions in Chapters 6 and 14).

Then, on the other hand, we have the observation of Herschel, since abundantly confirmed, that space is unusually vacant in the immediate neighborhood of condensed star-clusters and nebulæ, which, as far as it goes, might be taken as an indication that the assembled stars had been drawn together by their mutual attractions, and that the tendency to aggregation is still bringing new members toward the cluster. But in that case there must have been an original condensation of stars at that point in space. This could probably have been produced by the coagulation of a great nebula into stellar nuclei, a process which seems now to be taking place in the Orion Nebula.

A yet more remarkable globular star-cluster exists in the southern hemisphere, Omega Centauri. In this case the central condensation of stars presents an almost uniform blaze of light. Like the Hercules cluster, that in Centaurus is surrounded with stars scattered over a broad field and showing an appearance of radial arrangement. In fact, except for its greater richness, Omega Centauri is an exact duplicate of its northern rival. Each appears to an imaginative spectator as a veritable “city of suns.” Mathematics shrinks from the task of disentangling the maze of motions in such an assemblage. It would seem that the chance of collisions is not to be neglected, and this idea finds a certain degree of confirmation in the appearance of “temporary stars” which have more than once blazed out in, or close by, globular star-clusters.

This leads up to the notable fact, first established by Professor Bailey a few years ago, that such clusters are populous with variable stars. Omega Centauri and the Hercules cluster are especially remarkable in this respect. The variables found in them are all of short period and the changes of light show a noteworthy tendency to uniformity. The first thought is that these phenomena must be due to collisions among the crowded stars, but, if so, the encounters cannot be between the stars themselves, but probably between stars and meteor swarms revolving around them. Such periodic collisions might go on for ages without the meteors being exhausted by incorporation with the stars. This explanation appears all the more probable because one would naturally expect that flocks of meteors would abound in a close aggregation of stars. It is also consistent with Perrine’s discovery—that the globular star clusters are powdered with minute stars strewn thickly among the brighter ones.

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.

The whole question of star-drift has lately assumed a new phase, in consequence of the investigations of Kapteyn, Dyson, and Eddington on the “systematic motions of the stars.” This research will, it is hoped, lead to an understanding of the general law governing the movements of the whole body of stars constituting the visible universe. Taking about eleven hundred stars whose proper motions have been ascertained with an approach to certainty, and which are distributed in all parts of the sky, it has been shown that there exists an apparent double drift, in two independent streams, moving in different and nearly opposed directions. The apex of the motion of what is called “Stream I” is situated, according to Professor Kapteyn, in right ascension 85°, declination south 11°, which places it just south of the constellation Orion; while the apex of “Stream II” is in right ascension 260°, declination south 48°, placing it in the constellation Ara, south of Scorpio. The two apices differ very nearly 180° in right ascension and about 120° in declination. The discovery of these vast star-streams, if they really exist, is one of the most extraordinary in modern astronomy. It offers the correlation of stellar movements needed as the basis of a theory of those movements, but it seems far from revealing a physical cause for them. As projected against the celestial sphere the stars forming the two opposite streams appear intermingled, some obeying one tendency and some the other. As Professor Dyson has said, the hypothesis of this double movement is of a revolutionary character, and calls for further investigation. Indeed, it seems at first glance not less surprising than would be the observation that in a snow-storm the flakes over our heads were divided into two parties and driving across each other’s course in nearly opposite directions, as if urged by interpenetrating winds.

But whatever explanation may eventually be found for the motions of the stars, the knowledge of the existence of those motions must always afford a new charm to the contemplative observer of the heavens, for they impart a sense of life to the starry system that would otherwise be lacking. A stagnant universe, with every star fixed immovably in its place, would not content the imagination or satisfy our longing for ceaseless activity. The majestic grandeur of the evolutions of the celestial hosts, the inconceivable vastness of the fields of space in which they are executed, the countless numbers, the immeasurable distances, the involved convolutions, the flocking and the scattering, the interpenetrating marches and countermarches, the strange community of impulsion affecting stars that are wide apart in space and causing them to traverse the general movement about them like aides and despatch-bearers on a battle-field—all these arouse an intensity of interest which is heightened by the mystery behind them.