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.