Popular Science Monthly/Volume 1/September 1872/Motions of the Stars
AT the last meeting of the Royal Astronomical Society, Dr. Huggins, the eminent spectroscopist, made an extraordinary statement respecting the motions taking place among the stars. The results he announces are so wonderful that it will be well briefly to explain how they have been obtained, as well as their relation to what had formerly been known upon the subject.
Our readers are doubtless aware that the stars are not really fixed, but are known to be travelling swiftly through space. To ordinary observation the stars seem unmoving; nor indeed can the astronomer recognize any signs of motion save by prolonged observation. But, if the exact place of a star be carefully determined at any time, and again many years later, a measurable displacement can be recognized; year after year, and century after century, the motion thus determined proceeds, until at length the star may be removed by a considerable arc (or what is so regarded by astronomers accustomed to deal with the minutest displacements) from the position it had formerly occupied.
But, in general, these movements afford no means of estimating the real rate at which the stars are travelling through space. In the first place, a star might be moving with enormous rapidity toward or from the earth, and yet seem to be quite fixed on the star-vault—just as the light of a rapidly approaching or receding train seems to occupy an unchanging position if the train's course is at the moment in the direction of the line of sight. It is only what may be called the thwart-motion of the star that the astronomer can recognize by noting stellar displacements. But even this motion he cannot estimate—in miles per second, say—unless he knows how far off the sun is; and astronomers know in truth very little about stellar distances.
Now, it seems, at first sight, altogether hopeless to attempt to measure the rate at which a star is approaching or receding. No change of brightness could be looked for, nor indeed could any observed change be trusted as an evidence of changed distance, since stars are liable to real changes of brilliancy, much as our own sun is liable to be more or less spot-marked. But the distances of the stars are so enormous that no conceivable rate of approach or recession could affect their brilliancy discernibly. Only the most rapid thwart-motions yet recognized would carry a star over a space equal to the moon's seeming diameter in 500 years, so that a corresponding motion of recess or approach would only change a star's distance to about the same relative extent, and it is obvious that such a change could not make a star, even in that long period, change appreciably in brightness.
It will seem, then, utterly incredible that astronomers have learned not merely whether certain stars are receding or approaching, but have actually been enabled to determine respecting this kind of motion what they cannot determine respecting the more obvious thwart-motion, viz., the rate at which the motion takes place.
This is rendered possible by what is known of the nature of light. Light travels through space in waves, not as a direct emanation. Now, let us compare a star's action in emitting such waves with some known kind of wave-action, and we shall at once recognize the effects of very rapid motion on the star's part. Conceive a fixed paddle-wheel turning at a uniform rate in water, and that every blade as it reaches the water raises one wave, that wave being transmitted in a given direction. Then there would be a succession of waves separated from each other by a constant distance. But, suppose the paddle-wheel itself to be carried in the given direction. It is clear that, after one blade has raised its wave, the next blade, descending in the same time as before, will reach the water closer to the preceding wave than if the paddle-wheel had been at rest; for the moving wheel will have carried the blade closer, so that now a succession of waves will result as before, but they will have their crests closer together. And obviously, if the wheel were carried in the contrary direction, the wave-crests would be farther apart than if the wheel had been at rest.
Thus, reverting to the stars, we infer that if a star is approaching, the light which comes to us from it will have its waves closer together than if the star were at rest, and vice versa. Now, the distance between the wave-crests of light signifies a difference of color, the longer waves producing red and orange light; waves of medium length, yellow and green light; and the shorter waves producing blue, indigo, and violet light. So that, if a star were shining with pure red light, it might by approaching very rapidly be caused to appear yellow, or even blue or indigo, according to the rate of approach; while, if a star were shining with pure indigo light, it might by receding very rapidly be caused to appear green or yellow, or even orange or red.
But stars do not shine with pure-colored light, but with a mixture of all the colors of the rainbow; so that the attempt to estimate a star's rate of approach or recession by its color would fail, even though we knew the star's real color, and even though stars moved fast enough to produce color-changes. The spectroscopist has, however, a much more delicate means of dealing with the matter. The rainbow-tinted streak forming a star's spectrum is crossed by known dark lines; and these serve as veritable mile-marks for the spectroscopist. If one of these lines in the spectrum of any star is seen to be shifted toward the red end, the observer knows that the star is receding, and that swiftly; if the shift is toward the violet end, he knows that the star is swiftly approaching
Now, Dr. Huggins had been able nearly four years ago to apply this method to the case of the bright star Sirius, though his instrumental means were not then sufficient to render him quite certain as to the result. Still he was able to announce with some degree of confidence that Sirius is receding at a rate exceeding 20 miles per second. In order that he might extend the method to other stars, the Royal Society placed at his disposal a fine telescope, 15 inches in aperture, and specially adapted to gather as much light as possible with that aperture. Suitable spectroscopic appliances were also provided for the delicate work Dr. Huggins was to undertake. It was but last winter that the instrument was ready for work; but already Dr. Huggins has obtained the most wonderful news from the stars with its aid. He finds that many of the stars are travelling far more swiftly than had been supposed. Arcturus, for example, is travelling toward us at the rate of some 50 miles per second, and, as his thwart-motion is fully as great (for this star's distance has been estimated), the actual velocity with which he is speeding through space cannot be less than 70 miles per second. Other stars are moving with corresponding velocities.
But, amid the motions thus detected, Dr. Huggins has traced the signs of law. First he can trace a tendency among the stars in one part of the heavens to approach the earth, while the stars in the opposite part of the heavens are receding from us; and the stars which are approaching lie on that side of the heavens toward which Herschel long since taught us that the sun is travelling. But there are stars not obeying this simple law; and among these Dr. Huggins recognizes instances of that community of motion to which a modern student of the stars has given the name of star-drift. It happens, indeed, that one of the most remarkable of these instances relates to five well-known stars, which had been particularly pointed to as forming a drifting set. It had been asserted more than two years ago that certain five stars of the Plough or Charles's Wain—the stars known to astronomers as Beta, Gamma, Delta, Epsilon, and Zeta, of the Great Bear—are drifting bodily through space. The announcement seemed to many very daring, yet its author (trusting in the mathematical analysis of the evidence) expressed unquestioning confidence; he asserted, moreover, that whenever Dr. Huggins applied the new method of research, he would find that those five stars are either all approaching or all receding, and at the same rate, from the earth. The result has justified his confidence as well in his theory as in Dr. Huggins's mastery of the new method. Those five stars are found to be all receding from the earth at the rate of about 30 miles per second.
This result at once illustrates the interesting nature of Dr. Huggins's discoveries, and affords promise of future revelations even more interesting. The theories hitherto accepted respecting the constitution of the stellar universe have been tried against the views recently propounded, with a result decidedly in favor of the latter. We may feel assured that the matter will not rest here. A simple and decisive piece of evidence, such as that we have described, will invite many to examine afresh the theories respecting the stellar heavens which have so long been received unquestioningly. The theory of star-drift is associated with others equally novel, and which admit equally well of being put to test. We venture to predict that, before many years have passed, there will be recognized in the star-depths a variety of constitution and a complexity of arrangement startlingly contrasted with the general uniformity of structure recognized in the teachings at present accepted.—Spectator.