Science

IN 1835 Auguste Comte observed that there will probably never be any such thing as a science of sidereal astronomy; for without measuring the distances of the stars and ascertaining their actual motions, we cannot even be sure that the law of gravitation prevails beyond the limits of our solar system ; and up to 1835 all attempts at measuring stellar distances had proved abortive. In less than four years, however, the first step was taken toward founding a science of stellar astronomy, when Bessel ascertained the distance of the star 61 Cygni. The obstacles which he overcame were such as might well have seemed insurmountable, and it is but slowly and with uncertain steps that astronomers have been able to extend his researches. The measurement of the distance of 61 Cygni — the nearest star in the northern heavens — was indeed followed by the determination of its real transverse motion, which was found to amount to about fourteen hundred and fifty millions of miles per annum, or about forty miles per second. But a single fact like this in reality tells us next to nothing concerning the structure of the sidereal system, and the hosts of similar facts which we need are extremely hard to gather. Without knowing the distances of the stars, we cannot translate their apparent motions into their real motions ; and of the five thousand eight hundred and fifty lucid stars which can be seen by the naked eye, there are not more than twenty of which we have as yet been able to estimate the distance, while in only ten or eleven cases has the distance been satisfactorily determined. In the next place, even when we have obtained the amount of a star’s real transverse motion, as in the case of 61 Cygni, we have got but half-way toward a knowledge of its total actual motion. For obviously the star’s apparent transverse motion may be due either to a real motion which is at right angles to the line joining the earth and the star, or — which must be much oftener the case — it will be due to a real motion in a diagonal direction as referred to the same line. In the latter case we can ascertain but one of the elements of the diagonal motion, that, namely, which may be called the thwartmotion. The other element, namely, the motion towards the earth or away from it, cannot of course be determined by observation, as it does not affect the star’s apparent motion. When a railway-train at a long distance is directly approaching us, it has no apparent motion, it seems to stand still ; and if it is approaching us diagonally, all that part of its motion which is bringing it nearer to us is subtracted from the apparent motion it would have if viewed from a point at right angles to the track upon which it is running. In short, it is only thwart-motion which can make any object directly inform our eyes that it is changing its place.

This statement, however, is not rigorously correct. If a train is coming directly toward us as we stand in the depot, its approach is directly determinable, partly from comparison with neighboring fixed objects, partly from the increased size of its image upon the retina. When we look at the stars, however, we do not have these helps to our vision. If a given star is coming straight toward us at the rate of forty miles per second, a comparison with neighboring stars will not help us at such an enormous distance. And, as for intensified retinal impression, if a star were to move toward us at this rate of speed, a thousand years would hardly suffice to make an alteration of two per cent in its brilliancy.

To measure the rate of direct approach or recession of a star would, therefore, seem to be forever impossible. And as this element of direct approach or recession must enter into the motion of all stars whatever save the few which may be supposed to move precisely at right angles with reference to the earth’s position, it would appear to become questionable whether we can ever get any available knowledge of the behavior of the bodies constituting the stellar universe. So that Auguste Comte might seem, after all, to be justified in his bold assertion, that we are never to have a science of stellar astronomy.

But it is rash to seek to set limits to the possibilities of human achievement. Here, from the quarter least expected, a ray of the needed light has been shed upon this most difficult problem. As the doe, in the old fable, keeping her sound eye landward, was at last shot by archers passing in a boat, so Nature has here been forced to render up her secret in the most unlookedfor way. Through the amazing results obtained by spectrum analysis it has turned out that our heavier difficulty has become the lighter one, and that the direct approach or recession of a star is actually easier to measure than its thwart-motion ! Properly to exhibit the character of this most brilliant discovery, we must beg leave to recall to the reader a few considerations of which he may not otherwise bethink himself.

If the waves of a river, blown by the wind, are travelling in the same direction in which a swimmer is swimming, it is obvious that the white crests of any two consecutive waves will seem to the swimmer to follow each other more slowly than would be the case if he were swimming in a direction parallel to the direction of the wave-fronts ; while, if he were swimming in a direction contrary to the motion of the waves, the crests would pass him with an apparent increase of speed. And manifestly, the more rapidly the waves appeared to pass, the narrower would the waves seem to be ; and the slower the waves, the broader would they seem from crest to crest. Now the case is essentially the same whether the waves in question be those of a river, or those atmospheric waves which are known as sound, or those molecular waves which are known as light. Sound-waves travel, of course, much faster than the waves of water, namely, twelve hundred feet per second. Yet if from any source a sound of uniform pitch be emitted, the pitch of the sound will nevertheless seem to vary when the hearer approaches or recedes from the source of the sound, provided his rate of movement bears some appreciable ratio to the speed of the sound-waves. It is thus that when a railway-train is rapidly approaching us, its steam-whistle, though maintaining a uniform pitch, yet seems to sink by a fifth or an octave.

In the case of a star moving directly toward us or away from us, the same principle holds good ; yet it would seem impossible that the motion of a star, however rapid, could bear any appreciable proportion to the enormous velocity of light. A speed of forty miles per second, when compared with the speed of light, is as that of a snail to that of an express-train. Nevertheless, when we consider the inner structure of a ray of light, we find that assistance is obtainable from this quarter. It was some time since suggested by M. Doppler, that the color of a star may depend upon its motion. If a star can be supposed to approach us so swiftly that the red waves emitted by it will be so shortened as to produce upon our retinas the effect of orange light, and so on through the spectrum, then the star’s change of color, resulting from the loss of its red rays, will afford an index of the rate of speed at which it is nearing us. But this reasoning is fallacious, because it loses sight of an essential fact. Below the extreme red rays of the spectrum, there are numberless dark rays, with wave-lengths greater than are cognizable by our retinas. Obviously the general shortening of wavelengths occasioned by the star’s swift approach would so shorten these dark waves as to enable them to produce in us the sensation of red light, so that the color of the star would remain unaltered. And in the case of a receding star the result would be the same, since the lengthening of the waves answering to dark rays above the violet end of the spectrum would keep up the supply of the violet portion of the star’s light.

We cite this erroneous suggestion of M. Doppler, because it has proved suggestive of truth by reason of its very erroneousness, and because the reader, having duly considered it, will the better understand the ground upon which rests Mr. Huggins’s magnificent discovery. Though it is not true that the color of a distant object will change with its motion towards or away from the observer, it nevertheless happens that the lines of the spectrum cast by the object will be shifted in position as the object draws near or recedes. Here, as Mr. Proctor observes, in his lately published volume of “ Essays on Astronomy,” “ we have at once a most delicate means of detecting stellar movements of approach or recession. If in the spectrum of a star we can see a recognizable group of lines, or a line recognizable by its strength, and if in any way we can prove that this line does not hold the exact position which it has in the solar spectrum, then the change of position must be looked upon as due to the star’s motion towards or from the earth. The shifting of the spectrum bodily, which produces no change whatever in the star’s color, brings all the lines into new positions, and any one line, marked enough for ready examination, suffices as well as a hundred to determine the existence of such a change.” The rapid approach of a star, causing a general diminishing of wave-lengths, shifts the lines upward toward the violet end of the spectrum ; while when the star is swiftly receding, the lines are shifted down toward the red end. Four years ago Mr. Huggins, working upon these general principles, found that the F line in the spectrum of Sirius was displaced downwards, being removed by about one two hundred and fiftieth of an inch from the corresponding line in the spectrum of hydrogen. From this accurately measured displacement Mr. Huggins was enabled to calculate a recession of rather more than forty-one miles per second; which leads to the result, after taking into account the earth’s own motions, that Sirius is travelling away from us at the rate of twenty and five tenths miles per second.

Now Sirius does not move directly away from us, but diagonally, and as this is one of the few stars whose distance has been measured, we are able also to calculate its transverse motion ; so that, as a final result, we find that the star is moving through space in a given direction at the rate of thirty-three miles per second. In one case, then, the problem of stellar motion was solved by Mr. Huggins’s discovery of 1868.

But during the present year Mr. Huggins has obtained a large number of striking facts by this same ingenious method of research. He finds, for example, that Arcturus is approaching us at the rate of fifty miles per second, with a thwart-motion of about the same speed. And, having with equal success measured the motions of many other stars, he observes a general tendency among the stars in one portion of the heavens to approach the solar system, while in the opposite quarter the stars are receding from us. Moreover, the results obtained by Mr. Huggins throw welcome light upon the lately noticed phenomena of star-drift. For example, it is shown that five stars in the constellation of the Great Bear are all receding from the solar system at the common rate of thirty miles per second.

In view of these results, — obtained by attacking the great problem upon what was apparently its least vulnerable side, — we may reasonably hope erelong to be in possession of data sufficient to constitute a genuine science of sidereal dynamics. It can hardly be doubted that, with the aid of Mr. Huggins’s marvellous disclosures, and of other discoveries to be obtained by the prosecution of the same method, we shall eventually know enough of the true motions of the stars to be able to frame some theory of the mutual relations among the forces which impel this stupendous system of worlds. Already there begins to be detected a complexity of arrangement and behavior among the members of the starry universe sufficient to carry our speculations far beyond the point at which they were left by the painstaking researches of the elder and the younger Herschel.