Radial Motion of the Stars.— The most wonderful revelations of the spectroscope re late to the proper motions of the stars and lead to the measurement of the speed with which a star is moving to or from our system. This speed is called its radial motion. The possibil ity of determining it rests on the principle that the wave-length of light coming from a star is less when the star is approaching us and greater when the star is moving away from us. Hence, if, on measuring the refraction of a particular ray emitted by a star, we find it to be greater than its regular value, we know that the star is receding from us, and, in the contrary case, that it is approaching us.
It is very interesting that these measures agree as nearly as we could expect with obser vations on the position of the stars as to the direction in which our solar system is moving. Moreover, they have enabled Campbell to de termine with an approximation to certainty the speed of the motion, which is about 11 miles per second. The whole solar system, and our earth with it, has, therefore, been pursuing a journey through space during the whole of human history, nay for hundreds of thousands, possibly for millions of years. Even with this rapid and continuous motion it will probably take us half a million of years to reach the region in which Vega, file brightest star in the constellation Lyra, is now situated.
The proper motions of the stars naturally suggest the idea that these objects all have their proper orbits and are performing revolutions round some centre as the planets do around the sun. The idea of a universe of which this might be true—of a system in which the years were counted by millions of our years is very cap tivating. But truth compels us to say that up to the present time astronomical science fur nishes no sound basis for the reality of such a system: no resemblance of this sort can be traced Moreover the general trend of science is toward the conclusion that the duration of the universe, though very long, is finite and is not sufficient to enable a star to have made many revolutions round any centre.
Double Stars.—A careful watcher of the heavens will notice that there are several pairs of stars in the heavens— two such objects being so close together as to suggest that they must have some connection. The most striking case of this sort is in the Hyades, a V-shapel fig ure having the bright red star Aldebaran at one end of the W.* In the middle of one of the lines of the W' are two stars of the third or fourth magnitude, forming a very beautiful pair. This pair is easily to be seen in some part of the sky on any clear evening in autumn or winter. Another case of the same sort is
that of one of the brightest stars in the constel lation Capricornus, visible in summer and au tumn, which has a much fainter star alongside of it. A third case is in the constellation Lyra, one of whose stars of the fourth magnitude can be seen by the naked eye to be composed of two separate stars. But only a very good eye can distinguish them from a single star.
When the heavens are scanned with a tele scope, it is found that a great many stars which seem single to the eye really consist of two such objects in close proximity. Sometimes these are of the same brightness, but more com monly one is much brighter than the other. Very frequently the small companion is so faint and so close to the large one as to be lost in the glare of its light, except when very care fully examined by an experienced observer. Such pairs are called double stars.
The first question suggested by these objects is whether the two stars appear in such close proximity merely because they chance to be on the same straight line from us, or whether they are really in each other's neighborhood. The latter is known to be the case, for two reasons. In the first place these double stars are vastly more numerous than they would be if their proximity were merely accidental. Another conclusive proof is afforded by the fact that in many such cases the two stars are found to be revolving round each other, or the small one round the large one, as the case may be. The name Binary System is applied to such pairs. The most important and interesting conclusion drawn from the motions of these systems is that the law of gravitation extends to the stars. It is true that we could scarcely doubt such to be the case, even were no such proof available, but it is nevertheless of interest to have inde pendent evidence of it. The revolution of the iwo stars round each other is in all respects similar to that of the earth and planets round the sun, or that of the moon round the earth. The time occupied in one revolution may be but a few decades, and in one instance it is but 5.7 years, yet in the vast majority of cases, hundreds or even thousands of years are re quired. As exact observations have only been made on these bodies since the time of Sir Wil liam Herschel, there are less than 100 of which the orbits and periods are accurately deter mined. Thus although the whole number known amounts to upward of 20,000 the deter mination of the orbits goes on very slowly.