I have already mentioned that Sirius, though it seems, like other stars, so stationary that we speak of them as "fixed," is really sweeping along at the rate of 1000 miles a minute. Even this enormous velocity is exceeded in other cases. One, which is numbered as 1830 in Groombridge's Catalogue of the Stars, and is therefore known as "Groombridge's 1830," moves no less than 12,000 miles a minute, and Arcturus 22,000 miles a minute, or 32,000,000 of miles a day; and yet the distances of the Stars are so great that 1000 years would make hardly any difference in the appearance of the heavens.
Changes, however, there certainly would be. Even in the short time during which we have any observations, some are already on record. One of the most interesting is the fading of the 7th Pleiad, due, according to Ovid, to grief at the taking of Troy. Again, the "fiery Dogstar," as it used to be, is now, and has been for centuries, a clear white.
The star known as Nova Cygni—the "new star in the Constellation of the Swan"—was first observed on the 24th November 1876 by Dr. Schmidt of Athens, who had examined that part of the heavens only four days before, and is sure that no such star was visible then. At its brightest it was a brilliant star of the third magnitude, but this only lasted for a few days; in a week it had ceased to be a conspicuous object, and in a fortnight became invisible without a telescope. Its sudden splendour was probably due to a collision between two bodies, and was probably little, if at all, less than that of the Sun itself. It is still a mystery how so great a conflagration can have diminished so rapidly.
But though we speak of some stars as specially variable, they are no doubt all undergoing slow change. There was a time when they were not, and one will come when they will cease to shine. Each, indeed, has a life-history of its own. Some, doubtless, represent now what others once were, and what many will some day become.
For, in addition to the luminous heavenly bodies, we cannot doubt that there are countless others invisible to us, some from their greater distance or smaller size, but others, doubtless, from their feebler light; indeed, we know that there are many dark bodies which now emit no light, or comparatively little. Thus in the case of Procyon the existence of an invisible body is proved by the movement of the visible star. Again, I may refer to the curious phenomena presented by Algol, a bright star in the head of Medusa. The star shines without change for two days and thirteen hours; then in three hours and a half dwindles from a star of the second to one of the fourth magnitude; and then, in another three and a half hours, reassumes its original brilliancy. These changes led astronomers to infer the presence of an opaque body, which intercepts at regular intervals a part of the light emitted by Algol; and Vogel has now shown by the aid of the spectroscope that Algol does in fact revolve round a dark, and therefore invisible, companion. The spectroscope, in fact, makes known to us the presence of many stars which no telescope could reveal.
Thus the floor of heaven is not only "thick inlaid with patines of bright gold," but studded also with extinct stars, once probably as brilliant as our own Sun, but now dead and cold, as Helmholtz tells us that our Sun itself will be some seventeen millions of years hence.
Such dark bodies cannot of course be seen, and their existence, though we cannot doubt it, is a matter of calculation. In one case, however, the conclusion has received a most interesting confirmation. The movements of Sirius led mathematicians to conclude that it had also a mighty and massive neighbour, the relative position of which they calculated, though no such body had ever been seen. In February 1862, however, the Messrs. Alvan Clark of Cambridgeport were completing their 18-inch glass for the Chicago Observatory. "'Why, father,'" exclaimed the younger Clark, "'the star has a companion.' The father looked, and there was a faint star due east from the bright one, and distant about ten seconds. This was exactly the predicted direction for that time, though the discoverers knew nothing of it. As the news went round the world many observers turned their attention to Sirius; and it was then found that, though it had never before been noticed, the companion was really shown under favourable circumstances by any powerful telescope. It is, in fact, one-half of the size of Sirius, though only 1/10000th of the brightness."[72]
Stars are, we know, of different magnitudes and different degrees of glory. They are also of different colours. Most, indeed, are white, but some reddish, some ruddy, some intensely red; others, but fewer, green, blue, or violet. It is possible that the comparative rarity of these colours is due to the fact that our atmosphere especially absorbs green and blue, and it is remarkable that almost all of the green, blue, or violet stars are one of the pairs of a Double Star, and in every case the smaller one of the two, the larger being red, orange, or yellow. One of the most exquisite of these is β Cygni, a Double Star, the larger one being golden yellow, the smaller light blue. With a telescope the effect is very beautiful, but it must be magnificent if one could only see it from a lesser distance.
Double Stars occur in considerable numbers. In some cases indeed the relation may only be apparent, one being really far in front of the other. In very many cases, however, the association is real, and they revolve round one another. In some cases the period may extend to thousands of years; for the distance which separates them is enormous, and, even when with a powerful telescope it is indicated only by a narrow dark line, amounts to hundreds of millions of miles. The Pole Star itself is double. Andromeda is triple, with perhaps a fourth dark and therefore invisible companion. These dark bodies have a special interest, since it is impossible not to ask ourselves whether some at any rate of them may not be inhabited. In ε Lyræ there are two, each again being itself double. ξ Cancri, and probably also θ Orionis, consist of six stars, and from such a group we pass on to Star Clusters in which the number is very considerable. The cluster in Hercules consists of from 1000 to 4000. A stellar swarm in the Southern Cross contains several hundred stars of various colours, red, green, greenish blue, and blue closely thronged together, so that they have been compared to a "superb piece of fancy jewellery."[73]
The cluster in the Sword Handle of Perseus contains innumerable stars, many doubtless as brilliant as our Sun. We ourselves probably form a part of such a cluster. The Milky Way itself, as we know, entirely surrounds us; it is evident, therefore, that the Sun, and of course we ourselves, actually lie in it. It is, therefore, a Star Cluster, one of countless numbers, and containing our Sun as a single unit.