Sirius is in that portion of the heavens _from_ which the sun is advancing. Its proper motion is such that it is gaining upon the earth at the rate of about ten miles per second, and so it must overtake the sun after the lapse of great ages. Vega, on the other hand, is coming towards us from that part of the sky _towards_ which the sun is travelling. It should be about half a million years before the sun and Vega pass by one another. Those who have specially investigated this question say that, as regards the probability of a near approach, it is much more likely that Vega will be then so far to one side of the sun, that her brightness will not be much greater than it is at this moment. Considerations like these call up the chances of stellar collisions. Such possibilities need not, however, give rise to alarm; for the stars, as a rule, are at such great distances from each other, that the probability of relatively near approaches is slight. We thus see that the constellations do not in effect exist, and that there is in truth no real background to the sky. We find further that the stars are strewn through space at immense distances from each other, and are moving in various directions hither and thither. The sun, which is merely one of them, is moving also in a certain direction, carrying the solar system along with it. It seems, therefore, but natural to suppose that many a star may be surrounded by some planetary system in a way similar to ours, which accompanies it through space in the course of its celestial journeyings. [28] Vega, for instance, shines one hundred times more brightly than the sun would do, were it to be removed to the distance at which that star is from us. CHAPTER XXIII THE STARS--_continued_ The stars appear to us to be scattered about the sky without any orderly arrangement. Further, they are of varying degrees of brightness; some being extremely brilliant, whilst others can but barely be seen. The brightness of a star may arise from either of two causes. On the one hand, the body may be really very bright in itself; on the other hand, it may be situated comparatively near to us. Sometimes, indeed, both these circumstances may come into play together.