The following is a list of the stars whose proper motions exceed four seconds in a cen tury. In the first column is given the name by which the star is commonly known; in the second and third its position; in the fourth its magnitude, and in the last its proper motion in a year It is very remarkable that only three of these stars are visible to the naked eye, and these three are below the fourth magnitude. It will be seen that the annual motion of the most rapidly moving star is less than 12 seconds annually. If this star were to continue its course round the sky without ever stopping, it would take it more than 105,000 years to make the circuit of the heavens.
Proper Motion of the Sun.—Our sun being one of the stars, we may expect that it also has its own proper motion. But this motion cannot be directly observed. The only way in which we can detect it is by the relative motion of the other stars, just as we know that a rail way train is carrying us along when we see houses and trees seeming to pass by us. The motion of the sun was detected by observing that, notwithstanding the varied directions in which the stars appear to us to be moving, there is an average tendency to move from a point in the constellation Lyra toward the opposite region of the heavens south of Sirius. These two opposite points are near the Milky Way, though not in it From this general community of motion we infer that our sun, carrying the earth and all the bodies of the solar system with it, is moving toward the constellation .Lyra. The point toward which the motion is directed is called the solar apex. It was formerly supposed to be situated in the constel lation Hercules, but more recent investigations have placed it farther to the east. Its position is still uncertain by several degrees, and there is some difference of opinion among astron omers as to its exact location. But the best re sults seem to show its position, as defined by right ascension and declination, to be about the following: Right Ascension. 18 h. 30 in. Declination, 35' north.
This point is about 4° south of the bright star Lyra. We may, therefore, say that our solar system is constantly journeying toward that region of the heavens where Lyra is situated.
Star Streaming.— The earlier determina tions of the point of the heavens toward which our sun is moving were all based on the as sumption that there are no widespread sys tematic motions among the stars themselves, that is, that when large numbers of them are considered any general backward drift of them all must be the effect solely of our own motion through space. If a considerable number of
them are moving, or "streaming? together, the determination will be rendered inexact by this fact, although it is undoubtedly true that the direction and velocity of our motion as just described are Very approximately correct. Several instances of star streaming have recently been discovered. Ludendorff has found that five of the bright stars of Ursa Major, though they appear to us widely separated in the sky, are moving together in parallel paths, and Hertzsprung has shown that the Dog Star, Sirius, though from our position it seems so far removed from the others, is also moving with them. The centre of this cluster is about 100 light years from us: Sirius, only 8.6 light years away, is thus far out of the centre.
An even more interesting example was dis covered by Boss, who found that 39 stars, lying principally in the constellation Taurus, and which appear scattered over a considerable area of the sky, do, in fact, form a globular cluster which is drifting through space with a velocity of about 25 miles a second. Some 800,000 years ago, this swarm passed nearest our earth, its centre then being 73 light years away. It is now about twice as far away from us as this and at the expiration of 65,000,000 years its 39 scattered bright stars will be so distant that they will appear to us as a faint, round cluster about two-thirds as large as the full moon.
A discovery of more widespread importance, however, is that of Kapteyn, who finds evidence of two very extensive systematic star drifts, the larger being directed toward the star Xi Orionis, and the other in an opposite direction. The cause of this extensive streaming is not known; it has been suggested that it may arise from a falling in toward the centre of the outer stars of the cloud which surrounds us, and which, therefore, may change its shape periodically through intervals of time so great that all other eras hitherto considered in astronomy shrink to nearly nothing in com parison.