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Page:Popular Science Monthly Volume 57.djvu/519

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though this motion is, it would require nearly 150,000 years for the star to. make a complete circuit of the heavens if it moved round the sun uniformly at its present rate.

The fact that the stars move suggests a very natural analogy to the solar system. In the latter a number of planets revolve round the sun as their center, each planet continually describing the same orbit, while the various planets have different velocities. Around several of the planets revolve one or more satellites. Were civilized men ephemeral, observing the planets and satellites only for a few minutes, these bodies would be described as having proper motions of their own, as we find the stars to have. May it not then be that the stars also form a system; that each star is moving in a fixed orbit performing a revolution around some far-distant center in a period which may be hundreds of thousands or hundreds of millions of years? May it not be that there are systems of stars in which each star revolves around a center of its own while all these systems are in revolution around a single center?

This thought has been entertained by more than one contemplative astronomer. Lambert's magnificent conception of system upon system will be described hereafter. Mädler thought that he had obtained evidence of the revolution of the stars around Alcyone, the brightest of the Pleiades, as a center. But, as the proper motions of the stars are more carefully studied and their motion and direction more exactly ascertained, it becomes very clear that when considered on a large scale these conceptions are never realized in the actual universe as a whole. But there are isolated cases of systems of stars which are shown to be in some way connected by their having a common proper motion. We shall mention some of the more notable cases.

The Pleiades are found to move together with such exactness that up to the present time no difference in their proper motions has been detected. This is true not only of the six stars which we readily see with the naked eye, but of a much larger number of fainter ones made known by the telescope. It is an interesting fact, however, that a few stars apparently within the group do not partake of this motion, from which it may be inferred that they do not belong to the system. But there must be some motion among themselves, else the stars would ultimately fall together by their mutual attraction. The amount and nature of this motion cannot, however, be ascertained except by centuries of observation.

Another example of the same sort is seen in five out of the seven stars of Ursæ Major, or The Dipper. The stars are those lettered β, γ, δ, ε and ζ. All five have a proper motion in R. A. of nearly 8" per century, while in declination the movements are sometimes positive and sometimes negative: that is to say, some of the stars are apparently lessening their distance from the pole, while