CHAPTERS ON THE STARS. 509 



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 uni- 

 formly 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. Madler thought that he had obtained evi- 

 dence 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 cen- 

 turies of observation. 



Another example of the same sort is seen in five out of the seven 

 stars of Ursa? Major, or The Dipper. The stars are those lettered ft, y, 

 S, £ and 3. All five have a proper motion in E. A. of nearly 8" 

 per century, while in declination the movements are some- 

 times positive and sometimes negative; that is to say, some of 

 the stars are apparently lessening their distance from the pole, while 



