416 BINARY SYSTEMS. [SECT, xxxvn. 



an unbroken course of extreme brilliancy forming some of 

 the most admirable clusters of stars in the heavens. One 

 round y Sagittarii is an intense aggregate of stars, in some 

 parts of which they are so crowded as to exceed enumera- 

 tion ; at a very moderate estimate Sir John Herschel thinks 

 this group cannot contain fewer than a hundred thousand 

 stars. Other two groups between the constellations of the 

 Shield and Ophiuchus stand out like promontories of intense 

 brilliancy in the dark space that separates the starry 

 streams of the Milky Way. 



The distance of the fixed stars is too great to admit of 

 their exhibiting a sensible disc, but they must be spherical 

 if gravitation pervades all space, as there is every reason to 

 believe it does. With a powerful telescope the stars look 

 like points of light, their occultations by the moon are there- 

 fore instantaneous. Their twinkling arises from sudden 

 changes in the refractive power of the air, which would not 

 be sensible if they had discs like planets. Thus nothing 

 can be known of their distance from us, and from one 

 another, by their apparent diameters. 



Thousands of stars that seem to be only brilliant points 

 of light, when carefully examined, are found to be in reality 

 systems of two or more suns, many of which are known to 

 revolve about one another. These binary and multiple stars 

 are extremely remote, requiring powerful telescopes to show 

 them separately. The first catalogue of double stars, in 

 which their places and relative positions are given, was ac- 

 complished by the talents and industry of Sir William Her- 

 schel, who made so many great discoveries, and with whom 

 the idea of their combination in binary and multiple 

 systems originated ; and this important point he established 

 by the determination of the motion of the two stars of 

 Castor or a Geminorum, the largest and finest of all the 

 double stars in the northern hemisphere. 



Since that time the motions and revolutions of 50 or 60 



