DOUBLE STARS. 



stars whose existence has been ascertained by observation down to 

 the 7th magnitude inclusive, and supposing them to be scattered 

 fortuitously over the entire firmament, the chances against any 

 two of them having a position so close to each other as 4" would be 

 9570 to 1. But when this calculation was made, considerably 

 more than 100 cases of such duple juxta- position were ascertained 

 to exist. The same astronomer also calculated that the chances 

 against a third star falling within 32" of the first two would be 

 173524 to 1 ; yet the firmament presents at least four such triple 

 combinations. 



Among the most striking examples of double stars may be men- 

 tioned the bright star Castor, which, when sufficiently magnified, 

 is proved to consist of two stars between the third and fourth mag- 

 nitudes, within five seconds of each other. There are many, how- 

 ever, which are separated by intervals less than one second; 

 such as e Arietis, Atlas Pleiadum, y Coronce^ 77 and Herculis, 

 and r and \ Ophiuchi. 



51. Another argument against the supposition of mere fortuitous 

 optical juxta-position, unattended by any physical connection, is 

 derived from a circumstance which will be fully explained here- 

 after. Certain stars have been ascertained to have a proper 

 motion, that is, a motion exclusively belonging to each individual 

 star, in which the stars around it do not participate. Now, some 

 of the double stars have such a motion. If one individual of the 

 pair were affected by a proper motion, in which the other does not 

 participate, their separation at some subsequent epoch would 

 become inevitable, since one would necessarily move away from 

 the other. Now, no such separation has in any instance been 

 witnessed. It follows, therefore, that the proper motion of one 

 equally affects the other, and consequently, that their juxta-posi- 

 tion is real and not merely optical. 



52. The systematic observation of double stars, and their reduc- 

 tion to a catalogue with individual descriptions, commenced by 

 Sir "W. Herschel, has been continued with great activity and suc- 

 cess by Sir J. Herschel, Sir J. South, and Professor Struve, so that 

 the number of these objects now known, as to character and posi- 

 tion, amounts to several thousand, the individuals of each pair 

 being less than 32" asunder. They have been classed by Profes- 

 sor Struve according to their distances asunder, the first class 

 being separated by a distance not exceeding 1", the second 

 between 1" and 2", the third between 2" and 4", the fourth between 

 4" and 8", the fifth between 8" and 12", the sixth between 12" and 

 16", the seventh between 16" and 24", and the eighth between 24" 

 and 32". 



53. The double stars in the following table have been selected 



205 



