362 



THE STELLAR UNIVERSE. 



DOUBLE STARS. 



When the stars are examined individually by telescopes of a certain power, 

 it is found that many which to the naked eye appear to be single stars, are in 

 reality two stars placed so close together that they appear as one. These are 

 called double stars. 



A very limited number of these objects had been discovered before the 

 telescope had received the vast accession of power which was given to it by 

 the labor and genius of Sir William Herschel. That astronomer observed and 

 catalogued five hundred double stars, and subsequent observers, among whom 

 his son, Sir John Herschel, holds the foremost place, have augmented the 

 number to six thousand. 



The close apparent juxtaposition of two stars on the firmament is a phe- 

 nomenon which might be easily explained, and which could create no surprise. 

 Such an appearance would be produced by the accidental circumstance of the 

 lines of direction of the two stars as seen from the earth, forming a very small 

 angle, in which case, although the two stars might in reality be as far removed 

 from each other as any stars in the heavens, they would nevertheless appear 

 close together. The annexed diagram, fig. 1, will render this easily under- 

 stood. Let a and b be the two stars seen from c. The star a will be seen 

 relatively to b, as if it were at d, and the two objects will seem to be in close 

 juxtaposition, and if the angle under the lines c a and c b be less than the 

 sum of the apparent semi-diameters of the stars, they would actually appear 

 to touch. 



Fig. 1. 



If such objects were few in number, this mode of explaining them might be 

 admitted ; and such may in fact be the cause of the phenomenon in some in- 

 stances. The chances against such proximity of the lines of direction are so 

 great as to be utterly incompatible with the vast number of double stars that 

 have been discovered, even were there not, as there is, other conclusive proof 

 that this proximity and companionship is neither accidental nor merely ap- 

 parent, but that the connexion is real, and that the objects are united by a 

 physical bond analogous to that which attaches the planets to the sun. 



Among the most striking examples of double stars, may be mentioned the 

 bright star Castor, which, when sufficiently magnified, is proved to consist of 

 two stars between the third and fourth magnitudes, within five seconds of each 

 other. There are many, however, which are separated by intervals less than 

 one second, such as c Arietis, Atlas Pleiadum, y Corona, v and £ Herculis, and 

 r and * Ophiuchi. 



The telescopic appearance of double stars may be conceived from diagrams 

 of some of the more remarkable of the class, which have been given by Dr. 

 Dick, in his work on the heavens. 



Fig. 2 represents a telescopic view of * Bootis, with a magnifying power 

 of 200. This is considered to be a very beautiful double star, consisting of a 

 small and large one, the former blue, and the latter red. The smaller star ap- 

 pears about one third of the size of the larger, and separated from it by a space 

 equal to the diameter of the larger star. 



