140 THE EVOLUTION OF 



have appeared. With one possible exception the shortest 

 period so far recognized among double stars whose com- 

 ponents are separately visible in the telescope is that of 

 8 Equulei, which is completed in 5-7 years. From this 

 evidence there can be no doubt that if it were possible to 

 effect further refinement in the construction of telescopes 

 still closer binaries with shorter periods would be discov- 

 ered. We shall presently find very complete evidence of 

 the existence of binary systems, consisting of pairs of 

 stars so close as to lie far beyond the possibility of direct 

 observation. 



Before proceeding, however, it will be of interest to 

 inquire how we are to interpret the phenomenon of a 

 double star. 



When, as a consequence of the establishment of the 

 Copernican system of astronomy, it was clearly recognized 

 that the stars were suns, it was confidently believed that 

 each was, like the Sun, the centre of a planetary system 

 of its own, giving life to a system of revolving worlds. It 

 is difficult, however, to find a place for a double star in 

 such a scheme. It seems impossible for a planet to ac- 

 company a pair of suns, and at the same time to follow 

 so regular an orbit as to experience the slightly varying 

 conditions of temperature that are necessary to maintain 

 life. While but few double stars were known it was 

 possible to regard them as celestial monstrosities, objects 

 out of accord with the harmonious universe in which they 

 found a place, but in view of the increasing numbers that 

 have been discovered, their exceptional character can no 

 longer be maintained. A recent high authority, has gone 

 so far as to regard a double star as the normal condition 

 of stellar existence, and single stars, like the Sun, as 

 exceptions. 



How then have double stars come into existence ? A 



