DOUBLE STARS 139 



with the major axis of the relative ellipse, though not in 

 simple proportion. This law is illustrated in the increas- 

 ing lengths of the periods of revolution of the planets of 

 the solar system with their increasing distances from the 

 Sun. The period also depends upon the masses of the 

 stars. From these considerations we should expect that, 

 though irregularities would appear due to different systems 

 of double stars possessing different masses, upon the 

 whole, the closer the components of a double star, the 



labs. 



FIG. 4. Orbits of Sirius and companion. 



more rapid would be its revolution, and this is actually 

 found to be the case. The first binary systems discovered, 

 being naturally those in which the components are widely 

 separated, had long periods of revolution, and in many 

 instances no trace of relative motion has so far become 

 apparent, though other considerations leave little doubt 

 that the systems concerned are true binaries. With in- 

 creasing telescopic power, double stars with closer and 

 closer components have been recognized, and with these 

 discoveries correspondingly shorter periods of revolution 



