THE EVOLUTION OF DOUBLE STARS 133 



the pair are known as a binary star, and binary stars are 

 the only true doubles, or they may actually be very far 

 apart, their seeming proximity being apparent only, and 

 due to their lying by chance nearly in the same direction 

 in space. In this case they are said to form an optical 

 double^ and have no interest in the present study. The 

 two conditions are illustrated in figs, i and 2. If it were 



FIG. i. An optical double. 



possible to measure the distances of the components of a 

 double star from the Earth, it would obviously be possible 

 to determine in any case to which of the two classes it 

 belonged, but unfortunately it is only in rare instances 

 that the measurement of the distance of a star is possible, 

 and in no case has the application of this method to the 



FIG. 2. A binary star. 



examination of the binary character of double stars been 

 of practical use. 



There is, however, another way by which the binary 

 character of a double star may be recognized. If the 

 component stars are actually close together, and if the 

 laws of motion and gravitation that control the move- 

 ments of the bodies composing the solar system are also 

 applicable to them, they must attract each other with a 

 force varying inversely as the square of the distance 

 between them, and each must in consequence describe 

 an orbit round the other. The form of the orbit must 



