208 Trees, Stars, and Birds 



have not completed one revolution around their common 

 center of gravity. By means of the spectroscope it has 

 been discovered that each of these stars is double; so 

 we know that Castor is really a system of four stars. 



Many other instances of double stars are now known ; 

 that is, many stars which appear to the unaided eye as 

 single points of light are shown by the telescope to con- 

 sist of two stars. In some cases the two are not asso- 

 ciated, but merely happen to be in the same direction 

 from us, one of them much more distant than the other. 

 These are called " optical doubles." The binary star 

 consists of two that revolve around a common center 

 of gravity. Even these are in many cases farther apart 

 than are the earth and the sun ; but their distance from 

 the earth is so immense that they appear to us as one 

 point. Hundreds of stars that appear single in the best 

 telescopes are resolved into double stars by the spectro- 

 scope. Many thousands of stars are now known to 

 be binary, and nearly one hundred have been found to 

 consist of more than two stars each. Mizar, at the bend 

 of the handle of the Big Dipper, is quadruple ; that is, 

 it consists of four stars ; Polaris, the North Star, con- 

 sists of four stars ; and Theta Orionis, the star at the 

 middle of the sword of Orion, consists of six stars 

 enveloped in a nebula. 



Aquila, the Eagle. Starting with that star in the bowl 

 of the Little Dipper farthest from Polaris, a line drawn to 

 Vega and extended as much farther will come to Altair, 

 a first-magnitude star midway between two stars of 

 the third magnitude. These three belong to the con- 

 stellation Aquila, the Eagle. It is on the edge of the 



