ORBITS OF DOUBLE STARS 221 



dieted by mathematicians. It is twice as heavy as our 

 sun, but does not give a fiftieth part of the light. 

 Stars then may be double, or treble, or even quadruple 

 by nature, or by the accident of position. 



Comparing his own observations and such others as 

 he could procure, Herschel calculated that the one star 

 moved round the other, or that both moved round their 

 common centre of gravity in the following double 

 stars : 



Castor in about 342 years 2 months. 

 y Leonis in about 1200 years, 

 e Bootis in not less than 1681 years. 

 d Serpentis in about 375 years. 

 y Virginis in about 708 years. 1 



Another double star that he carefully examined was 

 Zeta Herculis. It presented him with a sight " which 

 is new in astronomy ; it is, the occultation of one star 

 by another." For twenty-one years he continued to 

 keep a watch on the star. After twenty years had 

 passed he could no longer perceive the smaller of the 

 two companions. The following year he found "the 

 apparent disk a little distorted; but there could not 

 be more than about f of the apparent diameter of 

 the small star wanting to a complete occultation." 

 But the observations made were not sufficient to 

 determine the nature of the motion that produced 

 these effects. The long period of 1681 years set down 

 against s Bootis, Herschel himself points out as subject 

 to uncertainties, which it will take long to clear up. 



1 "One thing very remarkable I must tell you, y Virginis is now a 

 single star in both the twenty-foot, and the seven-foot equatorial ! ! ! " 

 (Sir John Herschel, March 8, 1836). He means that one of the two 

 suns had eclipsed the other. 



