312 



POPULAR SCIENCE MONTHLY 



The phenomena of the red stars indicate that this final stage is 

 close at handj and curiously enough, in further testimony of the re- 

 markable power of the spectroscope, the total extinction of a star's light 

 does not always prove sufficient to place that star beyond the reach of 

 this instrument. It is true that a spectroscope cannot reveal the chem- 

 ical composition of a solid body which is devoid of intrinsic light, but 

 such a body may form a system with another object which is still lumin- 



FlG. 13. 

 LuNAE_ Crater TiiicopiiiLrs and suerouxdixg region. Photographed with the 



FOETY-INCH YeRKES TELESCOPE (RlTCHEY). 



ous, and its gravitational power may cause the luminous body to move in 

 an orbit. As we have already seen, the spectrosco|)e is capable of reveal- 

 ing the motions of such a body. From a knowledge of these motions and 

 the time in which the revolution is effected it is possible to determine 

 the mass and dimensions of the system, and in some special cases like 

 that of Algol, the diameter and density of the invisible component of 

 the pair. 



