SUN'S PLACE AMONG THE STARS — ADAMS 151 



Among the dwarf stars, on the other hand, the change of lumi- 

 nosity with temperature is very marked. As the temperature de- 

 creases the huninosity decreases regularly and then drops abruptly 

 as we reach the coolest stars. This is to be expected as the limits of 

 visible radiation are reached and the light of the stars goes out. 

 The dwarf stars are comparatively small, dense bodies, and the 

 lower the temperature the less massive and the denser they arc 

 found to be. So there is no increase of size as among the giants to 

 balance the effect of the decrease of surface brightness due to 

 decreased temperature. 



Our sun belongs to the sequence of dwarf star3 that forms an un- 

 broken chain between the faintest stars intrinsically of which we 

 have any knowledge, stars that give out less than 0.0001 part the 

 light of the sun, and the bright white stars with luminogities 50 or 

 more times that of the sun. Within this sequence the sun agrees well 

 with other stars of the same temperature: its luminosity seems to 

 be ^lightly less than that of the average star of its type but well 

 within the range that similar stars exhibit. In mass, spectral type, 

 and many other characteristics the sun can be almost precisely 

 matched by many of the stars which have already been observed. 

 As a typical dwarf star, therefore, we can reason that the sun in 

 its future history will pass through the evolutionary changes of 

 similar stars in the main sequence to which it belongs. What these 

 changes may be is very far from certain, for our theories of 3tellar 

 evolution are still in a most indefinite state. The probabilities are 

 that in the course of sufficient time the sun will radiate away much 

 of its mass, will decrease in temperature and luminosity, and arrive 

 at a condition similar to that of the faint red dwarf stars which are 

 our most frequent neighbors. The time required for such processes, 

 however, is almost incredibly long. It would take 40,000,000 million 

 years for the sun to lose half its present mass through radiation, 

 and it i^ quite possible that during a considerable part of that period 

 the output of light and heat would not differ seriously from that at 

 present. Every aspect of the study of the relationship of the sun 

 to the stars as a physical body leads to a time-scale of enormous 

 length, and it is clear that, whatever the future history of the earth, 

 its destiny will be defined by limitations quite other than those set 

 by a cold and inert sun. 



