1910.] 



RUSSELL— DISTANCES OF RED STARS. 



231 



As all the stars under consideration lie within the same limits 

 of apparent brightness, it follows that they are intrinsically fainter 

 the redder they are. The reddest average only one fiftieth as bright 

 as the sun. 



On the other hand, it is well known that many bright red stars, 

 such as Arcturus and Antares, are at great distances, and are 

 probably at least- one hundred times as bright as the sun. 



All this can be explained on the hypothesis (now well established 

 on other grounds) that the reddest stars are the lowest in effective 

 temperature. With the latest data regarding temperature and sur- 

 face brightness, it appears that the fainter red stars are somewhat 

 smaller, and presumably denser, than the sun, while the brighter 

 ones are very much larger than the sun, and presumably of very 

 small density. The latter class probably represent an early stage 

 in stellar evolution, and the former the latest stage that can be 

 observed. In the intermediate stages the star would be hotter, pass- 

 ing through orange and yellow to white, and back to red as it ap- 

 proaches extinction. On this hypothesis there ought to be two 

 distinct kinds of red and orange stars. The distribution of proper 

 motions among these stars supports this theory, showing that those 

 of given apparent brightness may be divided into two groups — one 

 of intrinsically faint stars relatively near our system, the other of 

 bright stars remote from it. 



PRI^XETON University, 

 April 20, 1910. 



