:>TELLAR EYOLUTIOX. 



311 



there are two well-defined classes of these older stars, which until recently 

 were not known to have any points in common except their red color. 

 These are the stars of Secchi's third and fourth types. In general 

 appearance their spectra are wholly unlike, particularly on account of 

 the absence from third class spectra of the broad dark bands due to the 

 absorption of carbon vapor, the most characteristic feature of the fourth 

 type. But in spite of this apparent dissimilarity, photographs recently 

 taken with the 40-inch Yerkes telescope show that in certain regions of 

 the spectrum stars of the two types are practically identical and are 

 thus probably more closely related than formerly appeared to be the 



Fig. 12. 

 Spectra of four red stars (Hale and Ellerman). Showing how the dark band 



DUE TO carbon INCREASES IN INTENSITY AS THE STAR COOLS. 



case. The measurements and reductions of a long series of photographs 

 of fourth type spectra now in progress at the Yerkes Observatory should 

 soon permit us to fonn an opinion of the nature of these interesting 

 stars. 



In both the third and fourth types it is easy to trace the successive 

 stages of development. In stars of the fourth type the signs of in- 

 creasing age are particularly striking. The carbon vapor which pro- 

 duces the broad dark bands becomes denser and denser, until it is not 

 difficult to imagine that through the further increase of such absorption 

 the light of the star might be completely extinguished (Fig. 12). 



