316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



rs CO Q2 QS C8 KO Kl K3 K4 KS K6 



Mount Wilson type. 



K7 MO Ml 



M2 



Figure 4. — Color index plotted against Mount Wilson spectral type for dwarfs. (Cour- 

 tesy Royal Observatory Bulletins, 1962.) 



owing to the shortage of electrons, and so more hydrogen atoms appear 

 in the line of sight. But it should be borne in mind that the hydrogen 

 lines are also sensitive to temperature, and it has not been established 

 that the stars in each pair actually have the same surface temperature ; 

 so that this conclusion may not turn out to be right in all cases. 



There is another curious effect which has been pointed out by 

 Wilson, which is that the stars whose spectra have been shown display 

 a big difference in color at one spectral type. The interesting thing 

 here is that it seems to be the subdwarf, which was marked "b" for 

 blue, that is relatively brighter in the blue band of the spectrum 

 although its temperature is about the same as that of the normal 

 dwarf marked "r" for red ; in spite of the appearance of the spectra, 

 on which the lines seemed to be equally strong within the very limited 

 degree of accuracy afforded by mere visual examination, the lines 

 must really be a little weaker in the subdwarf and cause the color 

 distribution to go in this direction although the subdwarf is no hotter, 

 and perhaps even a little cooler, than the normal dwarf. 



