ANALYSIS OF STARLIGHT — PAGEL 317 



Figure 4 illustrates how the effect shows up in the color index, 

 which has been plotted here on the y-axis against spectral type on the 

 a?-axis. The straight line represents the effect of variation in tempera- 

 ture alone for stars having the chemical composition of the Sun, or 

 Population I; such stars are tentatively identified here as stars 

 traveling in a circular orbit around the center of the Galaxy, and they 

 are shown as black dots. Ivnown subdwarfs are represented 

 by crosses, and the open circles represent stars that are moving in 

 elliptical orbits round the center of the Galaxy, and it is suspected 

 that some of these would actually turn out to be subdwarfs as well, 

 if their distances were better known. For the stars of spectral types 

 F and G, about as hot as the Sun, the dots, circles, and crosses are 

 distributed anyhow, but when we come to type K, there is a definite 

 tendency for the subdwarfs and other elliptical-orbit stars to lie be- 

 low the others in the diagram, that is to say their blue light is too 

 strong for their spectral types because of general faintness of the 

 lines. The general effect is in accord with theory and provides a pos- 

 sible method of judging the chemical composition of a star from 

 comparatively simple observations and with only a fairly rough 

 knowledge of its distance. 



Unfortunately the spectral types of stars can only be assessed rather 

 roughly, but we can confirm these conclusions for a limited niunber of 

 stars by appealing to photometric measurements in a relatively un- 

 popular region of the spectrum, the red and infrared, where absorption 

 lines are weak and so we have a better chance of judging the surface 

 temperatures of stars from observations of the color distribution. 



Figure 5. — R-1 color index plotted against B-V color index. (Courtesy Royal Observa- 

 tory Bulletins, 1962.) 



