ANALYSIS OF STARLIGHT — PAGEL 



313 



02 0-4- OS 



Colour Index. 



0-8 



-'fn 



Figure 3. — Color-luminosity diagram for stars within 64 light-years. 



or perhaps I should rather say to the left of, the main sequence, and 

 these are further examples of the stars that are known as subdwarf s ; 

 these stars are of considerable interest because they appear to be re- 

 lated to the dwarfs in the globular clusters. For one thing, they 

 appear to be going round the center of the Galaxy in elliptical orbits, 

 sometimes with quite liigh eccentricities and inclinations to the galactic 

 plane. Furthermore, when their spectra are examined, they tend 

 to show a deficiency in metals, which is another resemblance to the 

 stars in globular clusters. They seem to be interloping members of 

 the halo-type Population II, or possibly an intermediate class be- 

 tween the extreme population types. 



The fact that subdwarfs are shown lying below the main sequence 

 does not necessarily mean that they are peculiarly faint for their sur- 

 face temperature (that is to say, peculiarly small in size), because the 

 measured color that is plotted along the horizontal axis is itself 

 affected by the presence of dark absorption lines superimposed on the 

 continuous spectrum. It was noticed about 60 years ago by Sir Arthur 

 Schuster that the absorption lines are not uniformly distributed 

 throughout the spectrum of a star, but that they become systematically 

 stronger and more numerous as one goes from the red end to the blue 

 and on to the ultraviolet. Since the absorption lines remove some 

 of the energy that would otherwise be detected when we make our 

 measurements through yellow and blue glass filters, a star having 

 faint lines due to low abundance of the metals will seem to be rela- 

 tively brighter in the blue, as compared with the yellow, than another 

 star of the same surface temperature, but having liigher metal content 



