318 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



This is shown in figure 6, where a red and infrared color index 

 measured at the Lick Observatory, Calif., and at Mount Stromlo 

 Observatory in Australia is plotted on the vertical axis against the 

 ordinary blue-yellow color index on the horizontal axis. The vertical 

 axis is now essentially a measure of surface temperature, with cooler 

 stars toward the top and right of the diagram, and the tendency of 

 the subdwarfs and elliptic-orbit stars is to lie to the left of the line 

 defined by the normal dwarfs of Population I, just as we would expect 

 from the relative weakness of their dark absorption lines in the blue 

 part of the spectrum. These results are now being extended by 

 Gerald E. Kron, of the Lick Observatory, who is observing the bright- 

 nesses of the stars in six colors — infrared, red, green, blue, violet, and 

 ultraviolet, and his work should soon enable a clear and quite reliable 

 distinction to be made between the effects of surface temperature and 

 metal abundance on the measurements. 



CONCLUSION 



Probably more than enough has been said now about the somewhat 

 technical details of the interpretation of the brightnesses, colors, and 

 spectra of the stars, and it may be worth while to try to recapitulate 

 a little and especially to repeat some of the reasons why we consider 

 this work to be of general interest. We have seen some — I think — very 

 pleasing pictures of external galaxies that are believed to bear close 

 similarity to our own Milky Way system, and some of the inferences 

 that can be drawn from them as to the two main stellar populations 

 existing in the Galaxy at large and also in our own neighbor- 

 hood. At one extreme we have Population II distributed in a spheri- 

 cal halo around the center of the Galaxy, consisting of old stars and 

 devoid of interstellar matter. At the other extreme we have Popula- 

 tion I concentrated in the central plane of the Galaxy and especially in 

 spiral arms, in which new stars are still being formed by condensation 

 out of the interstellar medium. The examination of the color-lumi- 

 nosity diagrams of the clusters that are characteristic of the two 

 stellar populations, aided by a great deal of theory developed mainly 

 over the last 10 years, has led to a picture of the way in which stars 

 evolve in time which perhaps does not quite explain everything, but 

 does at least provide a framework into which a wide variety of ob- 

 servations can be fitted. This theory has been accompanied by the 

 suggestion of Fred Hoyle and his collaborators that the chemical 

 elements in the universe, and in particular on the Earth, were formed 

 by nuclear synthesis from hydrogen, helium, and neutrons in the in- 

 teriors of hot stars which then exploded as supernovae; this mecha- 

 nism scatters the newly formed heavy elements into the interstellar 

 medium, where they mix with the hydrogen already there and form 

 an enriched or contaminated medium — whichever way you prefer to 



