ANALYSIS OF STARLIGHT — PAGEL 

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Brightness 

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Co /out Index 



Figure 2. — Color-luminosity diagram for a globular cluster, M92. 



here we have a very old stellar population with an age of about 

 10,000 million years — almost as old as the Galaxy itself. We also 

 notice that the main sequence is somewhat lower in the diagram than 

 the main sequence of the Hyades; these stars are referred to as 

 "subdwarfs," and the existence of subdwarfs is closely related to the 

 evidence that we have that there is a big difference in cliemical 

 composition between the two stellar populations in the sense that the 

 old stars of Population II seem to have a much lower abimdance of 

 metals mixed in with their hydrogen than have the younger stars 

 of Population I. The difference between the two degrees of metal 

 abundance is considerable, generally a factor of about 100. 



To account for the difference in metal abundance between the 

 stars of different ages, Hoyle suggested some time ago that when 

 a star has gone through the giant phase of evolution, it runs out 

 of hydrogen to an ever-increasing extent and further nuclear reactions 

 have to occur involving heavier elements, particularly helium. Wlien 

 the Ulterior of the star has contracted and heated up sufficiently, 

 helium nuclei can fuse together with one another or with hydrogen 

 to produce a number of chemical elements up to atomic weight 56, 

 corresponding to iron. Still heavier elements, going all the way up 

 to uranium and beyond, can then be built up by capturing neutrons, 

 and in this way Hoyle, Geoffrey and Margaret Burbidge, and William 

 A. Fowler of California Institute of Teclinology have put forward 

 a detailed theory which is in tolerably good agreement with the relative 

 abundances of the heavy elements found in nature. A further idea is 

 that, at a late stage of evolution, many stars explode; this is the so- 

 called supernova outburst in which a star suddenly becomes about 



