ANALYSIS OF STARLIGHT — ^PAGEL 319 



look at it — from wliich new generations of stars will condense. This 

 picture is in general agreement with spectroscopic observations which 

 show that stars believed to be old, like the ones in globular clusters, 

 have a much lower admixture of metals in their atmospheres than 

 stars thought to be yomig, like the ones in the Hyades, although there 

 is surprisingly little difference in metal abundance between the oldest 

 and youngest clusters of Population I. Most stars in clusters, how- 

 ever, whether of the globular or the galactic variety, are a very long 

 way off and their distances and real brightnesses can only be estimated 

 by indirect methods that are often little better than guesses. Further- 

 more, the clusters undoubtedly contain stars that are intrinsically faint 

 and so cannot be observed at all. 



For these reasons, our ideas as to the nature of stars and the course 

 of stellar evolution need to be completed by examining the stars that 

 are near to us in space, especially those which are so near that we can 

 measure their distances directly. Except in the case of the Hyades, 

 such stars do not usually belong to clusters, but they may have escaped 

 from clusters in the past; furthermore, we can try to relate them to 

 the stars that are in clusters by observing their motions in space — or 

 in other words their orbits round the center of the Galaxy — and 

 their physical and chemical properties, and it is with this last question 

 that I have been primarily concerned. 



The problem of sorting out the true nature of a star from the 

 physical and chemical point of view is quite a complicated one, which 

 has to be tackled from various different angles. First there is the 

 theory of stellar structure, whose task it is to calculate the relation- 

 ship between the luminosities and surface temperatures of stars and 

 to predict how these quantities will change in the course of time, that 

 is to develop a picture of stellar evolution. Then there is the theory 

 of stellar surfaces, which says what happens to the atoms and electrons 

 in the atmosphere and tries to predict the distribution of energy in 

 the continuous spectrum and the intensities of the various absorption 

 lines, when the luminosity, surface temperature and chemical composi- 

 tion of the atmosphere are given. 



Wlien we try to verify these theories by making observations, we 

 come up against a number of difficulties. Ideally, we should like to 

 make direct physical measurements of the quantities discussed in the 

 theory, that is we should like to measure the total brightnesses and 

 surface temperatures of stars and then make a quantitative chemical 

 analysis of the line spectrum to determine the relative abundances of 

 the elements. But the amount of light reaching us from a star is very 

 small, and so we cannot usually examine either the continuous spec- 

 trum or the line spectrum in the amount of detail that we should like; 

 the only star that is really satisfactory in this respect is the Sun. 



