ANALYSIS OF STARLIGHT — PAGEL 303 



exist in the proportions in which we actually find them? The first 

 question has proved to be a somewhat elusive one, but the second 

 question — that is the one about the chemical elements — has made a 

 great deal of progress in the last few years as a result of observational 

 advances, and also very much as a result of the better miderstanding 

 that we now think we have of the way in which stars change or 

 "evolve" in the course of time, together with a coherent theory by 

 Fred Hoyle and several collaborators on the synthesis of the elements 

 by nuclear reactions in stars. We have seen that hydrogen is being 

 transformed into helimn in the interior of the Smi, and spectroscopic 

 analysis reveals that helium is about half as abmidant by weight in 

 the universe as hydrogen. "Wlien we look for the common heavier ele- 

 ments such as carbon, nitrogen, and oxygen, we find that there is much 

 less of them, just a few percent in comparison with hydrogen. Still 

 heavier elements, which astronomers tend to lump together in cavalier 

 fashion by referring to all of them as "metals", are even less common, 

 at least among the stars and Imninescent clouds of interstellar gas 

 that can be investigated with the spectroscope : all of them 2)ut together 

 on the average account for less than a tenth of 1 percent of the cosmic 

 mixture of elements. The situation here on the Earth, which consists 

 mostly of heavy elements, thus seems to be quite exceptional; pre- 

 sumably the hydrogen and helium, which are usually far and away 

 the most abundant elements, escaped at an early stage in the Earth's 

 history simply because they are so volatile. 



The relative proportions of the commoner metals like calcium, so- 

 dium, and iron do not appear to differ very radically from one another 

 in most stars ; indeed they seem to be present in rouglily similar propor- 

 tions to one another in the Earth, in meteorites, in stars, and in clouds 

 of interstellar gas. But the proportion of metals as a whole relative 

 to hydrogen, while always small, varies quite widely from one star to 

 another, ranging over a factor of maybe some hundreds. The varia- 

 tion in metal abundance appears to be related to the ages of stars, the 

 stages that they have reached in the course of their evolution, and 

 their distribution within the Galaxy ; and I should like to say some- 

 thing in a very general way about these things and about our pres- 

 ent ideas on the origin of the heavy elements, and then something 

 in a little more detail about such related spectroscopic investigations 

 concerning metal abundances and other characteristics among the 

 relatively nearby stars as are now being carried out at the Royal 

 Greenwich Observatory in Herstmonceux and elsewhere. 



GALACTIC STRUCTURE AND STELLAR POPULATIONS 



When we look up at the night sky we see a number of stars in all 

 directions; these are relatively near to us in space, the stars visible 

 to the naked eye being generally between 10 and 1,000 light-years 



