Differences in Stellar Composition
In the late 1950’s astronomers began to distinguish stars by their chemical composition. The older stars (those that formed when the universe was new) were deficient in the heavier elements. The older stars are Population II, because astronomers discovered them later than Population I, the stars nearby. Their lack of heavy elements requires Population II stars to reach high temperatures before they can begin burning hydrogen. Their light at the moment of emission is bluish-white. However the stars look red if they are far away and receding from us rapidly. Newer stars like our Sun are not deficient in the heavier elements. These stars burn their hydrogen at a lower temperature than Population II stars. They can do that because the heavier elements catalyze the nuclear reactions. This is why their light at the moment of emission is yellow or red.
NASA scientists recount this discovery as follows:
NASA scientists recount this discovery as follows:
However, a little more than a decade ago the astrophysicists started to measure the abundances of the elements in stars with good quantitative accuracy, and this changed the picture considerably. No longer was it a tenable assumption that the universe was chemically homogeneous. Two general types of abundance anomaly were found. On the one hand, it was found that the oldest stars in our galaxy, particularly those likely to be considerably older than the Sun, had a deficiency of heavier elements relative to hydrogen as compared to the Sun itself. This deficiency ranged from a factor of a hundred to a factor of a thousand in some extreme cases. The second type of anomaly concerned stars that in their overall composition might be quite similar to that of the Sun, but in which specific elements could be seen to be overabundant. Thus certain stars were observed to be enriched in carbon and other stars to be enriched in certain heavy elements.[i]
[i] Truran, J. W. and A. G. W. Cameron, Chapter 23, “Nucleosynthesis,” op. cit., p. 985.
The discovery of differences between stars led to understanding the processes that made the rest of the elements, those that were not present at the end of the first morning.
