Stellar Evolution and the Synthesis of the Elements.

The production of neutrons in stellar interiors will occur through exoergic (a, n) reactions on 017, Ne21, Mg25, and Mg26 as well as on C'2, as first suggested by Cameron With these reactions as possible neutron sources, an attempt is made to indicate a general, but still qualitative, basis for the synthesis of the elements in stars The importance of successive stages of interaction of the light nuclei with hydrogen and helium is stressed. In particular, the reaction Ne20 (p, )Na21( +)Ne22, followed by Ne21 (a, n)Mg24, is taken as the basis for alterations and modifications of the mechanisms of Hoyle and Cameron for the synthesis of the intermediate and heavy elements, respectively The use of this reaction obviates some of the difficulties encountered in processes in which a small proportion of hydrogen must be mixed into the stellar core at temperatures at which it will be very rapidly consumed A possible evolutionary scheme which is consistent with these element-building processes is outlined It is supposed that the synthesis of the heavy elements will take place in stars which have reached the cool giant stage in their evolutionary path. The M as well as the S stars may be important in this connection A rough estimate, based on the observed frequency of S stars, suggests that the rate of element synthesis in these stars is sufficient to produce the observed heavy-element abundances. Differences in chemical composition between population I, population II, and intermediate groups of stars are briefly discussed.