The Abundance of Helium in Stellar Interiors

Abstract

From the titles of the other contributions to this morning’s session, one deduces that the hard facts of theory are to be sandwiched between the loose speculations of direct observation. I shall therefore feel free to discuss theoretical implications from stellar structure unencumbered by what are, after all, but peripheral considerations.I should, however, begin with an apology. It is impossible to do even minimal justice to the vast number of investigations which have contributed to our present understanding (or lack of it). The choice of topics has been dictated in part by history, in part by current relevance, but mainly by personal whim. While much of the discussion centres, for obvious reasons, on Population II work, one or two problems associated with Population I will also be considered. Reference may be made to the exhaustive review article of Iben (1967) and also to the Warner Prize Lecture by Demarque (1968) for background and a more complete discussion of some of the points which will be touched upon here.I shall assume today that the ‘normal’ helium content of stars is of the order of 25±5% by mass. This leaves me free to pay particular attention to systems which do not appear to adhere to this generalization, systems for which abnormally high or low helium contents have been claimed. The main point of my talk is that there is no really convincing, concrete evidence from stellar interiors alone for helium contents far removed from normal. During the course of the past decade, this has come to be largely accepted. It is in a way sad to see the radicalism of one’s youth become the orthodoxy of middle age.