Fred’s Contributions to Stellar Evolution

Abstract

Fred began work on stellar structure after Hans Bethe and Carl-Friedrich von Weizsäcker had independently established that the thermonuclear fusion of hydrogen into helium is the primary source of the energy radiated by the Sun and other main sequence stars. A joint paper with Ray Lyttleton included this temperature-sensitive process explicitly in the energy equation, effectively vindicating the essentials of the theory of homogeneous gaseous stars presented in Sir Arthur Eddington’s celebrated monograph ‘The Internal Constitution of the Stars’. Agreement with the solar luminosity can be obtained with two alternative values for the hydrogen content. In a subsequent paper, Fred argued convincingly in favour of the case with a very high rather than a moderate fraction of hydrogen. An epoch-making joint paper with Martin Schwarzschild followed the evolution of a low mass star through nuclear processing, from the main sequence into the giant domain in the Hertzsprung-Russell diagram. The slowly growing, burnt-out core becomes degenerate and nearly isothermal, while the photospheric boundary condition forces the expanding envelope to become largely convective. At the top of the giant branch, the degenerate core becomes hot enough for the fusion of helium into carbon; the consequent secular instability, noted first in studies of white dwarfs, brings the star down to the ‘horizontal branch’, the location of the short-period globular cluster Cepheids. Two subsequent papers with Brian Haselgrove studied in further detail the structure of both main sequence and giant stars.