Hydrogen Flash in Stars

Abstract

DURING the past few years there has been some progress in understanding the way in which a star approaches the main sequence. In their studies of the pre-main sequence evolution of stars, Ezer and Cameron¹ assumed that the stellar material started with the highest possible adiabat consistent with the virial theorem (for a given temperature a high adiabat has a low density and a low adiabat has a high density). In later hydrodynamic collapse studies of the formation of a star with zero angular momentum, Hayashi² and Larson³ showed that much of the internal energy of the collapsing gas is radiated away, so that the stellar material must start on a much lower adiabat. The gas in the centre of such an object can only be heated by compression, so that it remains on a low adiabat. As the outer layers of the star fall onto this core, they undergo shock heating, due to gravitational potential energy release, which raises their adiabat, and allows the star to form a stable body in hydrostatic equilibrium prior to reaching the main sequence.