energy transportThe flow of heat from a hot to a cooler region by means of radiation, convection, or conduction. In stars the heat flows from the hot interior to the surface, where it is radiated away into space. The most important means of energy transport in most stars is radiative transport, in which high-energy photons lose energy to the hot plasma through scattering, mainly by free electrons, and by absorption through photoionization of ions; this occurs in the radiative zone of the star. (The equation of radiative transport is given at stellar structure.) Convection is another mode of energy transport in which collections of hot fluid in the convective zone of a star rise toward the surface, release their energy, and sink again to pick up more energy. Convection is a highly efficient process but the conditions must be right for its onset. Conduction is negligible in most stars but becomes important in very dense degenerate stars, such as white dwarfs.
The mode of energy transport has important effects on a star's evolution: radiation (and conduction) do not affect the shells of different composition within an evolved star, but convection mixes them to a uniform composition.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006