ON THE RELAXATION OF MATTER AND RADIATION IN THE STATE OF THERMODYNAMIC-EQUILIBRIUM

The possibility of using a two-temperature approximation, in which the radiation temperature T(r) is different from the matter temperature T, to calculate the rate of exchange of energy between matter and radiation near equilibrium is discussed based on a general formulation of the problem of the establishment of thermodynamic equilibrium between matter and radiation. It is shown that the temperatures T and T(r) calculated by Imshennik and Molodtsov (1990), in particular, have a precise physical meaning for arbitrary characteristics of nonequilibrium radiation. Other characteristics of relaxation processes-for example, typical relaxation times-take on a qualitative physical meaning. It is nonetheless asserted that the two-temperature approximation is effective in radiative gas dynamics for the general case of an arbitrary finite difference of the temperatures T(r) and T as a physically correct and simple, though rough, approximation for solving the problems of intrastellar radiative gas dynamics.