The radiation field is in general not Planckian in these calculations, so that a so-called multi-frequency description is necessary, analogous to the multi-group formulation of neutron transport calculations. An essential computational difference, however, is that at high temperatures and low densities, as often found in astrophysical applications, the radiation energy density may be comparable to the material energy density and is strongly coupled to it through the temperature-dependent opacity. Unless this coupling is treated implicitly, by solving the radiation transport and material energy equations simultaneously, prohibitively small time steps will often be required.
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