The initial planar expansion of a supernova can be described by a self-similar solution if the
progenitor star is sufficiently compact that radiative transfer effects do not play an important
role during this phase. The later free expansion of the gas leads to radial power laws for the
density and pressure profiles, although the effects of spherical geometry give some
inaccuracy in this result. If the initial expansion does set up power laws for the density and
pressure, effects of radiative diffusion can be described by a self-similar diffusion wave that�…