A blastwave (or blast wave) can be thought of as a shock wave (faster-than-sound) expanding (spherically, if allowed to) from a small volume. A mathematical model developed by several physicists, including Leonid Sedov and Geoffrey Taylor, describes what is termed Sedov-Taylor expansion (or Sedov expansion), expansion that is an adiabatic process, which basically holds true during a phase somewhat after the blast, when the amount of energy within the volume behind the shock front is not significantly changing. This model is useful for powerful explosions (including nuclear explosions) and, within astrophysics, for supernovae and in modeling supernova remnants. Sedov length is an astrophysical term for the distance from the initial blast at which the phase of Sedov expansion begins. There is a formula to approximate this distance, this approximation often what is referred to by the term.