The ejection of supersonic outflows drives the formation of strong shocks that propagate into the surrounding medium. Examples are blast waves that form in stellar and galactic explosions. The supernovae accompanying the death of a star, the afterglow emission that follows a GRB explosion, and the radio lobes observed in radio galaxies and blazars are clear signatures of those blast waves. When the energy released by the source is large such that E > (Mj + ρiV)c2, where Mj is the mass of the ejecta, ρi the ambient density and V the volume swept by the shock, the blast wave motion is relativistic. The most notable example is the afterglowshell in GRBs.

The structure formed in spherical explosions at early times is shown schematically in Fig. 8.1. It consists of a forward shock that propagates in the ambient medium, a reverse shock crossing the ejecta, and a contact discontinuity separating the shocked ejecta and the shocked ambient medium. This structure is clearly seen in the X-ray image of the SNR DEM L71 in Fig. 8.2. The early phases in the evolution of a blast wave are considered in detail in Section 8.6.

At sufficiently late times a major fraction of the explosion energy is contained in the shell of shocked ambient medium and the effect of the ejecta on the evolution of the forward shock can be ignored. This stage is well described by the impulsive blast wave model discussed in Section 8.2.