One of the most important applications of shock wave theory in astronomy is to the problem of a supernova exploding in an interstellar cloud. Supernovae dump around 1044 joules of thermal and kinetic energy of ejecta into a small region around the star on an astronomically minute timescale (a day or so). The shocked medium expands and sweeps up more gas, creating a large bubble in the interstellar medium (ISM). It is now known that the continuous injection of energy into the ISM in this way by successive supernovae is responsible for giving the ISM a ‘Swiss cheese’ structure, with bubbles of hot gas alternating with sheets and filaments of cooler material swept up in the cavity walls. Astronomical opinion is divided as to whether the net effect of supernova action is to suppress further star formation (through hot gas in the bubbles breaking out of the galactic potential) or to promote it (through enhanced fragmentation of gas swept up in the bubble walls). At any rate, the effect of supernovae is evidently fundamental to understanding the ISM in galaxies. Before we are in a position to estimate the global effects of successive supernovae, we must first construct an idealised model of a single supernova exploding in a uniform medium.

The title of the next section however indicates that the problem was first studied in another and more menacing context: the impetus for developing the elegant theory laid out below came from the need to model the effects of thermonuclear weapons exploding in the Earth's upper atmosphere.