Supernova radio remnants and the filling factor of the hot interstellar medium

If a hot (temperature of about one-million K), tenuous gas fills about 90% of interstellar space, then the observed number and surface brightness distribution of galactic radio remnants implies a galactic supernova rate of once about every 30 years. This is consistent with estimates based on observations of historical supernovae, pulsars, and supernovae in other spiral galaxies.