Supernovae versus models of the interstellar medium and the gaseous halo

An approximate theory and observational data are used to estimate Q, the fraction of volume or area occupied by supernova cavities, and the results are compared with observational determinations of Q. For Type I SNs the calculated values of Q(3D) are modest and do not violate observational data in the solar neighborhood. Type I SN rates should increase toward the Galactic interior, and it is suggested that the resulting increase in Q(3D) is responsible for the H I 'holes' observed in the centers of most spiral galaxies. For Type II SNs, which are correlated in space and time, the explosion cavities are large enough to break through the gaseous disk. The calculated values of Q(3D) for the Galaxy are much larger than unity. This is in violent disagreement with Galactic and extragalactic observational data. It is also predicted that either the mass injection rate into the halo is much larger than allowed by diffuse X-ray observations or that the Galaxy has a wind.