Stellar winds and repeated supernovae from an OB association will create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense, expanding shell of cool interstellar gas. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 5 x 10 7 yr. The supershellloses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10 7 yr and with a radius-100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Milky Way and other spiral and irregular galaxies from 21 em emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies.

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We present. models f<;>r ~emper~tu~e and ionization structure of low, uniform-density (n ~ 0.3 em-3) ~nterstellar gas. m a galactic disk which IS exposed to soft X-rays from supernova outbursts occurring randomly m space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ion~zed fraction x, temper~tu~e T! and their various observable moments. These time-dependent models predict a bimodal temperature d1stnbutwn of the gas with structure in x, T that agrees with various observations. Cold re~ions in the low-density gas may have the a~pearance of clouds in 21-cm absorption. The time-dependent model, m contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (T ~ 30° K), ionized (x ~ 0.1) regions.

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