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Published online by Cambridge University Press: 04 August 2017
A supernova explosion in the galactic disk creates a hot, low density cavity which persists much longer than typical lifetimes of supernova remnants. McCray and Snow (1979), Bruhweiler et al. (1980), Tomisaka, Habe and Ikeuchi (1980) and Kafatos et al. (1980) pointed out that repeated supernovae from a stellar association will produce an expanding shell of gas (R ≳ 100 pc) and offered extensive evidence for such shells -primarily in the form of H I shells.