Structure and Evolution of Wind-driven Circumstellar Shells

Properties of supernova shells are compared with those of 'bubbles' of hot gas surrounding early-type stars (referred to as interstellar bubbles). It is shown that the structure of an interstellar bubble is similar to that of a supernova shell but that the energy of a bubble is deposited gradually over the lifetime of the star in contrast to the immediate injection of the entire energy of a supernova shell. The relative importance of interstellar bubbles and supernova shells as sources of hot gas in the Galaxy is evaluated. It is concluded that the soft X-ray background in the Galaxy is more likely to be due to old supernova shells rather than to interstellar bubbles, but the bubbles may be relatively more important as a source of interstellar O VI absorption.