Stellar wind bubble

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Stellar wind bubble is the astronomical term usually used to describe a cavity light years across filled with hot gas blown into the interstellar medium by the high-velocity (several thousand km/s) stellar wind from a single massive star of type O or B. Weaker stellar winds still blow bubble structures, though. The heliosphere blown by the solar wind, within which all the major planets of the Solar System are embedded, is in fact a small example of a stellar wind bubble.

Stellar wind bubbles have a two-shock structure^[1]. The freely-expanding stellar wind hits an inner termination shock, where its kinetic energy is thermalized, producing 10⁶ K, X-ray emitting plasma. The hot, high-pressure, shocked wind expands, driving a shock into the surrounding interstellar gas. If the surrounding gas is dense enough (number densities $n > 0.1 cm - 3$ or so), the swept up gas radiatively cools far faster than the hot interior, forming a thin, relatively dense shell around the hot, shocked wind.