Early-type stars blow bubbles in the interstellar medium. The radii of the bubbles are typically 30 pc. Typical conditions in their interiors are temperatures of about 1 million K and densities of about 0.01 per cu cm. The dense shell of swept-up interstellar gas that surrounds them is likely to trap the ionization front and may also have an outer layer of H2. The column density of O VI in the interior is in accord with observations by the Copernicus ultraviolet telescope.
Castor et al. (Mon,) studied this question.
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