The discovery of a static, dense gaseous shell around the nearby red supergiant Betelgeuse raised doubts about the assumption that it was a fast-moving star with a powerful stellar wind driving a bow shock into its surroundings. These physically distinct structures cannot both be formed by hydrodynamic interaction of wind and interstellar medium. Hilding Neilson and colleagues describe a model in which Betelgeuse's wind is photo-ionized by radiation from external sources, and pressure induced by photo-ionization generates a standing shock in the neutral part of the wind. This forms an almost static shell, confining gas close to the star, where it can interact with ejecta from a future supernova explosion. This can explain the many supernovae with the signatures of circumstellar interaction.
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