  Betelgeuse, a nearby red supergiant, is a runaway star with a powerful
stellar wind that drives a bow shock into its surroundings. This picture has
been challenged by the discovery of a dense and almost static shell that is
three times closer to the star than the bow shock and has been decelerated by
some external force. The two physically distinct structures cannot both be
formed by the hydrodynamic interaction of the wind with the interstellar
medium. Here we report that a model in which Betelgeuse's wind is photoionized
by radiation from external sources can explain the static shell without
requiring a new understanding of the bow shock. Pressure from the photoionized
wind generates a standing shock in the neutral part of the wind and forms an
almost static, photoionization-confined shell. Other red supergiants should
have significantly more massive shells than Betelgeuse, because the
photoionization-confined shell traps up to 35 per cent of all mass lost during
the red supergiant phase, confining this gas close to the star until it
explodes. After the supernova explosion, massive shells dramatically affect the
supernova lightcurve, providing a natural explanation for the many supernovae
that have signatures of circumstellar interaction.
