Abstract
According to recent theoretical studies, the majority of single stars more massive than 30 M☉ successfully evolve into red supergiants but then lose most of their hydrogen envelopes and metamorphose into hot blue remnants. While they are cool, they become dynamically unstable as a result of high radiation pressure and partial ionization of the gases in their outer layers. It is shown here that these unstable red supergiant models repeatedly shrink and reexpand on a thermal timescale when perturbed by heavy bursts of mass loss. Consequently, they fill up the domain of yellow hypergiants on the Hertzsprung-Russell diagram and display very fast rates of evolution there, as observed.
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