Yellow Hypergiants as Dynamically Unstable Post–Red Supergiant Stars

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.