The yellow hypergiants

Abstract.

We list the main stellar data of known hypergiants and similarly luminous stars, and then concentrate on a review of the yellow hypergiants. These stars are post-red supergiants evolving along blueward loops in the Hertzsprung-Russell diagram. Their properties, their location in the Hertzsprung-Russell diagram and their occasional mass ejections are related to a region of atmospheric instability in the H-R diagram, the Yellow Void. The ‘bouncing against the border of the Void’ of three objects: $\rho$ Cas, HR8752 and IRC+10420, is described. The apparent atmospheric instability of yellow hypergiants is related to the atmospheric pulsations. There are indications that the approach to the Void is associated with an increased amplitude of the pulsation and with enhanced mass loss. The observed small-scale motion field is only apparently strongly supersonic; the observed large stochastic velocities are the quasi-stochastically varying thermal motions in the many hot sheets that occur in the wakes of many small shocks, while the real hydrodynamic velocity component is small and subsonic. This shock-wave field is also responsible for the observed rate of mass loss and for emission in the wings of H$\alpha$. Most yellow hypergiants have envelopes containing gas and dust, but a thick extended envelope, presumably dissipating and showing bipolar outflow, is only known around IRC+10420. At the interface of the bipolar wind and the interstellar medium one or more stationary shocks may develop as is observed in the case of IRC+10420 and suspected with $\rho$ Cas.