An obstacle to the late evolution of massive stars

During their evolution, massive stars first swell and thus become red supergiants, after which they shrink again to evolve to the blue supergiant phase. When, during this blueward loop, the effective photospheric temperature T_eff has increased to values of about 7000 K, the main part of the photosphere becomes moderately unstable. Observations of the yellow hypergiant star HD 217476=HR 8752 show that the approach to this instability region may lead to phases of enhanced mass loss and consequent reduced surface temperature, after which T_eff slowly increases until it again `bounces' against the atmospheric instability region. Two such bounces have been observed in the last 30 yr and a third seems imminent; frequent observations of this star are therefore useful. Other `yellow hypergiants', like rhoCas and IRC+10420, seem to show the same behaviour. Repeated observations of these stars may help us to understand how massive stars pass the instability region during their evolution towards the blue supergiant phase.