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 lu evolve to the blue super giant phase. When, during this blueward d loop, the effective photosph eric temperature T-eff has increased to values of about 7000 K, the ma in part of the photosphere becomes moderately unstable. Observations o f the yellow hypergiant star HD 217476 = HR 8752 show that the approac h to this instability region may lead to phases of enhanced mass loss and consequent reduced surface temperature, after which T-eff slowly i ncreases until it again 'bounces' against the atmospheric instability region. Two such bounces have been observed in the last 30 yr and a th ird seems imminent; frequent observations of this star are therefore u seful, Other 'yellow hypergiants', like rho Cas and IRC + 10420, seem to show the same behaviour. Repeated observations of these stars may h elp us to understand how massive stars pass the instability region dur ing their evolution towards the blue supergiant phase.