Massive stars are rare, unstable, and short-lived. They eject matter through stellar winds, outbursts, and, eventually, supernova explosions, and affect the chemical evolution of the galaxy as a whole. Their lives include various evolutionary stages, some of which happen very quickly on cosmic timescales – in tens of thousands of years. Stars in such short transition stages are very rare, but important for constraining stellar evolutionary models.
The PhD thesis focuses on yellow hypergiant stars. They are highly unstable, with significant variability, pulsations, and recurring mass-loss outbursts. We studied in detail two hypergiants in the Milky Way: V509 Cas and RW Cep. Long-term monitoring of V509 Cas reveals low-amplitude pulsations and unprecedented atmospheric stability. RW Cep is only beginning its journey as a yellow hypergiant, and we studied the kinematics of its recent outburst.
To place these individual cases in context, we examined the population of luminous yellow stars in the Milky Way. By identifying their memberships in stellar clusters and associations, we refined their distances and luminosities. This is a first step towards statistical population-level surveys, which will be possible with the upcoming large datasets from Gaia, 4MOST, and LSST.