The Complex Upper H-R Diagram — Shaped by Mass-Loss

Forty years ago, comparison of the H-R Diagrams for the luminous stars in our region of the Milky Way and the Large Magellanic Cloud revealed comparable populations of massive stars and the recognition of an empirical upper-luminosity boundary that was not predicted by theory or models at that time. The lack of red supergiants above a certain luminosity implied an upper mass for stars that could evolve to red supergiants with important implications for stellar evolution. We suggested then that the upper luminosity limit was due to mass loss including high mass loss episodes near the Eddington limit. Today, observations in the Galaxy and nearby resolved galaxies have revealed evolved stars of different types experiencing high mass loss, stars that characterize the upper luminosity limit, and provide clues to the origin of their high mass loss events — Luminous Blue Variables, warm hypergiants, B[e] supergiants and “supernova impstors”. In this review, their characteristics, mass loss, possible relationships, and eventual fate will be discussed.