The Cooler Supergiants (A to M): Crucial Signposts in the Lifecycles of Massive Stars

Abstract

The intermediate and late-type supergiants are the visually brightest stars. They are among the first stellar objects observed in other galaxies and provide our first clues to the conditions of massive star evolution in galaxies of different types. They are not as massive as the hottest and most luminous stars in the upper left of the HR diagram. Nevertheless, these somewhat lower mass stars (≈20−50 M⊙) with relatively cool temperatures play a major role in our efforts to understand massive star evolution. These supergiants are usually considered to be post hydrogen burning stars, and their relative numbers in the HR diagram provide essential comparisons with models for the later stages of massive star evolution. Most importantly, the most luminous cooler supergiants define the stability limit for massive stars in the HR diagram.