The Brightest Supergiants Predicted by Theory

The distribution of the brightest Galactic supergiants on the Hertzsprung-Russell diagram is studied here with the help of new theoretical evolutionary tracks for very massive stars. The shapes of the new tracks are determined largely by stellar wind mass loss during the main-sequence phase. In contrast to earlier theoretical work, the tracks for stars with the highest initial masses point down the main sequence and converge to essentially the same unique end product, which then evolves toward the region of red supergiants. Enhanced mass loss due to ionization-induced dynamical instability in the star's outer envelope eventually terminates the redward movement. In these two ways, mass loss imposes upper limits on the predicted luminosities for extreme OB supergiants, yellow hypergiants, and red supergiants. By adopting standard mass-loss rates for main-sequence stars, the new evolutionary tracks are found to account satisfactorily for the empirical supergiant luminosity limits determined by Humphreys & Davidson and by de Jager. Success is somewhat more mixed when main-sequence mass-loss rates 3 times larger (as suggested by some recent observational studies) are adopted.