The Brightest Stars (Geophysics and Astrophysics Monographs)

It is pointed out that there is an upper limit to the brightness of stars, beyond which no star can exist. It is shown that this limit coincides with the brightness limit for which the effective acceleration (i.e., the sum of the effects of gravitation and forces due to thermal, radiative, and turbulent pressure directed outward) drops to a very small value. In atmospheres of the most extreme stars, gravity waves can occur with very long wavelengths and elevated periods. It is suggested that it is these waves that are responsible for the instabilities of supergiant and hypergiant stars.