Stellar Winds and Mass Loss

The evolution of stars is strongly affected by mass loss, so we must first discuss the physics of stellar winds and the resulting mass-loss rates before continuing the description of post-MS stellar evolution. Stars may lose mass due to a stellar wind. The mass-loss rates are particularly high in luminous hot stars, where the wind is driven by radiation pressure in the spectral lines of ions, and in cool low gravity stars such as red giants and AGB stars. In these stars, the mass loss is driven by radiation pressure on dust grains, helped by the pulsation of the stars. The mass-loss rate of hot stars and cool supergiants is so large that it changes their evolution. Mass loss strips away the outer layers of stars. After severe mass loss, the original deeper layers that have been chemically modified appear at the surface. This allows observational tests of the predicted nuclear fusion and mixing processes. In this section, we discuss the two major mass-loss mechanisms. We also summarize expressions for the observed mass-loss rates in various types of stars.