On the Effect of Radiation Pressure in the Atmospheres of Early-Type Stars

The effects of radiation pressure on the stability of the atmospheres of the high-temperature stars are discussed and it is shown that these effects are not negligible for stars with T _e > 20,000 deg. An expression for the adiabatic gradient, $$d \text{log} T/d \text{log} p_g$$ , in a mixture of radiation and an ionizable gas in thermodynamic equilibrium is obtained. A recent method, proposed by Unsöld, of obtaining the adiabatic gradient from entropy considerations is extended to include the effect of the radiation field. It is shown that stars of high effective temperature and low surface gravity probably do not have stable atmospheres.