Evolution Through Helium Burning — Massive Stars

Abstract

After central hydrogen burning, the star has a helium core, which in the absence of energy sources tends to become isothermal. Indeed thermal equilibrium would require that the models consist of an isothermal helium core (of mass M_c = q_oM, radius R_c), surrounded by a hydrogen-rich envelope [of mass (1 — q_o)M] with hydrogen burning in a shell source at its bottom. Such models were discussed in detail in §30.5. We now once more consider the case of M = 3M_⊙, which is typical for stars on the upper part of the main sequence (say M > 2.5M_⊙). The possible solutions were comprised in a linear series of models consisting of 3 branches. This is shown in the first graph of Fig. 30.10, and again in Fig. 31.1, which also gives the position in the HR diagram.