Solar models

Here we describe only a few representative main sequence stellar models. One is for the zero age sun, that is, the sun as it was when it had just reached the main sequence and started to burn hydrogen. We also reproduce a model of the present sun, a star with spectral type G2 V, i.e. B − V ∼ 0.63 and Teff ∼ 5800 K, after it has burned hydrogen for about 4.5 × 109 years. In the next section we discuss the internal structures of a B0 star with Teff ∼ 30 000 K and an A0 type main sequence star with Teff ∼ 10 800 K. There are several basic differences between these stars. For the sun the nuclear energy production is due to the proton–proton chain, which approximately depends only on the fourth power of temperature and is therefore not strongly concentrated towards the center. We do not have a convective core in the sun, but we do have an outer hydrogen convection zone in the region where hydrogen and helium are partially ionized. The opacity in the central regions of the sun is due mainly to bound-free and free-free transitions, though at the base of the outer convection zone many strong lines of the heavy elements like C, N, O and Fe also increase the opacity.

In Table 13.1 we reproduce the temperature and pressure stratifications of the zero age sun. In Table 13.2 we give the values for the present sun as given by Bahcall and Ulrich (1987). The central temperature of the sun was around 13 million degrees when it first arrived on the main sequence;…