Microphysics: Equation of state

The equation of state is one of the three fundamental ingredients used to c onstruct stellar models. The plasma of the interiors of stars such as the S un is only slightly non-ideal. However, the extraordinary accuracy of the h elioseismological data requires refined equations of state. It turned out t o be necessary to include a Coulomb correction, commonly evaluated in the D ebye-Huckel approximation. Higher-order non-ideal effects have implications as well, both for plasma physics and for solar physics. As a typical examp le, the recently studied thermodynamic consequence of excited states in com pound particles is discussed. This effect is of considerable relevance in t he helioseismic determination of the helium abundance in the solar convecti on zone.