The Sun as an equation-of-state laboratory

Thanks to helioseismology, the equation of state of the plasma of the solar interior can be diagnosed. Although the gas in the solar interior is only weakly coupled and weakly degenerate, the great observational accuracy of t he helioseismic measurements puts strong constraints on the nonideal part o f the equation of state. The helioseismic verification of major nonideal ef fects in the equation of state of solar matter has become well established. The dominant contribution is the Coulomb pressure, conventionally describe d in the Debye-Huckel approximation. However, recently the increased precis ion of the helioseismic diagnosis has brought significant observational pro gress beyond the Debye-Huckel approximation. This is illustrated with the s ubtle effect of excited states in bound systems, in particular hydrogen.