[en] In this study of stellar structure, evolution, stability, and pulsation or explosion, there are three very vital pieces of physical information needed. We assume the composition is known from observations of assumption. To construct a model of a star we then need to know the nuclear generation rates which give the luminosity the star emits, the pressure and energy equation of state which determines the flow of radiation through the star. It is the equation of state and opacity that we will be discussing in the next two lectures