The properties of opacity at high temperature (and low pressure) are controlled by the physical process of ionization. Other physical properties of the gas, including thermodynamic quantities such as the specific heats (which are important for transport of energy), are also significantly affected by the ionization process. In order to have a clear understanding of the physics of certain regions in the Sun, it is important to have a quantitative model of ionization. This leads us to consider an equation originally derived by Saha (1921) for ionization equilibrium.