Stellar evolution theory using p-p chain fusion, CNO, and triple alpha process fusion, the physics indicate a star will expand its size and slowly evolve into a red giant star, e.g. the Sun as the star burns its hydrogen. Equations of state (EOS) are developed for this cycle showing density, pressure, and temperature changes and the ideal gas law (PV=NRT) shows the stellar expansion. Stellar evolution theory is used to explain the H-R star diagram plots obtained for various star clusters like main sequence, main sequence turn-off point (e.g. M67) and red giant branch stars.