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Specific heat (specific heat capacity) is the ratio per unit mass of a substance's increased temperature to heat energy added to the substance. For a gas, the ratio also depends upon its pressure and volume, which are affected by the added heat as well as the pressure or container that is holding the gas together and gas specific heat values are generally cited specifying two particular cases: for heat being added while simultaneously adjusting the pressure such that the volume remains constant (CV), and for heat being added while simultaneously adjusting the volume such that the pressure remains constant (CP). The ratio of these two ratios (CV/CP) is a useful value, which is known as the adiabatic index (γ). An index of 5/3 is well-known as a reasonable model of monatomic gases, a common state within stars. The possible indexes are used in simple stellar structure models to determine whether convection is taking place (mixing length theory) and can be used in polytrope equations of state in modeling convective stars.
Solids, which do not fill a volume in the manner of gases, generally have a single cited specific heat capacity.