Developed by the Indian astrophysicist Meghnad Saha in 1920, this formula describes the degree of ionization of a gas as a function of the temperature T, density, and ionization energy.

Let X = n_e / n be the ionization fraction, i.e., the electron density relative to the total density of atoms and ions. Then the Saha equation is

where h is Planck's constant and I is the ionization energy.

External links

• a detailed derivation
• lecture notes