A formula that relates the fraction of ionized atoms in a star’s atmosphere to the temperature of the gas and the electron density (i.e. the number of free electrons per unit volume in the atmosphere). It provides an understanding of the relative prominence of spectral lines of different atoms and ions in the spectra of stars, and enables their temperatures to be derived from their spectral type. The equation was derived by the Indian astrophysicist and nuclear physicist Meghnad Saha (1894–1956).