mathematical relationship between the observed spectra (spectrum) of stars (star) and their temperatures. The equation was stated first in 1920 by the Indian astrophysicist Meghnad N. Saha (Saha, Meghnad N.). It expresses how the state of ionization of any particular element in a star changes with varying temperatures and pressures. The spectrum of a star is directly related to the relative numbers of atoms (atom) and ions (ion) it contains because each atom or ion can absorb or emit radiation of a particular set of wavelengths.

The Saha equation is

where N_{i + 1} and N_i are the number of atoms in the (i + 1)th and ith ionization states, respectively; U_{i + 1} and U_i describe how energy is partitioned among the (i + 1)th and ith ionization states; E_{i + 1} and E_i are the energies of the ionization states; N_e is the number of electrons (electron); and T is the temperature. The other quantities in the equation are physical constants: m_e is the mass of the electron, k is the Boltzmann constant, and h is Planck's constant.