In physics, the Saha ionization equation is an expression that relates the ionization state of a gas in thermal equilibrium to the temperature and pressure. The equation is a result of combining ideas of quantum mechanics and statistical mechanics and is used to explain the spectral classification of stars. The expression was developed by Indian physicist Meghnad Saha in 1920. Description For a gas at a high enough temperature (here measured in energy units, i.e. keV or J) and/or density, the thermal collisions of the atoms will ionize some of the atoms, making an ionized gas. When several or more of the electrons that are normally bound to the atom in orbits around the atomic nucleus are freed, they form an independent electron gas cloud co-existing with the surrounding gas of atomic ions and neutral atoms. In turn, this generates an electric field, where the motion of charges generates currents, making a localised magnetic field, and creates the state of matter called plasma. The Saha equation describes the degree of ionization for any gas in thermal equilibrium as a function of the temperature, density, and ionization energies of the atoms. The Saha equation only holds for weakly ionized plasmas for which the Debye length is large. This means that the screening of the Coulomb interaction of ions and electrons by other ions and electrons is negligible. The subsequent lowering of the ionization potentials and the "cutoff" of the partition function is therefore also negligible. For a gas composed of a single atomic species, the Saha equation is written: where: is the density of atoms in the i-th state of ionization, that is with i electrons removed. is the degeneracy of states for the i-ions is the energy required to remove i electrons from a neutral atom, creating an i-level ion. is the electron density is the thermal de Broglie wavelength of an electron is the mass of an electron is the temperature of the gas is Planck's constant The expression is the energy required to remove the electron. In the case where only one level of ionization is important, we have and defining the total density n as , the Saha equation simplifies to: where is the energy of ionization. Particle densities The Saha equation is useful for determining the ratio of particle densities for two different ionization levels. The most useful form of the Saha equation for this purpose