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The **Saha equation** (**Saha ionization equation** or **Saha-Langmuir equation**)
gives the relative count of ions in a particular
**ionization state** (in this case, number of "missing" electrons)
expected based upon the temperature
and other factors regarding a gas. It is used in stellar models.
One form:

N^{i+1}Z^{i+1}2 ———— = ———— ———— (2π m_{e}kT)^{3/2}e^{-χi/kT}N^{i}Z^{i}n_{e}h^{3}

- N
^{i+1}, N^{i}- number density of two successive ionization states i and i+1. - Z
^{i+1}, Z^{i}- partition function for the same (which differs for each state of ionization). - n
^{e}- number-density of free electrons. - h - Planck constant.
- m
_{e}- mass of an electron. - k - Boltzmann constant.
- T - temperature.
- χ
_{i}- ionization potential (energy needed to ionize) at ionization state i.

number density

state of ionization

temperature