An Etymological Dictionary of Astronomy and Astrophysics
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1) An equation that expresses the relative number (per unit volume) of → excited atoms in different states as a function of the temperature for a gas in → thermal equilibrium: N_u/N_l = (g_u/g_l) exp (-ΔE/kT_ex), where N_u and N_l are the upper level and lower level populations respectively, g_u and g_l the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h  → Planck's constant.

→ Boltzmann's constant; → equation.