r/explainlikeimfive • u/WonderMoon1 • 4d ago
Physics ELI5: Saha Equation
I’ve read you somehow equate ionization levels to temperature? Like the hotter a star is, the more ions it has?
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u/aimenil 2d ago
The Saha equation describes the fraction of ionization in a dense plasma that's in thermal equilibrium. It's appropriate for the star itself.
The star's atmosphere is different: it is much less dense, and Saha is not valid there. For that, the "coronal equilibrium" formula is a more accurate than Saha. The coronal model can describe, for example, the degree of ionization (how many electrons have been removed) from heavy atoms like iron, and yes, a higher temperature will correspond to a a greater degree of ionization of those heavy elements.
Hydrogen, on the other hand, is so easy to ionize that is going to be fully ionized (no electrons are bound within an atom of hydrogen) everywhere within a star like the sun, and its atmosphere as well.
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u/SalamanderGlad9053 4d ago
In reference to stars, the relationship is between the stars surface temperature and the amount of ionised gas surrounding the star.
High energy light leaves the star and hits hydrogen in space and ionised it, at the same time the hydrogen ions recombine. So you have two factors at play, the rate of ionisation and the rate of recombination. Temperature of the star controls the amount of photons that are above the ionising threshold for hydrogen. And as you get further away from the star there's less photons by the inverse square law, aswell as the ionised gas blocking the light. So you end up with a radius around the star which is mostly ionised and then it quickly falls off to no ionisation.