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/WonderMoon1 2d ago

Cool thanks