An equation that made the letters physical

Meghnad Saha asks what happens to an atom in a star’s atmosphere when the heat rises. Electrons leave. Lines appear and vanish not because the star is made of something else, but because the same atoms are in a different state. He writes the equilibrium. The Harvard letters can be temperatures.

He is not at Harvard. The spectra he needs are already classified. The equation travels. Payne will carry it across the Atlantic and into abundances. That is 1925. This is 1920.

The Saha equation remains the first tool for reading a stellar atmosphere. It does not yet say the stars are hydrogen. It says why Cannon’s sequence is not a mineral collection.