The main thing that Hertzsprung-Russel diagram did was that it demonstrated the relationship between temperature and luminosity (“brightness”) in stars, thus providing evidence for/about stellar evolution. Specifically, most stars will increase in magnitude and temperature over their lifetimes until they reach a “turning point”, after which they will hold steady in luminosity but decrease in temperature (this is because they’re expanding to become giants or supergiants).
These days, the Hertzsprung-Russel diagram is primarily used to estimate the distance to stars, by translating their temperature (obtained from looking at their color spectrum) into an absolute magnitude (how bright they *should* be) and comparing that against their apparent magnitude (how bright they *are*).
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