we now call the Hertzsprung-Russell diagram, or H-R diagram for short because who the heck has the time to keep saying “Hertzsprung-Russell?” They found that stars follow very specific paths as they age, always getting brighter and hotter with time. And the more massive stars evolve much faster than the smaller ones (which makes sense: with more mass, there’s more gravity, which means the fusion rates go that much faster, and the whole game plays out in fast-forward). So by looking at a star and measuring its mass, you can pinpoint where on the evolutionary track it is, how old it is, and how
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