
There are 250 billion stars in our galaxy alone. Many are much like the sun, labeled with the Latin sol for “sun” in this diagram. But many more are not quite what we might expect stars to be like, after living under the light of a white G2 star our whole lives.
Wait a second. White G2? Since when is the sun white? And what the heck does G2 mean?
I’m talking about its spectral type—a classification system that organizes stars by their temperatures, determined by what they’re made of. The sequence is O, B, A, F, G, K, and M, in order from hottest to coolest. The sun is a fairly cool star.
But the thing is, the spectral types don’t actually tell you anything about how bright the star is, how big it is, how luminous it is…I could go on.
So how can we make things easy for ourselves and classify stars according to spectral type, size, and luminosity all at the same time?
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