Spectral Class and Lines
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Transcript: The spectral lines that appear in a stellar sequence depend on temperature. Helium takes a larger temperature to ionize than hydrogen, and this effects the visibility of helium compared to hydrogen. Going from hotter stars to cooler stars: O and B stars show ionized helium and neutral helium, A and F stars show mostly neutral hydrogen atoms, G stars show neutral hydrogen and some ionized calcium, K stars show neutral metal atoms of various species, and the coolest M stars show neutral metals and molecular lines.
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