How does that happen? Is its fuel more energetic in some sense or does the gravity-vs-pressure equilibrium simply favor lower densities for a total mass of 20M☉?
You're right on both counts! Betelgeuse is burning helium now (post main-sequence). The higher output of helium fusion inflates the outer atmosphere like a giant floppy balloon.
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How does that happen? Is its fuel more energetic in some sense or does the gravity-vs-pressure equilibrium simply favor lower densities for a total mass of 20M☉?
You're right on both counts! Betelgeuse is burning helium now (post main-sequence). The higher output of helium fusion inflates the outer atmosphere like a giant floppy balloon.