Why are the Supergiant regions (Class I) very sparse on a typical HR diagram?
Answer
Stars spend only a few million years in that highly energetic state
The observation that Supergiant regions (Class I) on the Hertzsprung-Russell diagram are sparsely populated does not imply they are nonexistent, but rather reflects the duration of that evolutionary phase. Stars in the supergiant state are extremely massive and highly energetic, consuming their fuel supply at an incredible rate. This causes them to transition through this phase very quickly, perhaps only lasting a few million years. Statistically, far fewer stars will be caught in this brief, luminous stage compared to the extended, billions-of-years lifespan spent on the main sequence.
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