What primarily powers brown dwarfs instead of sustained hydrogen fusion?
Answer
Slow gravitational contraction and cooling
Brown dwarfs are fundamentally defined by their inability to maintain the self-sustaining nuclear fusion of ordinary hydrogen ($ ext{}^1 ext{H}$) into helium that powers true main-sequence stars. Since they cannot sustain this core reaction, their primary source of internal energy over long timescales comes from the slow, steady release of gravitational potential energy as they contract over billions of years. This process causes them to cool down progressively, becoming dimmer and redder as time passes, essentially acting as cooling remnants of their formation stage.
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