Which nuclear reaction can massive brown dwarfs, exceeding 13 Jupiter masses, briefly engage in?
Answer
Deuterium ($ ext{}^2 ext{H}$) fusion
While brown dwarfs fail to sustain the high temperatures needed for ordinary hydrogen ($ ext{}^1 ext{H}$) fusion, the more massive examples, generally those above approximately 13 Jupiter masses ($M_J$), possess enough gravitational compression to briefly fuse deuterium ($ ext{}^2 ext{H}$). Deuterium fusion requires significantly lower temperatures and pressures than $ ext{}^1 ext{H}$ fusion, allowing these substellar objects to generate a temporary burst of internal heat. However, this supply of deuterium is minute, leading to burning that lasts only for millions of years before the object settles into cooling via gravitational contraction.
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