How do Brown Dwarfs generate light and heat if they cannot sustain core hydrogen fusion?

Answer

Through slow gravitational contraction and brief deuterium fusion

Brown dwarfs generate energy from gravitational contraction and, if massive enough, by fusing deuterium, a heavy isotope of hydrogen, but this energy output is limited and fades over time.

How do Brown Dwarfs generate light and heat if they cannot sustain core hydrogen fusion?
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