What prevents a failed protostar (brown dwarf) from replenishing energy lost to space over cosmic timescales?

Answer

It lacks the internal thermonuclear furnace for hydrogen burning

Unlike a true star that achieves equilibrium by burning hydrogen steadily, a brown dwarf lacks the necessary internal thermonuclear furnace to replenish the energy it continuously loses to space, leading to a long, slow decline.

What prevents a failed protostar (brown dwarf) from replenishing energy lost to space over cosmic timescales?
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