What allows a brown dwarf, despite being a 'failed star,' to temporarily generate internal heat and glow?

Answer

The ability to temporarily fuse deuterium

Brown dwarfs, which are slightly more massive than $13$ Jupiter masses, possess enough internal heat and pressure to temporarily fuse deuterium, a heavier isotope of hydrogen, providing a brief glow before fading.

What allows a brown dwarf, despite being a 'failed star,' to temporarily generate internal heat and glow?
astronomystarprotostarstellar evolutionmass