Ahh, stars. Giant furnaces of nuclear fusion. Doing the stuff our Sun does, burning hydrogen fuel into helium (among other things) and emitting lots of visible light and energy in the process.
But when we take a look at brown dwarfs, they aren't like normal (i.e., main sequence) stars like our Sun. Instead of burning hydrogen into helium for their fuel, brown dwarfs don't generate enough pressure to make that happen; they can only burn hydrogen into deuterium.
Let's go over what the differences here are. A hydrogen nucleus is just a proton, with a mass of 938.272 MeV/c2. (I use these units…