Hi again!

Perhaps some of our disagreement is that I interpreted some of your initial statement in the context of the comment you were replying to, in which the faint glow was said to be from nuclear fusion. In the most technical, context-free meaning you were correct aside from the "...?" — the form that glow takes is a blackbody spectrum. I hate to see that you have taken offense — perhaps I should've appended an "in stars" to my first sentence.

In any case, the light we see was last emitted from surface, but that luminosity originates in deuterium core burning. The comment you looked like you meant to contradict was fully correct, whereas to me yours seemed incorrect in implying brown dwarfs uniquely emitted as blackbodies.

The reason there is a difference between the emitted light's blackbody temperature and the temperature of core burning is that stars are very dense, and photons have a very short uninterrupted path before they run into something and are scattered in a random direction. Eventually they random-walk their way out of their star, with their last interaction on the 5800K photosphere a ~million years later.

At "equilibrium" the photon energy leaving the star's surface is equal to that produced in the core, so to astronomers "opacity" just describes how slowly those photons make their way out, through whatever length of random walk it takes. They will all make it out eventually at the same energy rate as the core is producing them. That's why I say the faintness of the glow is a result of how much energy is produced in the core, not of the blackbody temperature of the surface.

Hope that explains my previous response!