Brown giants

The size of a gas giant is not important in this equation. The determining factor is the gas giant's Mass. According to accepted theories, the critical mass required in order to be able to commence Hydrogen fusion is 0.084 Mo. A Brown Dwarf must therefore have a mass less than this. The lower limit is more difficult to pinpoint, but a Brown Dwarf is usually regarded as having a mass between 10 Mj and 84 Mj, in other words an object that masses below 1% of the Suns mass or which has 10 times the mass of Jupiter.

Central Temperature

The central temperature must be less than 3 million degrees (critical temperature required for substantial nuclear reaction). The temperature is dependent on the mass, and will be lower for lower mass objects.

Surface Temperature

The temperature of the outermost part of a Brown Dwarf is expected to be around 1000 K, though this will of course depend on its age. It will cool down as it get older. Nuclear fusion may take place at the beginning of its life, but this cannot be sustained very long.

Luminosity

Brown dwarfs are believed to shine with only the heat of gravitational contraction for a comparatively short time after birth and then darken as they cool down. Because of their low surface temperature Brown Dwarfs are not very luminous.

Other Oddities
Brown Dwarfs with hotter outer atmosperes emmit X-Rays, in much the same way as stars. There have also been documented cases of activity that resembles solar flares. Much of this has occured in larger Brown Dwarfs.