A brown dwarf is a celestial object with the following properties
- Mass between that of a planet and a star
- Fusion cannot happen but deuterium fusion is possible
- Convective nature - The same composition all over
- Some brown dwarves exert extremely strong magnetic fields
- This is due to them emitting radio waves (we do not know how this happens)
Brown dwarves are not hot and heavy enough to fuse hydrogen. However, they are powered by deuterium fusion until the core gets hotter and denser (by contraction)
It will not reach the point where it can begin fusing hydrogen because the core is held from contraction due to electron degeneracy pressure