What is a brown dwarf?

1.there no such thing.
2.a star no longer undergoing nuclear fusion.
3.a star right as it undergoes nuclear fusion.
4.an object that is big, but not massive enough to undergo brackets fusion

3. A star right as it undergoes nuclear fusion.

Explanation: A brown dwarf is a celestial object intermediate in size between a giant planet and a small star, believed to emit mainly infrared radiation.

i think the answer is a, hope this : )