What is a Brown Dwarf?
Simply, a Brown Dwarf is a Failed Star. We know that all the stars start their journey from the part of a nebula. The gaseous cloud start to collapse due to its own gravity and keep on collapsing until the defending forces against gravity become powerful enough to equalize or cancel gravity. These defending forces are thermal energy produced due to collision of particles, angular momentum (spinning) of the mass and finally the energy produced from the hydrogen fusion.
It all depends upon mass of the original cloud which started its journey from the part of nebula. If the mass at the stage of a protostar is about 10% of the our Sun (written as MASS = 0.01M SUN) or less, there is not enough heat and ion density in the core to start the hydrogen fusion in the core and hence this star can't shine and instead starts to cool down slowly from this stage. In other words this mass couldn't reach the stage of a shining star and hence called the failed star.
There is a term called degeneracy pressure, which is, due to the laws of quantum mechanics "two electrons can't be at same place under the same state". Due to degeneracy pressure the core stops contracting before the hydrogen even starts to fuse into helium.
Most of the Brown Dwarfs are of the size from 9 Jupiters to 80 Jupiters (Note Sun = 1000 Jupiters in Mass)
So we can say that the Brown Dwarfs are neither Stars nor Planets but something in between. They emit infrared light and can be observed near other shining stars.
Brown Dwarfs are classified into categories such as, L -Dwarfs are young and hot Brown Dwarfs and T -Dwarfs are old and cold Brown Dwarfs.
Brown Dwarf Gliese 229B as seen near the other shining star shown in the image below;
