 It is generally considered that brown dwarfs are formed just like regular stars. Stars form inside concentrations of cold interstellar gas and dust known as a molecular cloud. The process begins when the denser parts of the cloud collapse under their own gravity. Angular momentum turns the cloud into a rotating disk with the center forming into a protostar. The whole process takes around 10 million years. At first the protostar has about 1% of its final mass. But the star continues to grow as infalling material is accreted. Densities and temperatures rise dramatically. This continues until either the cloud material is exhausted and we have a brown dwarf, or thermonuclear fusion begins in its core and we have a young star. You can see why brown dwarfs are sometimes referred to as failed stars.