A brown dwarf is not a star. It is more like a gas giant like Jupiter but larger. It does not have the mass to begin fusion. The closest thing to the stellar object in the movie would probably be a neutron star. That is if a neutron star were to somehow break apart or if a piece was ejected during its formation.
If a piece of a brown dwarf star even a quarter the size of the one that hit the moon in this movie actually hit the moon, it would punch right through it and keep on going, especially at the oblique angle of the impact.