What makes a star a brown dwarf?
ASTRONOMERS can label an object as a ‘brown dwarf’ in one of two ways: by its mass or by its origin. As defined by mass, brown dwarfs are objects too lightweight to sustain fusion of hydrogen nuclei in their centres but massive enough to generate significant energy by gravitational contraction. That gives limits of 80 Jupiters on the high end and several Jupiters on the low end. However, Robert A. Brown of the Space Telescope Science Institute in Baltimore believes that the critical distinction between planets and brown dwarfs is their mode of formation. By his criteria, brown dwarfs form in the same way as stars, as condensation in an interstellar gas cloud. They differ from stars in that they are too low in mass for hydrogen fusion to begin. Planets, on the other hand, form from material in a disc around a star. Theory puts a lower limit on brown dwarfs, because the lowest mass of objects that can form from protostellar clouds is about 20 Jupiters, according to Alan Boss of the Carne
