Differences between heavy and light brown dwarfs stars
Lithium: Brown dwarfs, unlike stars with low mass, contain lithium. This is due to the fact that stars are enough for fusion reactions of the temperature quickly exhaust its initial reserves of lithium. The collision kernel of lithium-7 and two free protons formed the nucleus of helium-4. The temperature required for this reaction is slightly lower than the temperature at which fusion is possible with the participation of hydrogen. Convection in stars is the cause of complete depletion of lithium, which is from the cold outer layers gradually into the hot interior, and it burns. Consequently, the presence of lithium lines in the spectra of the candidates for brown dwarfs is a good sign of their subzvezdnoy structure. Such an approach to distinguish brown dwarfs and low mass stars was first proposed by Rafael Rebola and his colleagues and was named test lithium-ion.
At the same time, the lithium is composed of very young stars, despite everything still does not burn it. More heavy stars like our Sun, contain lithium in the upper atmosphere, which is too cold for the reactions to his participation. But such a star easily distinguishable from the brown dwarfs in size. On the other hand, the heavy brown dwarfs (on the order of 65-80 MJ) can deplete reserves of lithium in the early years of his life, that is about half a billion years. Thus, lithium test is not perfect.
Methane: Unlike stars, brown dwarfs, some in the final period of its existence, rather coldly, that for a long time to accumulate in the atmosphere foreseeable amount of methane. An example may serve as Gliese 229.
Brightness: main sequence stars, ostyvaya, eventually reaching a minimum brightness, which they can sustain stable thermonuclear reactions. This value is the brightness of an average of at least 0.01% brightness of the Sun. Brown dwarfs tuskneyut and gradually cools down during its life cycle. Just old dwarfs are too dull to be stars.