Brown dwarfs are the link between stars and planets and are not massive enough like stars on the main sequence and sometimes called as failed stars. As a result of their mass-poor they don't start the hydrogen fusion. But brown dwarfs heavier than 13 Jupiter masses do fuse deuterium.
Massive brown dwarfs (heavier than 65 jupiter masses) don’t fuse deuterium, they fuse lithium and this help to detect brown dwarfs because lithium is generally present in brown dwarfs and not in low-mass stars with exception of very young stars.
Another thing that atypical for stars but normal for (older) brown dwarfs is methan. This feature was helping the scientist to confirm the first brown dwarf at Gliese 229B.
Brown dwarfs generally cool and darken steadily over their lifetimes and their luminosity is very low but a remarkable property of brown dwarfs is that they are all roughly the same radius, more or less the radius of Jupiter. And this can make distinguishing them from planets difficult.
November 1995 - Astronomers found with the Hubble space telecope the first clear evidence of a brown dwarf in the Gliese 229 system, 19 light years away.
July 2000 - The Chandra X-ray Observatory captured the first flare from a brown dwarf (LP 944-20).
April 2003 - Scientist detected with Chandra X-rays from a low mass brown dwarf (TWA 5B), 180 light years away in the southern constellation Hydra.
August 2003 - Astronomers detected with the Gemini South telescope in Chile that the closest known brown dwarf in the Epsilon Indi system has a companion.
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