A type of star that's not massive enough for normal stellar nuclear reactions to occur. Like stars, they form from a cooling cloud of hydrogen gas but with a mass range between 15x and 80x that of Jupiter are not massive enough to sustain hydrogen nuclear fusion. However, they are believed to be able to fuse deuterium and lithium.
The nearest known brown dwarf is binary system Luhman 16 in the constellation Vela. It lies 6.5 light-years distant and shines very dimly at apparent magnitude +15.0. Each star has a mass of only 0.04x that of the Sun with surface temperatures of 1300K.