Brown dwarf

In Astronomy a brown dwarf is an astronomical object that does not have the minimum mass -- eighty times the mass of Planet Jupiter -- required to begin Hydrogen fusion, like traditional stars. However, object larger than thirteen times the mass of Jupiter do contain small amounts of the element Lithium, the third lightest element on the Periodic Table. Very small amounts of Lithium were produced during the Big Bang, and the temperature and pressure requirements for Lithium fusion are low enough for Lithium in the core of these objects to fuse.

Brown dwarfs do produce some energy through fusion. But it is so little energy that there outer atmospheres are planet-like, not star like. They are so dim they glow in the infrared.