Science journalist: Brown dwarfs are celestial objects with more mass than planets but less mass than stars  They are identified by their mass and whether or not lithium is present in their atmospheres  Stars at least as massive as the Sun have lithium remaining in their atmospheres because the mixing of elements in their internal nuclear furnaces is incomplete  Stars with less mass than the Sun have no lithium because the element has been fully mixed into their nuclear furnaces and consumed
A brown dwarf does not have a fully functional nuclear furnace and so its lithium cannot be consumed
No celestial object that has no lithium in its atmosphere is a brown dwarf.