Brown dwarfs are substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses . Brown dwarfs heavier than about are thought to fuse deuterium and those above , fuse lithium as well. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.