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A useful criterion for telling brown dwarfs from low mass stars is obtained through testing for lithium, which is generally present in the former and not in the latter. Stars, which achieve high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium 7 and a proton producing two helium 4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in the stars ensures that lithium in the whole volume of the star is depleted, resulting in an easily observable change in the star spectral lines. In contrast, most brown dwarfs do not have the high temperature necessary for hydrogen fusion, and thus the reaction destroying lithium does not occur.

It follows that a presence of lithium is a test for low-mass radiant objects that are suspected of being brown dwarfs. If lithium is present it means that there has been no hydrogen fusion, and the object is a brown dwarf (though it could also be that it is a star, but too young and has not had time to deplete all of its lithium). If lithium is absent, the object can be a high mass brown dwarf (that reached temperatures enough for lithium but not for hydrogen fusion), or a low mass M star (that has hydrogen fusion).

Brown dwarfs continue to glow in the red and infrared after their deuterium is exhausted. This glow is from the leftover heat generated by their formation and by the earlier deuterium and lithium fusion. The atmospheres of known brown dwarfs range over temperatures from about 800 to 2500 °C. All brown dwarfs cool over time; more massive objects cool more slowly than lower mass objects.

Several hundred brown dwarfs have been detected, and they are thought to be the most common type of object in the Milky Way galaxy. Gas giant planets that form directly from a collapsing nebula rather than accreting from a protostellar disk like other planets are more properly termed brown dwarfs.

Recent observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely violent, comparable to but far exceeding Jupiter's famous storms.