The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test. Heavier stars like the Sun can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size. Brown dwarfs at the high end of their mass range (60–75 M) can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 M can burn off their lithium by the time they are half a billion years old; thus, this test is not perfect.