The presence of the element lithium acts as a chronometer for stars and brown dwarfs. Objects of solar mass and below which have lithium in their atmospheres must be relatively young, since the element is eventually destroyed by fusing with protons. The presence of the lithium absorption line at 6,708 Å has often been used to confirm the youth of suspected pre-main-sequence stars. Very low-mass stars and brown dwarfs evolve so slowly and are cold enough that their lithium may be intact even at a substantial age. Thus, the appearance of lithium below a certain mass has yielded the ages of stellar clusters such as the Pleiades.