lithium

Lithium (Li) is the element with atomic number 3, symbol Li. The most abundant isotope, with mass number 7, is stable, as is that with mass number 6. Lithium burns at a temperature less than that of hydrogen burning, suggesting that main sequence stars quickly deplete themselves of it, faster than would brown dwarfs, and its presence has been used to identify the latter.

Lithium is thought to have been formed in the Big Bang, but though observed hydrogen and helium abundances reflect current models of relic abundances, the abundance of lithium does not. The discrepancy between the expected abundance and that indicated by observation is termed the primordial lithium problem or cosmological lithium problem. The oldest observed stars (Population II) appear to have not enough lithium, suggesting by some means it is burned within the stars. Some other stars appear to have too much, suggesting some significant later source. A possible source is cosmic ray spallation in the interstellar medium, i.e., fission produced by cosmic rays. Another possibility is neutrino-induced processes in core collapse supernovae.

The Sun shows a low abundance, seemingly inconsistent with the much larger abundance found in meteorites, this discrepancy termed the solar lithium problem. Both these problems retain research interest.

Lithium in stars is detected through a spectral feature, a spectral line at 6708 Å.