lithium depletion boundary

The term lithium depletion boundary (LDB) essentially refers to a line placed on an H-R diagram (HRD) that distinguishes the stars with high lithium abundance from those with low. HRDs of young stellar clusters tend to show regions containing the stars with lithium or the stars with little lithium. The position of such a line is a clue to the age of the cluster that works well determining the age of clusters on the order of 20 million to 200 million years old and is possibly the best means of age determination within this range. This is useful because age determination of stars and clusters of stars is generally inexact and challenging.

Pre-main-sequence stars (PMSs) fuse their lithium (lithium burning) when they reach a certain temperature (on the order of 3 million K), essentially eliminating it rather quickly. The temperature of a stellar core rises as the star shrinks due to gravity (gravitational collapse with the Kelvin-Helmholtz mechanism) until hydrogen burning produces the pressure to stabilize the star, i.e., to maintain its size. The more massive ("earlier") the star, the sooner the lithium burning happens and the lowest mass stars (future brown dwarfs and red dwarfs) do this much later than early stars. The mass at which this is occurring, i.e., above which lithium no longer shows in such stars' spectrums is related to their age and in a coeval stellar cluster, the age of all its stars. The presence of lithium shows in the spectrum because PMS stars have convection throughout and none of its elements are absent from the surface.