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Technically Speaking: Survival of Lithium in Stars
Hydrogen fusion destroys lithium in stars. The amount of lithium that can survive is a function
of how strongly the material of the star is mixed down to the core fusion region by convection.
Protostars are convective, so stars start off with a strongly mixed interior, but the initial core
temperature in the protostar is not high enough to destroy lithium. The lightest stars
(red dwarfs) remain convective once
on the main sequence, so lithium is mixed down to
the fusion region and destroyed in red dwarfs.
Because these stars are cool, it takes longer to destroy the lithium,
but calculations indicate that lithium could survive no longer than 100 million years in the lightest true
star.
For more massive stars like the Sun, as the protostar contracts the center becomes radiative and
the region of mixing due to convection begins to retreat toward the surface
at about the same time that the fusion reactions switch on. Whether all the lithium is destroyed
depends on how fast this happens. If the retreat is fast enough to separate a well-mixed surface from the
interior, a tiny amount
of the lithium will survive. In the case of the Sun, it appears that about 1% of its
original lithium was preserved in this manner, leaving the Sun with a very small concentration of
lithium.
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