A similar scenario may be sketched for 7Li. As a weakly bound nuclide, it is easily destroyed when cycled through stars except if it can be kept in the cooler, outer layers. The high lithium abundances observed in the few "super-lithium-rich red giants" provide direct evidence that at least some stars can synthesize post-BBN lithium and bring it to the surface. But, an unsolved issue is how much of this newly-synthesized lithium is actually returned to the ISM rather than mixed back into the interior and destroyed.
With these caveats in mind, in Figure 4 lithium abundances are shown as a function of metallicity from a compilation by V. V. Smith (private communication). Since the quest for nearly primordial lithium is restricted to the oldest, most metal-poor stars in the Galaxy, stars that have had the most time to redistribute - and destroy or dilute - their surface lithium abundances, it is unclear whether the "plateau" at low metallicities is representative of the primordial abundance of lithium. Although it seems clear that the lithium abundance in the Galaxy has increased since BBN, a quantitatively reliable estimate of its primordial abundance eludes us at present. Given this state of affairs, the most fruitful approach is to learn about stellar structure and evolution by comparing the BBN-predicted lithium abundance to those abundances inferred from observations of the oldest stars, rather than to attempt to use the stellar observations to constrain the BBN-inferred baryon density. Concentrating on the low-metallicity, nearly primordial data, it seems that [Li] 12 + log(Li/H) 2.2 ± 0.1. This estimate will be compared to the BBN-predicted lithium abundance using D as a baryometer and, to the BBN-predicted lithium abundance using the CBR-inferred baryon density. Any tension between these BBN-predicted abundances and that inferred from the Galactic data may provide hints of nonstandard stellar astrophysics.
Figure 4. Lithium abundances, log (Li) [Li] 12 +log(Li/H) versus metallicity (on a log scale relative to solar) from a compilation of stellar observations by V. V. Smith.