The lithium discrepancy

In order to measure the amount of lithium produced during Big Bang nucleosynthesis (BBN), researchers look at very old, chemically primitive stars that formed from nearly primordial gas. However, the amount of lithium in these "Spite-plateau" stars (green) is much less than has been inferred by combining BBN with measurements of the cosmic microwave background made using WMAP (yellow band). This mismatch has prompted some researchers to question whether our understanding of stellar astrophysics may be at fault. For example, one proposal suggests that some lithium-7 gets destroyed due to mixing or diffusion processes. Those Spite-plateau stars that have surface temperatures between 5700 and 6400 K have uniform abundances of lithium because the shallow convective envelopes of these warm stars do not penetrate to depths where the temperature exceeds that for lithium-7 to be destroyed (T_destruct ∼ 2.5 × 10⁶ K). The envelopes of cooler stars (data points towards the left of the graph) do extend to such depths, so their surfaces have lost lithium to nuclear reactions. If the warm stars gradually circulate lithium from the convective envelope to depths where T > T_destruct, then their surfaces may also slowly lose their lithium.

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