Astronomers solve riddle of missing lithium
European astronomers have found the solution to a mystery that has been infuriating astronomers for years: the riddle of the missing lithium. Some time ago, scientists calculated how much hydrogen, helium and lithium were produced in the aftermath of the big bang. Lithium can also be measured in the oldest, metal-poor stars, which are made from matter similar to the primordial matter. And therein lies the problem: the predicted abundance of lithium in the new-born universe is two to three times higher than that measured in the oldest stars. Ever since the discrepancy was discovered, astronomers have been trying to work out where the missing lithium is. Now a team of European astronomers seems to have solved this riddle, thanks to analyses of a cluster of ancient stars using the European Southern Observatory's Very Large Telescope in Chile. Their findings are reported in the journal Nature. Diffusive processes alter the relative abundance of elements in stars, as gravity slowly but surely forces heavier elements to sink into the depths of the star, out of visibility. 'The effects of diffusion are expected to be more pronounced in old, very metal-poor stars,' said Andreas Korn of Uppsala University. 'Given their greater age, diffusion has had more time to produce sizeable effects than in younger stars like the Sun.' The stars in the cluster studied are all of the same age and started off with the same chemical composition. However, they are at different evolutionary stages, allowing the researchers to see how this affects the atmospheric levels of various elements. Their observations clearly show that as the stars evolve, the proportion of lithium in the atmosphere of the stars at first increases and then drops off sharply, as predicted by diffusion models with extra mixing. This means that the amount of lithium found in the atmosphere of old stars does not necessarily reflect the amount of lithium in the gases which formed the star. 'Once this effect is corrected for, the abundance of lithium measured in old, unevolved stars agrees with the cosmologically predicted value,' said Dr Korn. 'The cosmological lithium discrepancy is thus largely removed.' However, as is so often the case in science, resolving one problem simply raises further questions. In this case, theoreticians now have to identify the physical processes in the stars that are the origin of the extra mixing.
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