Abstract
The ages of the oldest stars in the Galaxy indicate when star formation began, and provide a minimum age for the Universe. Radioactive dating of meteoritic material1 and stars2 relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232Th (half-life 14 Gyr) has been used to date Galactic stars2,3,4, but it decays by only a factor of two over the lifetime of the Universe. 238U (half-life 4.5 Gyr) is in principle a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously not been detected in stars4,5,6,7. Here we report a measurement of this line in the very metal-poor star CS31082-0018, a star which is strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H) = -13.7 ± 0.14 ± 0.12 yields an age of 12.5 ± 3 Gyr, though this is still model dependent. The observation of this cosmochronometer gives the most direct age determination of the Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly precise lower limit to the age of the Universe.
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Acknowledgements
We thank the ESO staff for assistance during the observations, J.-F. Wyart and L. Tchang-Brillet for helping us to obtain the best current atomic data for Th and U, and G. Simon for providing the unpublished infrared colours for CS 31082-001from the DENIS18 survey. Partial financial support for this work was obtained from the US NSF (to T.C.B.), and from the Carlsberg Foundation and Julie Damms Studiefond (to J.A. and B.N.).
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Cayrel, R., Hill, V., Beers, T. et al. Measurement of stellar age from uranium decay. Nature 409, 691–692 (2001). https://doi.org/10.1038/35055507
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DOI: https://doi.org/10.1038/35055507
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