1. Observatoire de Paris-Meudon, DASGAL, F 75014 Paris, France
2. European Southern Observatory, D 85748 Garching bei München, Germany
3. Michigan State University, East Lansing, Michigan 48824, USA
4. Universidade de São Paulo, São Paulo, BR 01060-970, Brazil
5. Observatoire de Paris-Meudon, DASGAL, F 92195 Meudon Cedex, France
6. GRAAL, Université Montpellier-2, F 34095 Montpellier Cedex, France
7. University of Copenhagen, Astronomical Observatory, DK 2100, Copenhagen, Denmark
8. Osservatorio Astronomico di Trieste, I-34131 Trieste, Italy
9. European Southern Observatory, Casilla 19001, Santiago 19, Chile
10. Lund Observatory, Lund University, S 22100 Lund, Sweden

Correspondence to: R. Cayrel¹ Correspondence should be addressed to R.C. (e-mail: Email: Roger.Cayrel@obspm.fr).

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 stars^{2, 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 stars^{4, 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.

1. Observatoire de Paris-Meudon, DASGAL, F 75014 Paris, France
2. European Southern Observatory, D 85748 Garching bei München, Germany
3. Michigan State University, East Lansing, Michigan 48824, USA
4. Universidade de São Paulo, São Paulo, BR 01060-970, Brazil
5. Observatoire de Paris-Meudon, DASGAL, F 92195 Meudon Cedex, France
6. GRAAL, Université Montpellier-2, F 34095 Montpellier Cedex, France
7. University of Copenhagen, Astronomical Observatory, DK 2100, Copenhagen, Denmark
8. Osservatorio Astronomico di Trieste, I-34131 Trieste, Italy
9. European Southern Observatory, Casilla 19001, Santiago 19, Chile
10. Lund Observatory, Lund University, S 22100 Lund, Sweden

Correspondence to: R. Cayrel¹ Correspondence should be addressed to R.C. (e-mail: Email: Roger.Cayrel@obspm.fr).