1. Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. Department of Astronomy and Space Physics, Uppsala University, Box 524, SE – 75120 Uppsala, Sweden
3. Research School of Astonomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, Australian Capital Territory 2611, Australia
4. Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
5. Universitäts–Sternwarte München, Scheinerstrasse 1, D-81679 München, Germany
6. Instituto de Astronomia, Geofísica e Cie¸ncias Atmosféricas, Universidade de São Paulo, Departamento de Astronomia, 05508-900 São Paulo, Brazil

Correspondence to: N. Christlieb^1,2 Correspondence and requests for materials should be addressed to N.C. (e-mail: Email: nchristlieb@hs.uni-hamburg.de).

Abstract

The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts^{1, 2}; if totally metal-free ('population III') stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun^{3, 4}, leading to the suggestion^{5, 6} that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist—that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.