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An extremely primitive star in the Galactic halo


The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium1; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as ‘metallicity’. A number of very metal-poor stars has been found2,3, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen4,5,6. For theoretical reasons7,8 and because of an observed absence of stars with Z < 1.5 × 10−5, it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10−8 to 1.5 × 10−6 (ref. 8), although competing theories claiming the contrary do exist9. (We use ‘low-mass’ here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10−7, which is 4.5 × 10−5 times that of the Sun10) and a chemical pattern typical of classical extremely metal-poor stars2,3—that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion11. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.

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Figure 1: Observed spectra of SDSS J102915+172927.
Figure 2: Lithium abundance of SDSS J102915+172927 compared to that of other metal-poor stars.


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The spectra were secured through X-Shooter Guaranteed Time Observations (GTOs) and ESO Director's Discretionary Time at the ESO VLT Kueyen 8.2-m telescope. E.C. is a Gliese fellow.

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Authors and Affiliations



E.C. developed the code for the analysis of the SDSS spectra, selected the targets for high resolution follow-up, performed the chemical analysis of X-Shooter and UVES spectra and was mainly responsible for writing the paper. P.B. supervised the project and was the Principal Investigator of the ESO proposals. P.F. reduced the X-Shooter data and cross-checked the chemical analysis. L.S. reduced the UVES data. L.M. performed the X-Shooter observations. M.S. and F.S. cross-checked the chemical analysis and contributed to writing the paper. H.-G.L. provided the codes for 3D hydrodynamical simulations and spectral synthesis. R.C. was the main inspirer of this project. S.Z. was responsible for the interpretation of the kinematical data. F.H. and S.R. were respectively the French and Italian Principal Investigators of X-Shooter, who were granted the GTO and decided to invest it in this project. P.M., V.H. and all the other authors contributed to the astrophysical interpretation and to the final version of the paper.

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Correspondence to Elisabetta Caffau.

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The authors declare no competing financial interests.

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Caffau, E., Bonifacio, P., François, P. et al. An extremely primitive star in the Galactic halo. Nature 477, 67–69 (2011).

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