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The oldest stars in the Milky Way

In the framework of modern cosmology, the first stars formed in the cores of dark matter overdensities of a few million solar masses (M⊙), when small density fluctuations present in the early universe first attained large amplitudes. Then, the Jeans gravitational instability triggered a run-away collapse, enabling these cores to reach stellar densities, provided that the baryonic matter was able to efficiently radiate away the heat gained from gravitational potential energy during the collapse. Thanks to recent enormous progress in knowledge of the microwave cosmological background (in particular observations by the NASA Wilkinson Microwave Anisotropy Probe), the chemical composition of matter produced during primordial nucleosynthesis in the first 15 min after the Big Bang is completely constrained, and one can confidently identify the chemical composition of the matter from which the first stars were born. Stars with such a composition are called, for historical reasons, Population III stars. The nature of the first stars has aroused the curiosity of theoreticians and observers alike. Important progress has been made on both sides in the last ten years. A brief account of these recent advances is presented here. When and where the first stars formed is now fairly well understood. One major uncertainty that remains however concerns the distribution of masses of the first stars. It is almost certain that these masses were above the mass required to have such stars still radiating today, i.e. above 0.9 times the mass of the Sun, according to stellar evolution computations. This is unfortunate for observers, who have endeavoured for many decades to find even a single example of a Population III star. However, in the course of their searches, they have at least observed stars with chemical compositions that are very close to the primordial composition. A few stars have been identified, polluted by only 3 × 10−6 in mass by elements not produced by the primordial nucleosynthesis, but by supernova nucleosynthesis. The study of the chemical composition of this tiny fraction of matter is a wonderfully rich source of information on true Population III objects, which synthesized the elements composing it, and then ejected these elements into the surrounding primordial medium, producing the matter from which were born the still observable low-mass stars scrutinized by observers.