Les premières étoiles

Abstract

The oldest stars of the Galaxy are quite different from common stars, like our Sun. Understanding why it is so, requires to open the question in a cosmological perspective. After the Big Bang, and for at least 300 000 years, the Universe was nearly uniform, and had a very simple chemical composition formed during the hot phase of the Big Bang: only hydrogen, helium and traces of other light elements, deuterium, 3 He, and 7 Li. This composition is known as “primordial”. At a later time, about one or two billion years after the Big Bang, condensations developped at all scales, the smallest ones being stars. The most massive stars, reaching very high temperatures at their center, transformed their initial composition by thermonuclear reactions, producing all common elements observed in the solar system, carbon, nitrogen, oxygen, etc. These elements were dispersed into the interstellar medium by mass-ejection at the final stage of evolution of these massive stars, and recycled by subsequent generations of stars. The first stars must have been formed with the primordial composition, whereas later generations had an increasing proportion of elements produced by stellar nucleosynthesis. Intensive searches of stars with no, or very little elements produced by stellar nucleosynthesis have been performed during the last 20 years. Actually more than 100 stars were discovered with a very low proportion of such elements, one thousandth of the proportion in the Sun (in which they amount to about 1.7% by mass), or less. But no star was found with less that 1/10,000 of the solar proportion. So no “primordial” star has been observed yet. The reason why is still an open question.