Employing the the worlds largest optical telescopes we identify old stars by means of their chemically primitive nature. These so-called “metal-poor” stars contain only tiny amounts of heavy elements (“metals”) that are heavier than hydrogen and helium. They formed before the Universe was significantly enriched in chemical elements, as we know it today. Hence, the oldest stars in the Milky Way are stellar fossils and make possible to reconstruct the chemical evolution of the Galaxy from the earliest times until today.
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