The first generations of stars

Extremely metal-poor (EMP) stars are descendants of massive stars from the earliest stellar generations. Their chemical composition reflects the early phases of the Universe, which allows us to place constraints on the masses of these first-generation massive stars and also allows us to better understand the formation of low-mass stars from a gas cloud depleted in metals. EMP stars are a minor component of the stellar population (about a thousandth) and special care has to be invested to find them. We exploited the Sloan Digital Sky Survey (SDSS) to search for EMP candidates with the colours of Turn-Off stars. We observed them at medium- and high-resolution with the ESO/VLT spectrographs X-Shooter and UVES to derive their chemical composition. We present the preliminary results on the metallicity distribution function, on the analysis of strontium, on the carbon enhanced stars, and on our effort to analyse molecular bands with 3D hydrodynamical models.