The first generations of stars in the Local Group

The different projects of this thesis contribute to improving our understanding of the early chemical evolution of the galaxies.The standard cosmological model states an hierarchical formation of the galaxies, the dwarf galaxies being the first stellar systems to form in the early Universe, merging to build larger galaxies over time. Observing the oldest stars in different environments can therefore answer many questions for the models of galaxy formation. Do the old stellar populations of the dwarf galaxies share the same chemical abundance patterns as the Milky Way halo ?These oldest and extremely metal-poor stars (EMP, [Fe/H]