Star Formation Histories in the nearby Universe: the HST Legacy

The aim of this thesis is to study star-forming galaxies in the local Universe, and in particular the distribution of their stellar populations and their star formation histories, to understand how the clustering of star formation evolves both in space and time. The data I analyzed have been acquired with the Hubble Space Telescope, whose spatial resolution and sensitivity allow to measure individual stars with the highest possible accuracy even in crowded and relatively distant galaxies. Thanks to its exquisite performances, HST is still the most powerful facility for this kind of studies. The method I used to derive the star formation histories of the examined galaxies is based on the color-magnitude diagram, one of the best information desks on the evolution of a galaxy or stellar system. More specifically, I applied to the observational color-magnitude diagrams the synthetic CMD technique, a reliable tool to explore the detailed star formation history of nearby galaxies and refine stellar evolution models by comparing them with the data. This method was implemented in the code SFERA, which I contributed to improve and that will be extensively described in this thesis. Within this framework, I discussed the results obtained for three galaxies of different morphological type which have been targeted by the HST Treasury program LEGUS, a large international collaboration aimed to investigate and connect the different scales of star formation, from young stellar clusters to local Universe galaxies. The galaxies I analyze are DDO 68, a dwarf irregular, NGC 4449, a Magellanic irregular, and NGC 7793, a flocculent spiral, studied both in the UV and optical bands, in order to recover their star formation histories from very recent to older epochs and to understand whether and how the star formation process may depend on the morphological, dynamical and environmental properties of the galaxies.