STARS carries a state-of-the-art experiment designed to make fundamental measurements of unprecedented quality, enabling us to probe a large variety of stars from their cores to their transition regions. By testing and calibrating the central theory in astrophysics, STARS will create a new era in stellar physics, providing, for the first time, detailed information about the internal structure of the stars, about their dynamics and their associated surface activity, which has far-reaching implications for the whole of astronomy and cosmology.