Heidelberg Joint Astronomical Colloquium

Stars and star clusters are the fundamental visible building blocks of galaxies at present days as well as in the early universe. Today, stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars and star clusters. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Together with the thermodynamic properties of the gas it regulates the fragmentation behavior in star forming clouds. I discuss our current understanding of present-day star formation and speculate about the implications for the first and second generation of stars in the universe. I argue that the masses of zero-metallicity stars were smaller than previously thought and that the first stars typically formed as members of binary or higher-order multiple stellar systems.