Extra-galactic star formation revealed

High angular resolution observations of nearby galaxies in the optical using ground-based and space-based telescopes have not only revealed the presence of young stellar clusters, but also allowed to study their properties in various dynamical environments. These studies have shown that young massive clusters (YMCs) have typical masses of a few 1000 M-circle dot and sizes of a few parsec irrespective of their site of formation (such as bulges, spiral arms, starburst rings, or mergers). This points toward a universal formation mechanism for these stellar clusters. Observations of the dust and gas content in high redshift galaxies allows one to study the reservoir for star formation in the early universe. These studies reveal extremely high star formation rates of a few 1000 M(circle dot)yr(-1), while the distribution of the molecular gas still seems to be comparable to what is observed in the local universe. The detection of considerable amounts of molecular gas via its CO lines in the highest redshifted QSOs known today (up to z=6.4) indicates that star formation in the early universe has already produced considerable amounts of metals.