A significant fraction of galaxies show the signatures of recent large-scale star formation activity. Such galaxies are known widely as ``starburst galaxies''. These are generally characterized by relatively blue colors and strong H II-region-type emission-line spectra (due to a large number of O and B-type stars) and relatively strong radio emission (due to recent supernova remnants). In some cases, the starburst is apparently confined to an unresolved region at the galactic center, which looks very much like an active nucleus. These ``nuclear starbursts'' are typically around 10 times brighter than the giant HII-region complexes seen in normal spirals and are thus distinct from otherwise inactive late-type spirals. The relationship between AGNs and nuclear starbursts is not clear; there is some speculation that there is evolution between the two types of phenomena. A more extreme view is that they are different manifestations of the same phenomenon.
Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)