Extreme star formation is the violent, luminous star formation that occurs in starbursts and luminous infrared galaxies. It is the formation of super star clusters that may eventually become globular clusters. It is the source of galactic winds and metal enrichment in galaxies. It is probably what most star formation in the universe was like several gigayears ago.
The process of star formation and its associated microphysics is most easily studied in the local universe where we can examine the process of star formation in detail. While there are regions in the Galaxy that may qualify as extreme star formation, most extreme systems are extragalactic. Advances in the study of extragalactic star formation during the next decade are likely to come from improvements in spatial resolution and sensitivity, particularly in the infrared and submillimeter parts of the spectrum. The refurbished HST, forthcoming JWST, and ground-based adaptive optics systems will make fundamental contributions to our understanding of the stellar content of extreme star forming regions. Herschel, ALMA, CARMA, Plateau de Bure, SMA, and SOFIA are the far-infrared, submillimeter, and millimeter telescopes that will deliver images and spectra of molecular gas in galaxies, enabling the study of the earliest stages of star formation, and the regulation of star formation by feedback into molecular clouds. With the subarcsecond and milliarcsecond resolutions now possible we can study the star formation process in other galaxies on the parsec spatial scales of molecular cores, young clusters, and Stromgren spheres.
This review is an attempt to distill a very active area of research on extreme star formation, covering both the stellar content and studies of the star-forming gas, and to project this research into the observations of the next decade. The field is a remarkably broad one, because in the process of star birth and cluster evolution, stars and gas are physically interrelated. The observations discussed here cover the range from ultraviolet spectroscopy of hot stars to millimeter line imaging of cold molecular clouds. The focus will be on star formation in the local universe where individual star-forming regions can be resolved, and the star formation process itself can be studied. Star formation in the early universe, where extreme star formation may have been more the norm than the exception, is covered elsewhere in this volume in contributions by Tom Abel and Alice Shapley.