In normal galaxies like the Milky Way, the specific star-formation rates are typically of order 0.1Gyr−1, which implies star-formation time scales (defined as the ratio between the total stellar mass and the current star-formation rate) that are comparable to the age of the Universe. There are, however, systems in which the (specific) star-formation rates are 10 or even 100 times higher, with implied star-formation time scales as short as 108 years. These galaxies are referred to as starbursts. The star-formation activity in such systems (at least in the most massive ones) is often concentrated in small regions, with sizes typically about 1 kpc, much smaller than the disk sizes in normal spiral galaxies.
Because of the large current star-formation rate, a starburst contains a large number of young stars. Indeed, for blue starbursts where the star-formation regions are not obscured by dust, their spectra generally have strong blue continuum produced by massive stars, and show strong emission lines from HII regions produced by the UV photons of O and B stars (see Fig. 1). Since the formation of stars is, in general, associated with the production of large amounts of dust, most of the strong starbursts are not observed directly via their strong UV emission. Rather, the UV photons produced by the young stars are absorbed by dust and re-emitted in the far-infrared. In extreme cases these starbursting galaxies emit the great majority of their light in the infrared, giving rise to the population of infrared luminous galaxies (LIRGs) discovered in the 1980s with the Infrared Astronomical Satellite (IRAS). A LIRG is defined as a galaxy with a far-infrared luminosity exceeding 
. If its far-infrared luminosity exceeds 1012 
it is called an ultraluminous infrared galaxy (ULIRG).
The fact that starbursts are typically confined to a small region (usually the nucleus) of the starbursting galaxy, combined with their high star-formation rates, requires a large amount of cold gas to be accumulated in a small region in a short time. The most efficient way of achieving this is through mergers of gas-rich galaxies, where the interstellar media of the merging systems can be strongly compressed and concentrated by tidal interactions This scenario is supported by the observation that massive starbursts (in particular ULIRGs) are almost exclusively found in strongly interacting systems with peculiar morphologies.
Fig.1. Spectra of different types of galaxies from the ultraviolet to the near-infrared. From ellipticals to late-type spirals, the blue continuum and emission lines become systematically stronger. For early-type galaxies, which lack hot, young stars, most of the light emerges at the longest wavelengths, where one sees absorption lines characteristic of cool K stars. In the blue, the spectrum of early-type galaxies show strong H and K absorption lines of calcium and the G band, characteristic of solar type stars. Such galaxies emit little light at wavelengths shorter than 4000A˚ and have no emission lines. In contrast, late-type galaxies and starbursts emit most of their light in the blue and near-ultraviolet. This light is produced by hot young stars, which also heat and ionize the interstellar medium giving rise to strong emission lines. [Based on data kindly provided by S. Charlot
