GALAXIES, STARBURST JOHN P. HUCHRA Starburst galaxies are galaxies presently undergoing an intense period of star formation. Because all late type galaxies (spiral and Magellanic irregular galaxies) are forming stars at some rate, to qualify as a true starburst, the current rate of star formation must greatly exceed the rate calculated if the total mass of the galaxy were turned into stars at a constant rate over the age of the galaxy (which is usually taken to be nearly the age of the universe, 10-20 billion years). In 1968, Beatrice Tinsley showed that the properties of normal galaxies could be explained by models in which their stellar populations are formed by either constant or declining star formation rates over the age of the universe. However, in 1973, Leonard Searle, Wallace Sargent, and William Bagnuolo noted that the optical colors of the bluest galaxies (which had been discovered in surveys for compact and peculiar galaxies, by Fritz Zwicky and in surveys for objects with ultraviolet excesses, by Guillermo Haro and B.E. Markarian) were bluer than could be explained by models with even constant star formation. Subsequent models by the author and by Richard Larson and Tinsley in the late 1970s showed that the observed properties of such galaxies could easily be explained by superposing a burst of star formation on an older population. In typical starburst galaxies, 1-10% of the mass of the galaxy is involved in a star formation episode that lasts -0.1% of the age of the universe. Starburst galaxies fall into three basic classes: (1) extragalactic HII regions, first identified as such by Sargent and Searle in 1970; (2) clumpy irregular galaxies, a subset of the Magellanic irregulars; (3) starburst nuclei, often just called starburst galaxies. In all of these objects, the energy output is dominated by recently formed hot young stars (of spectral classes O and B and ages less than or equal to 10 million years). The presence of hot young stars is evidenced at optical wavelengths by the blue color of such galaxies and by the presence of strong emission lines from gas ionized by such stars (see Fig. 1). Recent star formation is also usually evidenced in the infrared by strong thermal emission from warm dust that has been heated by the hot stars. (The exception to this might occur if the metal content of the interstellar medium in the galaxy is so low that dust cannot form effectively.) The Infrared Astronomical Satellite (IRAS) whole-sky survey has detected nearly 250,000 high galactic latitude sources at a wavelength of 60 **, most of which are galaxies and a large fraction (10-30%) of which could be classified as starburst galaxies. In some extreme cases, a signature of starburst activity can also be seen at radio wavelengths as extended synchrotron emission from young supernova remnants, produced as massive young stars reach the end points of their evolution. The three classes of starburst galaxies differ in their morphology, average luminosity, spectroscopic properties, and probably in the mechanisms responsible for inducing their bursts of star formation. Extragalactic HII regions are generally of low luminosity (**** of our galaxy's), very low metal abundance (*** of the solar abundance ratios), and "compact" morphology. As the name implies, they can best be described as isolated giant HII regions. In the most extreme cases, Markarian 36 and Markarian 116=IZw 18, the majority of the stars formed (by mass) have been formed in the last few tens of millions of years. The clumpy irregular galaxies are Magellanic irregular galaxies that are forming stars in a majority of the volume of the galaxy at the same time. NGC 4214 (Fig. 2a) and NGC 4449 are examples of irregular galaxies that qualify as starbursts. These galaxies have intermediate luminosities (***** of our galaxy's), and somewhat low metal abundances (*** solar). The starburst nuclei galaxies, which is somewhat of a misnomer because star formation is often proceeding vigorously well outside the galaxy's nucleus, are usually spiral galaxies and are very often members of interacting or disturbed systems. They are luminous (1-10 times our galaxy) and metal-rich. Examples of this type of starburst galaxy include NGC 7714=Markarian 538, NGC 3690= Markarian 171 (Fig. 2b), and M82. These galaxies contain large amounts of warm and hot dust and thus emit strongly in the far infrared. In many (for example, NGC 6240 and NGC 3690) strong OH maser emission has been detected at the 18-cm radio wavelength, another indicator of violent star formation activity. Several different mechanisms have been proposed to explain bursts of star formation in galaxies. The most likely cause is dynamical interaction. A majority of the starburst galaxies uncovered in early surveys of galaxy colors and in such surveys as that done by the IRAS satellite are in interacting systems and appear to be kinematically disturbed (see Fig. 2a). Dynamical interactions can cause star formation by shocking the gas and even by gas transfer. Other processes that can be responsible for extreme star formation rates in galaxies are exceptionally strong spiral density waves (the spiral shock patterns that Lin proposed to explain the very regular spiral patterns of many bright spiral galaxies) or the infall of intergalactic gas clouds (as opposed to another galaxy). It is probable that all of these mechanisms operate. Additional Reading Kunth, D., Thuan, T.X., and Tran Than Van, J., eds.(1985). Star-Forming Dwarf Galaxies. Editions Frontieres, Gif sur Yvette. Lonsdale-Persson, C.J., ed.(1987). Star formation in galaxies. NASA Conference Publication No. 2466. Shields, G.A.(1990). Extragalactic HII regions. Ann. Rev. Astron. Ap. 28 525. Thuan, T.X., Montmerle, T., and Tran Than Van, J., eds.(1987). Starbursts and Galaxy Evolution. Editions Frontieres, Gir sur Yvette. Vorontsov-Vel'yaminov, B.A.(1987). Extragalactic Astronomy. Harwood Academic Publishers, London.