Starburst Galaxy

Billions of large, essentially independent groups of stars exist in the universe. These are called galaxies. A galaxy is labeled a starburst galaxy if an exceptionally high rate of star formation is found to be taking place within it. This often occurs in galaxies that are in the process of or have recently undergone a merging or collision with another galaxy. Although astronomers do not know exactly what causes starbursts in galaxies, or what creates observed ripples in gas and dust of the outer regions of some of them, both phenomena occur in collisions between galaxies. The gravitational pull of the stars of two galaxies passing close to one another seems to cause increased star formation activity and the rippling effect.

First identified by an excess of infrared (heat) radiation from dust within galaxies, violent star formation is usually associated with very distant galaxies. However, it is occurring in some of the nearby galaxies such as the closest starburst galaxy, NGC 253. Within a region 1,000 light-years (about six quadrillion miles) across, the Hubble Space Telescope (HST) high resolution camera has shown very bright star clusters, dust lanes, which trace regions of dense gas, and filaments of glowing gas in the starburst core of this galaxy. The HST has identified several regions of intense star formation, which include a bright, super-compact star cluster. This confirms the theory that stars are often born in dense clusters within starbursts. In NGC 253, dense gas coexists with and obscures the starburst core. Other measurements revealed unusual motions of the gas in the nucleus of NGC 253, which seem to indicate a fast-rotating ring of cold gas as well as gas flowing outward from the nucleus. Similar features have been found in other starburst galaxies.

Ground-based telescopes have shown that the core of one starburst galaxy contains massive clumps of young stars and nebulous ripples in its outermost stellar disk. These observations were made long after any collision, but it is expected that this galaxy, and all starburst galaxies, will eventually settle down to reduced star formation levels, and may one day resemble normal spiral galaxies similar to the Milky Way.