Starburst galaxies make stars so fast they burn through their fuel in a cosmic blink
The Milky Way turns roughly three Suns' worth of gas into new stars each year. A starburst galaxy can manage a hundred or more. At that pace its whole gas supply is gone in around a hundred million years, a brief frenzy usually set off by a collision or close pass with another galaxy.
What makes a starburst is not simply a high number but a mismatch in timing. An ordinary galaxy would take one to two billion years to use up its molecular gas; a starburst would drain it perhaps ten times faster, well within the galaxy's own lifetime or even a single rotation. Astronomers often call a galaxy a starburst when it is forming stars at least three times faster than its long-run average. In the nearby universe they make up only about 1 percent of star-forming galaxies, though they grow more common the farther back we look.
The trigger is usually gravity. When galaxies merge or brush past each other, tidal forces and colliding gas clouds rob gas of its spin, so it plunges inward and compresses into stars. Such galaxies often trail long tidal tails. Bars across a spiral's centre can funnel gas toward the core and do the same job. M82 is the classic case: a close encounter with its neighbour M81 set it off, and radio maps show streams of hydrogen still linking the pair, plus a crowd of young supernova remnants in M82's heart. The colliding Antennae galaxies are another.
Inside, conditions are ferocious. Abundant gas builds massive stars whose ultraviolet light ionises the hydrogen around them, and many soon explode as supernovae. Some starbursts are so dusty that the starlight is absorbed and re-emitted as infrared, making them glow brighter than a trillion Suns in the most extreme types. Others are compact blue dwarfs nearly free of dust, among them IZw18, the most metal-poor galaxy known, and the Green Peas, spotted by volunteers in the Galaxy Zoo project.
The party ends when the fuel runs out or stellar winds blow it away, leaving a quiet post-starburst galaxy. Because many of the most distant galaxies in the Hubble Deep Field are starbursts, nearby examples offer a close-up of what the young universe was doing.
Source: Starburst galaxy
