Starburst Galaxy

A galaxy that is experiencing a period of intense star-forming activity. Such activity may span the whole galaxy, but is often confined to the nucleus. While this burst of activity can last for 10 million years or more, on a cosmic scale it is really a brief period compared to the 10-billion-year-or-so overall lifetime of a typical galaxy. During a starburst stars form at a rate that is 10s and even 100s of times greater than the normal rate of star formation in a galaxy. The burst occurs over a region that is a few thousand light-years in diameter.

Starbursts may appear as ultraviolet bright galaxies or may be identified by strong optical emission lines or excess radio emission, but the largest number to date has been found from far-infrared emission (as by the Infrared Astronomical Satellite). This is because many starbursts are quite dusty, and when the dust absorbs the ultraviolet and visible light from the massive stars, it radiates in the far-infrared (typically at 0.03–0.1 mm). Starbursts can be triggered by tidal interactions and mergers between galaxies, and the energy output of the massive stars and supernovae can be strong enough to drive a wind sweeping gas completely out of the galaxy (and terminating the starburst). Starburst galaxies are useful guides for the conditions expected in genuinely young galaxies in the early Universe, when star formation was more intense than is usually found in present-day galaxies.

The most popular theory in astrophysics is that the starburst is triggered by a close encounter or even collision with another galaxy. The collision of galaxies sends shock waves rushing through the affected galaxies. These shock waves (manifested as bursts of radiation, gravitational field distortions, etc.) then push on the giant molecular clouds (GMCs) of dust and gas, causing the GMCs to form a few hundred new stars. Many of the newly formed stars are very massive and very bright, making a starburst galaxy among the most luminous types of galaxies.

The massive new stars use up their nuclear fuel very quickly and then explode as supernovas. The exploding supernovas send out bursts of radiation and material, a process that helps stimulate the formation of other new stars, which in turn experience a supernova. This star formation chain reaction can propagate through the central region of a galaxy, where most of the interstellar gas is located. The starburst ends when most of this interstellar gas is used up or blown away.

Questions to Ponder

• How can a collision between galaxies produce a starburst galaxy?
• How far is the nearest Starburst Galaxy from the Milky Way?