Starburst Galaxies: Hotbeds of Star Formation

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Hubble Space Telescope looked at a pair of colliding galaxies that are tangling as they interact. The shock of the collision has produced blue streamers that look like clouds. They are actually giant starburst regions, where clusters of hot, massive young stars are being born. Starirth clusters are just one of the many effects of galaxy collision. NASA/ESA/STScI

The universe is filled with galaxies, which are themselves filled with stars. At some point in its life, each galaxy bristled with star formation in vast clouds of hydrogen gas. Even today, some galaxies seem to have more than the usual amount of star birth activity and astronomers want to know why.

There were so many stars being born in some galaxies in earlier times that they probably looked like cosmic fireworks bursts. Astronomers refer to these hotbeds of star birth as "starburst galaxies." They have unusually high rates of star formation, and those bursts last for a short time during the galaxy's long life. That's because star formation burns through the gas reserves of the galaxy very quickly.

It is likely that the sudden burst of star birth is triggered by a specific event. In most cases, a galaxy merger does the trick. That's when two or more galaxies mesh together in a long gravitational dance and eventually meld together. During the merger, the gases of all the galaxies involved are mixed together. The collision sends shock waves through those gas clouds, which compress the gases and set off bursts of star formation. 

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The space between galaxies may look empty, but it's not. It's filled with gases and sometimes streamers of stars that seem to arc between galaxies. Space Telescope Science Institute

Properties of Starburst Galaxies

Starburst galaxies are not a "new" type of galaxy, but rather simply a galaxy (or mingled galaxies) in a particular phase of their evolution. Even so, there are some properties that show up in most starburst galaxies:

  • a very rapid star formation rate. These galaxies will produce stars at rates well above the average rate of most "regular" galaxies;
  • availability of gas and dust. Some galaxies may have higher than normal star-formation rates simply due to their high volumes of gas and dust. However, some starburst galaxies do not a have the reserves to justify why they would have such high rates of star formation, so mergers may not be the only explanation;
  • star formation rate is inconsistent with the age of the galaxy. The main point is that the current rate of star formation could not have been constant since the formation of the galaxy given its age. An older galaxy simply wouldn't have enough gas left over to keep up starbirth action for billions of years. In some starburst galaxies astronomers see a sudden burst of star birth, and often the explanation is a merger or chance encounter with another galaxy.

Astronomers sometimes also compare the rate of star formation in a galaxy relative to its rotational period. If, for example, the galaxy exhausts all of its available gas during one rotation of the galaxy (given the high star formation rate), then it can be considered a starburst galaxy. The Milky Way rotates once every 220 million years; some galaxies go much slower, others faster.

Another widely accepted method to see if a galaxy is a starburst is to compare the star formation rate against the age of the universe. If the current rate would exhaust all of the available gas in less time than 13.7 billion years, then it's possible that a given galaxy may be in a starburst state. 

Gas in galaxy collisions
Annotated image showing dazzling eyelid-like features bursting with stars in galaxy IC 2163. A tsunami of stars and gas triggered by a glancing collision with galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy. M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

Types of Starburst Galaxies

Starburst activity can occur in galaxies ranging from spirals to irregulars. Astronomers who study these objects classify them into sub-types that help describe their ages and other characteristics. Starburst galaxy types include:

  • Wolf-Rayet galaxies: defined by their ratio of bright stars that fall into the Wolf-Rayet classification. Galaxies of this type have regions of high stellar wind, driven by the Wolf-Rayet stars. Those stellar monsters are incredibly massive and luminous and have very high rates of mass loss. The winds that they produce can collide with regions of gas and drive rapid star formation.
  • Blue compact galaxies: low mass galaxies that were once thought to be young galaxies, just beginning to form stars. However, they usually contain populations of very old stars. That usually is a good clue that the galaxy is quite old. Astronomers now suspect that blue compact galaxies are actually the result of mergers between galaxies of varying ages. Once they collide, starburst activity ramps up and lights up the galaxies.
  • Luminous infrared galaxies: dim, hidden galaxies that are difficult to study because they contain high levels of dust that can obscure observation. Typically infrared radiation detected by telescopes is used to penetrate the dust. That provides clues to increased star formation. Some of these objects have been found to contain multiple supermassive black holes, which can shut down star formation. The increase in star birth in such galaxies has to be the result of a recent galaxy merger.

    Cause of Increased Star Formation

    Although the merger of galaxies is pinpointed as the main cause of star birth in these galaxies, the exact processes are not well understood. Partially, this is due to the fact that starburst galaxies come in many shapes and sizes, so there may be more than one condition that leads to increased star formation. However, for a starburst galaxy to even form, there must be lots of gas available to generate the new stars. Also, something must disturb the gas, to begin the gravitational collapse process that leads to the creation of new objects. Those two requirements led astronomers to suspect galaxy mergers and shock waves as two processes that can lead to starburst galaxies. 

    Centaurus A galaxy has a massive black hole at its heart that is actively gobbling up material. The actions of such active galactic nuclei may play a role in starbursts in galaxies.  ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)

    Two other possibilities for the cause of starburst galaxies include:

    • Active Galactic Nuclei (AGN): Virtually all galaxies contain a supermassive black hole in their core. Some galaxies appear to be in a state of high activity, where the central black hole is ejecting massive amounts of energy. There is a great deal of evidence to show that the presence of such a black hole can dampen star formation activity. However, in the case of these so-called active galactic nuclei, they can also, under the right conditions, trigger rapid star formation as the accretion of matter in a disk and its eventual ejection away from the black hole can create shockwaves that could trigger star formation.
    • High supernova rates: Supernovae are violent events. If the rate of explosions increases due to the presence of a very high number of aging stars in a compact area, the resulting shockwaves can begin a rapid increase in star formation. However, this such an event to occur the conditions would have to be ideal; more so than in the other possibilities listed here.
    The Crab Nebula
    A supernova can push clouds of nearby gas around spur limited amounts of starbirth. This supernova is shown in a Hubble Space Telescope view of the Crab Nebula supernova remnant. NASA/ESA/STScI

    Starburst galaxies remain an active area of investigation by astronomers. The more they find, the better scientists can describe the actual conditions that lead up to the bright bursts of star formation that populate these galaxies. 

    Key Takeaways

    • Starburst galaxies are galaxies where high rates of star formation has occurred very quickly.
    • Nearly all types of galaxies can undergo starburst events if conditions are right.
    • Astronomers know that starburst galaxies are often involved in mergers that mingle stars and gas. Shock waves push the gas, which sets off the starburst activity.

    Edited and updated by Carolyn Collins Petersen.