What are Starburst Galaxies?

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

There are many different shapes, sizes and types of galaxies. While some galaxies may look similar to others, each one is unique. 

Some of the most visually appealing galaxies are known as starburst galaxies. The name comes from the rapid bursts of star formation that light them up. In fact, they have unusually high rates of star formation, which lasts for a short time during the galaxy's long life.

This is because rapid rates of rate of star formation burn through the gas reserves of the galaxy in short amount of time (relatively speaking).

It is likely that the sudden burst was triggered by an event such as a galaxy merger, which mingles the gases of two or more galaxies. The collision sends shock waves through the gas clouds, which sets off starbirth activity.

Properties of Starburst Galaxies

Starburst galaxies are not actually a "new" type of galaxy, but rather simply a galaxy in a particular state. Even so, there are a general set of properties that are generally viewed as the main identifiers for starburst galaxies:

  • A very rapid star formation rate. These galaxies will produce stars at rates well above average for galaxies in general.
  • Short term 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, starburst galaxies do not have the reserves to justify why they would have such high rates of star formation.
  • Star formation rate is inconsistent with age of the galaxy. This really follows from the other two properties. The main point here is that the current rate of star formation could not have been constant since the formation of the galaxy given its age. Or, to put it another way, there has been a dramatic increase in the star formation rate in the recent pas.

    Astronomers sometimes evaluate the rate of star formation in a galaxy relative to its rotational period. That is, if 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.

    Another widely accepted metric 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. This is a less stringent condition than the one above and sometimes leads to debate.

    Types of Starburst Galaxies

    As has already been discussed, starburst galaxies themselves are not a unique galaxy form, but rather is a set of properties that can exist in galaxies ranging form spirals to irregulars. Even so, there has been some attempt to break down starburst galaxies into more specific types. These include:

    • Wolf-Rayet galaxies: These galaxies are 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. These 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: These low mass galaxies were once thought to be young galaxies, just beginning to form stars. However, they usually contain populations of old stars, inconsistent with this model. It is now believed that blue compact galaxies are actually the result of mergers, and it is this event that has sparked the increased star formation.
    • Luminous infrared galaxies: These systems are difficult to study because they contain high levels of dust that can obscure observation. Typically infrared radiation is used to penetrate the dust. Doing so reveals signatures that indicate lots of star formation. Because at least some of these objects have been found to contain multiple supermassive black holes, it is believed that the increase in star formation is the result of a recent galaxy merger.

      Cause of Increased Star Formation

      The exact cause in the increased star formation that characterizes starburst galaxies is 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.

      Some possibilities for the cause of starburst galaxies include:

      • Galaxy mergers: One of the best ways to instigate star formation is to collide giant pillars of gas. This has been observed to happen quite readily when two or more galaxies merge. If two such galaxies contain high levels of free gas the resulting collision may initiate a starburst galaxy.
      • High supernova rates: Supernovae are violent events. And should the rate of explosions increase, 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.
      • 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. These so called Active Galactic Nuclei (AGN) can also, under the right conditions, trigger rapid star formation. While the ejection of material is actually away from the galaxy, the accretion of matter onto the black hole can create shockwaves that could trigger star formation.

        Edited and updated by Carolyn Collins Petersen.

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        Your Citation
        Millis, John P., Ph.D. "What are Starburst Galaxies?" ThoughtCo, Mar. 2, 2017, thoughtco.com/what-are-starburst-galaxies-3072050. Millis, John P., Ph.D. (2017, March 2). What are Starburst Galaxies? Retrieved from https://www.thoughtco.com/what-are-starburst-galaxies-3072050 Millis, John P., Ph.D. "What are Starburst Galaxies?" ThoughtCo. https://www.thoughtco.com/what-are-starburst-galaxies-3072050 (accessed November 20, 2017).