A starburst galaxy is a galaxy

Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 in the process of an exceptionally high rate of star formation

Star formation

Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies. The rate of star formation is so great for a galaxy undergoing a starburst that, if the rate was sustained, the gas reservoirs from which stars are formed would be used up on timescales much shorter than the age
of the galaxy. For this reason, it is presumed that starbursts are temporary. Well-known starburst galaxies include M82

Messier 82

Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major...

, NGC 4038/NGC 4039

Antennae Galaxies

The Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus. They are currently going through a phase of starburst. They were discovered by William Herschel in 1785...

 (the Antennae Galaxies), and IC 10

IC 10

IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by Lewis Swift in 1887. Nicholas U. Mayall was the first to suggest that the object is extragalactic in 1935. Edwin Hubble suspected it might belong to the Local Group of galaxies, but its status remained uncertain for...

.

Starburst definitions

Several definitions of the term starburst galaxy exist and there is no strict definition on which all astronomer

Astronomer

An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s agree. However, many generally agree that the definition must in some way be related to these three factors:

1. the rate at which the galaxy is currently converting gas into stars (the star-formation rate, or SFR)
2. the available quantity of gas from which stars can be formed
3. comparison of the timescale of star formation with the age or rotation period of the galaxy.

Commonly used definitions include:

• Continued star-formation with the current SFR would exhaust the available gas reservoir in much less than the age of the Universe (the Hubble Time). This is sometimes referred to as a "true" starburst.
• Continued star-formation with the current SFR would exhaust the available gas reservoir in much less than the dynamical timescale of the galaxy (perhaps one rotation period in a disk type galaxy).
• The current SFR, normalised by the past-averaged SFR is much greater than unity. This ratio is referred to as the birthrate parameter.

Starburst triggering mechanisms

Essentially to ignite a starburst, it is necessary to concentrate a large amount of cool molecular gas in a small volume. Such concentrations and perturbations are strongly suspected to cause global starburst phenomena in major galaxy mergers, although the exact mechanisms are not fully understood.

Observational surveys have long since shown that there is often a burst of disk star-formation in merging and interacting pairs of galaxies. It is also currently believed that nearby interactions between galaxies that don't actually merge can trigger unstable rotation modes, such as the bar instability, that cause gas to be funneled towards the nucleus, igniting bursts of star formation near the galactic nucleus.

Types of starburst

Classifying the starburst category itself isn't easy since starburst galaxies don't represent a specific type in themselves. Starbursts can occur in disk galaxies

Spiral galaxy

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...

, and irregular galaxies

Irregular galaxy

An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...

 often exhibit knots of starburst, often spread throughout the irregular galaxy. However, several different subtypes of starburst are currently under discussion among galactic astronomers:

• Blue compact galaxies (BCGs). These galaxies are often low mass, low metallicity, dust-free objects. Because they are dust-free and contain a large number of hot, young stars, they are often blue in optical and ultraviolet colours. It was initially thought that BCGs were genuinely young galaxies in the process of forming their first generation of stars, thus explaining their low metal content. However old stellar populations have been found in most BCGs and it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGs show signs of recent mergers and/or close interactions. Well-studied BCGs include IZw18
  I Zwicky 18
  I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted...
  
   (the most metal poor galaxy known), ESO338-IG04 and Haro11.
  • Blue compact dwarf galaxies (BCD galaxies) are small compact galaxies
  • Pea galaxy
    Pea galaxy
    A Pea galaxy, also referred to as a Pea or Green Pea, is a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation...
    
    (Pea galaxies) are small compact galaxies resembling primordial starbursts. They were found by citizen scientists taking part in the Galaxy Zoo
    Galaxy Zoo
    Galaxy Zoo is an online astronomy project which invites members of the public to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research. An improved version—Galaxy...
    
     project.

• Luminous infrared galaxies
  Luminous infrared galaxy
  A luminous infrared galaxy , is a galactic body whose defining characteristic is in emitting more than 1011 solar luminosities in the far-infrared part of the electromagnetic spectrum. A more luminous system, emitting more than 1012 solar luminosities in the far-infrared, is called ultraluminous...
  
  (LIRGs)
  • Ultra-luminous Infrared Galaxies
    Luminous infrared galaxy
    A luminous infrared galaxy , is a galactic body whose defining characteristic is in emitting more than 1011 solar luminosities in the far-infrared part of the electromagnetic spectrum. A more luminous system, emitting more than 1012 solar luminosities in the far-infrared, is called ultraluminous...
    
    (ULIRGs). These galaxies are generally extremely dusty objects. The ultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in the infrared spectrum at wavelengths of around 100 micrometres. This explains the extreme red colours associated with ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation and some astronomers believe ULIRGs to be powered (at least in part) by active galactic nuclei
    Active galactic nucleus
    An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
    
     (AGN). X-ray observations
    X-ray astronomy
    X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
    
     of many ULIRGs that penetrate the dust suggest that many starburst are double cored systems, lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers. Well-studied ULIRGs include Arp 220
    Arp 220
    Arp 220 is the result of a collision between two galaxies which are now in the process of merging. Located 250 million light-years away in the constellation Serpens, it is the 220th object in Halton Arp's Atlas of Peculiar Galaxies.-Features:...
    
    .
  • Hyperluminous Infrared galaxies
    Luminous infrared galaxy
    A luminous infrared galaxy , is a galactic body whose defining characteristic is in emitting more than 1011 solar luminosities in the far-infrared part of the electromagnetic spectrum. A more luminous system, emitting more than 1012 solar luminosities in the far-infrared, is called ultraluminous...
    
    (HLIRGs)

• Wolf-Rayet galaxies (WR galaxies), galaxy where a large portion of the bright stars are Wolf-Rayet star
  Wolf-Rayet star
  Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
  
  s.

The ingredients of a starburst

Firstly, a starburst must have a large supply of gas available to form stars. The burst itself may be triggered by a close encounter with another galaxy (such as M81/M82), a collision with another galaxy (such as the Antennae), or by another process which forces material into the center of the galaxy (such as a stellar bar).

Inside the starburst is quite an extreme environment. The large amounts of gas mean that very massive stars are formed. Young, hot stars ionize the gas (mainly hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

) around them creating H II region

H II region

An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...

s. Groups of very hot stars are known as OB associations. These stars burn very bright and very fast, and are quite likely to explode at the end of their lives as supernovae.

After the supernova explosion, the ejected material expands and becomes a supernova remnant

Supernova remnant

A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...

. These remnants interact with the surrounding environment within the starburst (the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

) and can be the site of naturally occurring masers

Astrophysical maser

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum...

.

Studying nearby starburst galaxies can help us determine the history of galaxy formation and evolution. Large numbers of the very distant galaxies seen, for example, in the Hubble Deep Field

Hubble Deep Field

The Hubble Deep Field is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area 2.5 arcminutes across, two parts in a million of the whole sky, which is equivalent in angular size to a 65 mm tennis...

 are known to be starbursts, but they are too far away to be studied in any detail. Observing nearby examples and exploring their characteristics can give us an idea of what was happening in the early universe as the light we see from these distant galaxies left them when the universe was much younger (see redshift

Redshift

In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

). Unfortunately however, starburst galaxies seem to be quite rare in our local universe, and are more common further away - indicating that there were more of them billions of years ago. All galaxies were closer together then, and therefore more likely to be influenced by each other's gravity. More frequent encounters produced more starbursts as galactic forms evolved with the expanding universe.

Well-known starbursts

M82

Messier 82

Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major...

 is the archetypal starburst galaxy. Its high level of star formation is due to a close encounter with the nearby spiral M81. Maps of the regions made with radio telescopes show large streams of neutral hydrogen connecting the two galaxies, also as a result of the encounter. Radio images of the central regions of M82 also show a large number of young supernova remnants, left behind when the more massive stars created in the starburst came to the end of their lives. The Antennae is another well-known starburst system, made famous by a stunning Hubble picture, released in 1997.

See also

• Active galaxy
  Active galactic nucleus
  An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
  
  
• Baby Boom Galaxy
  Baby Boom Galaxy
  The Baby Boom Galaxy is a starburst galaxy located 12.2 billion light years away. Discovered by NASA's Spitzer Science Center at the California Institute of Technology, the galaxy is notable for being the new record holder for the brightest starburst galaxy in the very distant universe, with...
  
  
• Blue compact dwarf galaxy
  Blue compact dwarf galaxy
  In astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, hot, massive stars. These stars cause the galaxy to appear blue in color. Since they exist mostly in clusters, blue compact dwarf galaxies don't have an exact uniform shape...
  
  
• Messier 82
  Messier 82
  Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major...
  
  
• Pea galaxy
  Pea galaxy
  A Pea galaxy, also referred to as a Pea or Green Pea, is a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation...
  
  
• Starburst

External links