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An irregular galaxy is a
galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...
that does not have a distinct regular shape, unlike a
spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...
. Irregular galaxies do not fall into any of the regular classes of the
Hubble sequence The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tun ...
, and they are often chaotic in appearance, with neither a
nuclear bulge In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges ...
nor any trace of
spiral arm Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...
but not enough to place it cleanly into the Hubble sequence. **Subtypes with some spiral structure are called Sm galaxies **Subtypes without spiral structure are called Im galaxies. * An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. * A dI-galaxy (or dIrr) is a
dwarf Dwarf or dwarves may refer to: Common uses *Dwarf (folklore), a being from Germanic mythology and folklore * Dwarf, a person or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a humanoid ...
irregular galaxy. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of
metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...
and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys. Some of the irregular galaxies, especially of the Magellanic type, are small spiral galaxies that are being distorted by the gravity of a larger neighbor.


Magellanic Clouds

The Magellanic Cloud galaxies were once classified as irregular galaxies. The
Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50  kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...
has since been re-classified as type SBm (barred
Magellanic spiral Magellanic spiral galaxies are (usually) dwarf galaxies which are classified as the type Sm (and SAm, SBm, SABm). They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They ca ...
). The
Small Magellanic Cloud The Small Magellanic Cloud (SMC), or Nubecula Minor, is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of ...
remains classified as an irregular galaxy of type Im under current
galaxy morphological classification Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most f ...
, although it does contain a bar structure.


Gallery

File:Feeling blue ESO 338-4.jpg,
Blue compact dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...
ESO 338-4. File:A frenzy of stars IC 4710.jpg, IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo. File:Surveying the cosmos.jpg, ESO 486-21 is a spiral galaxy with a somewhat irregular and ill-defined structure. File:Spotlight on IC 3583.jpg, Irregular galaxy IC 3583 has been found to have a bar of stars running through its center. File:A lopsided lynx NGC 2337.jpg, NGC 2337 is an irregular galaxy that resides 25 million light-years away in the constellation of Lynx. File:A distinctly disorganised dwarf.jpg, UGC 4459 is an irregular dwarf galaxy located approximately 11 million light-years away in the constellation of
Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...
. File:Meeting the neighbours.jpg, Dwarf irregular galaxy known as PGC 18431. File:A spattering of blue.jpg, IC 559 is classified as a type Sm galaxy. File:A cosmic optical illusion.jpg, Irregular dwarf galaxy
PGC 16389 PGC may refer to: *Pennsylvania Game Commission *Persian Gulf Cup, Iran's highest association football league * PGC (gene) * PGC 1000714, a ring galaxy * PGC-1α, a protein which is the master regulator of mitochondrial biogenesis *Playwrights Guild ...
covers its neighboring galaxy APMBGC 252+125-117.


See also

*
Dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...
*
Dwarf elliptical galaxy Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. Examples "Dwarf elliptical" galaxie ...
*
Peculiar galaxy A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar. Astronomers have identified two types of peculiar galaxies: ''interacting galaxies'' and ''active ga ...
*
Galaxy morphological classification Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most f ...


References


External links

{{Authority control Galaxy morphological types