An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an
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 t ...
, 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 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 a ...
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
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''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 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 ...
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
IC 4710 is a galaxy in southern constellation of Pavo, roughly 34 million light-years away.
Discovered on August 18, 1900 by astronomer DeLisle Stewart, this galaxy is composed of bright stars, with bright pockets — marking bursts of new star ...
lies roughly 25 million light-years away in the southern constellation of Pavo.
File:Surveying the cosmos.jpg,
ESO 486-21
The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental research organisation made up of 16 member states for ground-based ast ...
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
NGC commonly refers to:
* New General Catalogue of Nebulae and Clusters of Stars, a catalogue of deep sky objects in astronomy
NGC may also refer to:
Companies
* NGC Corporation, name of US electric company Dynegy, Inc. from 1995 to 1998
* Nati ...
is an irregular galaxy that resides 25 million light-years away in the constellation of Lynx.
File:A distinctly disorganised dwarf.jpg,
UGC 4459
UGC may refer to:
Science and technology
* Universal gravitational constant G, in physics
* Uppsala General Catalogue, an astronomical catalogue of galaxies
* UGC, a codon for cysteine
* Unique games conjecture, a conjecture in computational ...
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 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 o ...
.
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 ...
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 ...
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 ...