An irregular galaxy is a

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

that does not have a distinct regular shape, unlike a

spiral In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

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 three main galaxy morphological classification, classes of galaxy described by Edwin Hubble in his Hub ...

. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of

spiral arm Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms i ...

structure. This absence of structure in an irregular galaxy leads to little density waves in these galaxies. This makes irregular galaxies prime areas to study

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

without the effects of density waves. Collectively they are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust. This is not necessarily true for dwarf irregulars. Irregular galaxies may also be formed in galaxy collisions. Irregular galaxies are commonly small, about one tenth the mass of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy, though there are also unusual cases of large irregulars like UGC 6697. Due to their small sizes, they are prone to environmental effects like colliding with large galaxies and intergalactic clouds.

Types

There are three major types of irregular galaxies: * An Irr-I galaxy (Irr I) is an irregular galaxy that features some

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 as ...

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, dwarfs or dwarves may refer to: Common uses *Dwarf (folklore), a supernatural being from Germanic folklore * Dwarf, a human or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a sh ...

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 the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

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. These types of irregular galaxies also have high amounts of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. Irregular galaxies are considered late-type along with

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''

as opposed to early type

elliptical galaxies An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Re ...

. 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''

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) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...

has since been re-classified as type SBm (barred

Magellanic spiral A Magellanic spiral galaxy is a spiral galaxy with only one spiral arm. Magellanic spiral galaxies are classified as the type Sm (with sub-categories SAm, SBm, SABm); the prototype galaxy and namesake for Magellanic spirals is the Large Magell ...

). The

Small Magellanic Cloud The Small Magellanic Cloud (SMC) 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 approximately 7 bill ...

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 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 Lynx constellation. 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 M ...

. 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 covers its neighboring galaxy APMBGC 252+125-117.

References

External links

{{Authority control Galaxy morphological types

Types

Magellanic Clouds

Gallery

See also

References

External links