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. 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
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 w ...
nor any trace of spiral arm structure.
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 are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing 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 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 that are being distorted by the gravity of a larger neighbor.
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). 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 ...
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 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.
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 Gui ...
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 ...