Editing Hubble sequence (section)

===Irregulars===
[[Image:large.mc.arp.750pix.jpg|thumb|The [[Large Magellanic Cloud]] (LMC) – a dwarf [[irregular galaxy]]]]
{{see also|Irregular galaxy}}
Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed [[irregular galaxy|irregular galaxies]]. Hubble defined two classes of irregular galaxy:<ref name="longair">{{cite book |last=Longair |first=M.S. |author-link=Malcolm Longair |title=Galaxy Formation |date=1998 |publisher=Springer |location=New York |isbn=3-540-63785-0}}</ref>
* Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
* Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters
In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the [[Magellanic Clouds]] – two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure<ref>{{cite journal |last1=de&nbsp;Vaucouleurs |first1=G. |author-link1=Gérard de Vaucouleurs |date=1955 |title=Studies of Magellanic Clouds. I. Dimensions and structure of the Large Cloud |journal=[[The Astronomical Journal]] |volume=160 |pages=126–140 |bibcode=1955AJ.....60..126D |doi=10.1086/107173 |last2=Oemler |first2=Augustus Jr. |last3=Butcher |first3=Harvey R. |last4=Gunn |first4=James E.}}</ref> in the [[Large Magellanic Cloud]] led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the [[Small Magellanic Cloud]] (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.

Examples of irregular galaxies: [[Messier 82|M82]], [[NGC 1427A]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]].