What is Irregular galaxy?
[[File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg|thumb|230px|NGC 1427A, an example of an irregular galaxy about 52 million [[light year]]s distant.]] [[File:HubbleTuningFork.jpg|thumb|230px|Tuning-fork style diagram of the [[Hubble]] sequence]] An '''irregular galaxy''' is a [[galaxy]] that does not have a distinct regular shape, like a [[spiral galaxy|spiral]] or an [[elliptical galaxy]].<ref>Butz, Stephen D. 2002. ''Science of Earth systems.'' Cengage Learning, 107 {{ISBN|978-0-7668-3391-3}}</ref> Irregular galaxies do not fall into any of the regular types of the [[Hubble sequence]]. They do not have a [[wikt:bulge|bulge]] in the centre, nor any trace of [[spiral]] structure.<ref>Morgan W.W. & Mayall N.U. 1957. A spectral classification of galaxies. ''Publications of the Astronomical Society of the Pacific.'' '''69''' (409): 291&ndash;303.</ref> They are quite common: they may make up a quarter of all galaxies. Most were once spiral or elliptical galaxies but were deformed by gravitational pull. Irregular galaxies also contain abundant amounts of gas and dust. There are three sorts of irregular galaxies:<ref>Gallagher J.S. & Hunter D.A. 1984. Structure and evolution of irregular galaxies. ''Annual Review of Astronomy and Astrophysics.'' '''22''': 37-74. {{doi|10.1146/annurev.aa.22.090184.000345}}</ref> #An Irr-I galaxy ('''Irr I''') is an irregular galaxy that features some [[structure]] but not enough to place it cleanly into the Hubble sequence. Some of the irregular galaxies are small [[spiral galaxies]] that are being distorted by the gravity of a larger neighbour. #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. #Dwarf irregulars,<ref>Grebel, Eva K. 2004. The evolutionary history of Local Group irregular galaxies. In McWilliam, Andrew & Rauch, Michael (eds) ''Origin and evolution of the elements''. Cambridge University Press, 234-254. {{ISBN|978-0-521-75578-8}}</ref> This type