Editing Irregular galaxy (section)

{{Short description|Class of galaxy}}
[[File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg|thumb|[[NGC 1427A]], an example of an irregular galaxy. It is an Irr-I category galaxy about 52 Mly distant.]]

An '''irregular galaxy''' is a [[galaxy]] that does not have a distinct regular shape, unlike a [[Spiral galaxy|spiral]] or an [[elliptical galaxy]].<ref>Butz, Stephen D. (2002). ''Science of Earth Systems.'' Cengage Learning. p. 107. {{ISBN|978-0-7668-3391-3}}.</ref> 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 [[bulge (astronomy)|nuclear bulge]] nor any trace of [[spiral arm]] 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> 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]] without the effects of density waves.<ref>{{Cite web |title=Star Formation in Irregular Galaxies |url=https://ned.ipac.caltech.edu/level5/Hunter/Hunter1.html#:~:text=Without%20spiral%20density%20waves%20to,from%20the%20spiral%20density%20wave. |access-date=2025-01-27 |website=ned.ipac.caltech.edu}}</ref>

Collectively they are thought to make up about a quarter of all galaxies.<ref>{{Cite web |url=https://www.mssl.ucl.ac.uk/www_astro/outreach/School/Galaxies/irreg.html |access-date=2025-01-27 |website=www.mssl.ucl.ac.uk |title=Irregular galaxies}}</ref> 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.<ref>{{cite web |url=http://www.le.ac.uk/ph/faulkes/web/galaxies/r_ga_irregular.html |title=Faulkes Telescope Educational Guide – Irregular Galaxies |url-status=dead |archive-date=17 October 2022 |archive-url=https://web.archive.org/web/20221017082000/https://www.le.ac.uk/ph/faulkes/web/galaxies/r_ga_irregular.html }}</ref> This is not necessarily true for dwarf irregulars.<ref>Walter, F. et al. Astophys J '''661''', 102–114, 2007</ref> Irregular galaxies may also be formed in galaxy collisions.<ref>{{Cite web |date=2020-09-21 |title=Types – NASA Science |url=https://science.nasa.gov/universe/galaxies/types/ |access-date=2025-01-27 |language=en-US}}</ref>

Irregular galaxies are commonly small, about one tenth the mass of the [[Milky Way]] 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 [[Interacting galaxy|colliding]] with large galaxies and intergalactic clouds.<ref>{{cite book |author-link1=Debra Elmegreen |last1=Elmegreen |first1=Debra Meloy |author-link2=Bruce Elmegreen |first2=Bruce G. |last2=Elmegreen |chapter=Galaxies |title=Space Sciences |editor-first=Pat |editor-last=Dasch |volume=2 |series=Planetary Science and Astronomy |publisher=[[Gale (publisher)|Macmillan Reference USA]] |year=2002 |pages=50–56 |via=Gale Virtual Reference Library |chapter-url-access=subscription |chapter-url=https://go.galegroup.com/ps/i.do?p=GVRL&sw=w&u=mcc_pv&v=2.1&it=r&id=GALE%7CCX3408800138&asid=f1f899f2eb1f500bc3341c4f13abb896 |access-date=25 September 2017 }}</ref>