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An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy.^[1] 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 structure.^[2] 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.^[3]

Collectively they are thought to make up about a quarter of all galaxies.^[4] 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.^[5] This is not necessarily true for dwarf irregulars.^[6] Irregular galaxies may also be formed in galaxy collisions.^[7]

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 colliding with large galaxies and intergalactic clouds.^[8]

Types

There are three major types of irregular galaxies:^[9]

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.
- 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.^[10] 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 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. ^[11]

Irregular galaxies are considered late-type along with spiral galaxies as opposed to early type elliptical galaxies. ^[12]

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.

Magellanic Clouds

The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm (barred Magellanic spiral).^[13] The Small Magellanic Cloud remains classified as an irregular galaxy of type Im under current galaxy morphological classification, although it does contain a bar structure.

Gallery

Blue compact dwarf galaxy ESO 338-4^[14]
IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo.^[15]
ESO 486-21 is a spiral galaxy with a somewhat irregular and ill-defined structure.^[16]
Irregular galaxy IC 3583 has been found to have a bar of stars running through its center.^[17]
NGC 2337 is an irregular galaxy that resides 25 million light-years away in the Lynx constellation.^[18]
UGC 4459 is an irregular dwarf galaxy located approximately 11 million light-years away in the constellation of Ursa Major.^[19]
Dwarf irregular galaxy known as PGC 18431^[20]
IC 559 is classified as a type Sm galaxy.^[21]
Irregular dwarf galaxy PGC 16389 covers its neighboring galaxy APMBGC 252+125-117.^[22]

References

^ Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. ISBN 978-0-7668-3391-3.
^ Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303.
^ "Star Formation in Irregular Galaxies". ned.ipac.caltech.edu. Retrieved 2025-08-06.
^ "Irregular galaxies". www.mssl.ucl.ac.uk. Retrieved 2025-08-06.
^ "Faulkes Telescope Educational Guide – Irregular Galaxies". Archived from the original on 17 October 2022.
^ Walter, F. et al. Astophys J 661, 102–114, 2007
^ "Types – NASA Science". 2025-08-06. Retrieved 2025-08-06.
^ Elmegreen, Debra Meloy; Elmegreen, Bruce G. (2002). "Galaxies". In Dasch, Pat (ed.). Space Sciences. Planetary Science and Astronomy. Vol. 2. Macmillan Reference USA. pp. 50–56. Retrieved 25 September 2017 – via Gale Virtual Reference Library.
^ 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
^ 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. p. 234–254. ISBN 978-0-521-75578-8.
^ Rivas, Jonathan; Flores-Velazquez, Jeysen; Wheeler, Coral (2025-08-06). "Major Merger between a Classical Dwarf Irregular Galaxy and its Massive Ultra-Faint Satellite". American Astronomical Society Meeting Abstracts. 243: 404.18. Bibcode:2024AAS...24340418R.
^ "Hubble Looks at a Late-type Galaxy – NASA Science". 2025-08-06. Retrieved 2025-08-06.
^ Corso, G. and Buscombe, W. The Observatory, 90, 229–233 (1970) On the spiral structure of the Large Magellanic Cloud
^ "Feeling blue". www.spacetelescope.org. Archived from the original on 5 November 2018. Retrieved 5 November 2018.
^ "A frenzy of stars". www.spacetelescope.org. Archived from the original on 25 October 2020. Retrieved 26 February 2018.
^ "Surveying the cosmos". www.spacetelescope.org. Archived from the original on 18 July 2020. Retrieved 19 June 2017.
^ "Spotlight on IC 3583". www.spacetelescope.org. Archived from the original on 17 August 2019. Retrieved 28 November 2016.
^ "A lopsided lynx". www.spacetelescope.org. Archived from the original on 21 November 2020. Retrieved 9 August 2016.
^ "A distinctly disorganised dwarf". Archived from the original on 25 October 2020. Retrieved 29 March 2016.
^ "Meeting the neighbours". www.spacetelescope.org. ESA/Hubble. Archived from the original on 27 November 2020. Retrieved 8 June 2015.
^ "A spattering of blue". www.spacetelescope.org. ESA/Hubble. Archived from the original on 5 June 2019. Retrieved 8 September 2014.
^ "A cosmic optical illusion". ESA/Hubble Picture of the Week. Archived from the original on 7 January 2018. Retrieved 20 August 2013.

External links

[1] Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. ISBN 978-0-7668-3391-3.

[2] Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303.

[3] "Star Formation in Irregular Galaxies". ned.ipac.caltech.edu. Retrieved 2025-08-06.

[4] "Irregular galaxies". www.mssl.ucl.ac.uk. Retrieved 2025-08-06.

[5] "Faulkes Telescope Educational Guide – Irregular Galaxies". Archived from the original on 17 October 2022.

[6] Walter, F. et al. Astophys J 661, 102–114, 2007

[7] "Types – NASA Science". 2025-08-06. Retrieved 2025-08-06.

[8] Elmegreen, Debra Meloy; Elmegreen, Bruce G. (2002). "Galaxies". In Dasch, Pat (ed.). Space Sciences. Planetary Science and Astronomy. Vol. 2. Macmillan Reference USA. pp. 50–56. Retrieved 25 September 2017 – via Gale Virtual Reference Library.

[9] 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

[10] 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. p. 234–254. ISBN 978-0-521-75578-8.

[11] Rivas, Jonathan; Flores-Velazquez, Jeysen; Wheeler, Coral (2025-08-06). "Major Merger between a Classical Dwarf Irregular Galaxy and its Massive Ultra-Faint Satellite". American Astronomical Society Meeting Abstracts. 243: 404.18. Bibcode:2024AAS...24340418R.

[12] "Hubble Looks at a Late-type Galaxy – NASA Science". 2025-08-06. Retrieved 2025-08-06.

[13] Corso, G. and Buscombe, W. The Observatory, 90, 229–233 (1970) On the spiral structure of the Large Magellanic Cloud

[14] "Feeling blue". www.spacetelescope.org. Archived from the original on 5 November 2018. Retrieved 5 November 2018.

[15] "A frenzy of stars". www.spacetelescope.org. Archived from the original on 25 October 2020. Retrieved 26 February 2018.

[16] "Surveying the cosmos". www.spacetelescope.org. Archived from the original on 18 July 2020. Retrieved 19 June 2017.

[17] "Spotlight on IC 3583". www.spacetelescope.org. Archived from the original on 17 August 2019. Retrieved 28 November 2016.

[18] "A lopsided lynx". www.spacetelescope.org. Archived from the original on 21 November 2020. Retrieved 9 August 2016.

[19] "A distinctly disorganised dwarf". Archived from the original on 25 October 2020. Retrieved 29 March 2016.

[20] "Meeting the neighbours". www.spacetelescope.org. ESA/Hubble. Archived from the original on 27 November 2020. Retrieved 8 June 2015.

[21] "A spattering of blue". www.spacetelescope.org. ESA/Hubble. Archived from the original on 5 June 2019. Retrieved 8 September 2014.

[22] "A cosmic optical illusion". ESA/Hubble Picture of the Week. Archived from the original on 7 January 2018. Retrieved 20 August 2013.

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