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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.〔Butz, Stephen D. (2002). ''Science of Earth Systems.'' Cengage Learning. p. 107. ISBN 978-0-7668-3391-3.〕 The shape of an irregular galaxy is uncommon – they 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.〔Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." ''Publications of the Astronomical Society of the Pacific.'' 69 (409): 291–303.〕 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 disorders in gravitational pull. Irregular galaxies may contain abundant〔(Faulkes Telescope Educational Guide - Galaxies - Irregulars )〕 amounts of gas and dust. This is not necessarily true for Dwarf Irregulars.〔Walter, F. et al. Astophys J 661, 102 - 114, 2007〕 ==Types== There are three major types of irregular galaxies:〔Gallagher, J. S. & Hunter, D. A. (1984). "Structure and Evolution of Irregular Galaxies." ''Annual Review of Astronomy and Astrophysics.'' 22: 37-74. 〕 * 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 dIrrs) is a dwarf irregular galaxy〔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.〕 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「irregular galaxy」の詳細全文を読む スポンサード リンク
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