Class of galaxy that has spiral structures extending from their cores.
This article is about the class of galaxy. For the type of logic puzzle, see Spiral Galaxies (puzzle).
An example of a spiral galaxy, the Messier 77 (also known as NGC 1068)
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Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae[1] and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,[2] extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.[3]
The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.[4] The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope.[5]
Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe.[6] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[7]
^Hubble, E.P. (1936). The realm of the nebulae. Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Alt URL(pp. 124–151)
^D. Mihalas (1968). Galactic Astronomy. W. H. Freeman. ISBN 978-0-7167-0326-6.
^"Hubble and Galaxy Zoo Find Bars and Baby Galaxies Don't Mix". Science Daily. 16 January 2014.
^"Ripples in a Galactic Pond". Scientific American. October 2005. Archived from the original on 6 September 2013.
^R. A. Benjamin; E. Churchwell; B. L. Babler; R. Indebetouw; M. R. Meade; B. A. Whitney; C. Watson; M. G. Wolfire; M. J. Wolff; R. Ignace; T. M. Bania; S. Bracker; D. P. Clemens; L. Chomiuk; M. Cohen; J. M. Dickey; J. M. Jackson; H. A. Kobulnicky; E. P. Mercer; J. S. Mathis; S. R. Stolovy; B. Uzpen (September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy". The Astrophysical Journal Letters. 630 (2): L149–L152. arXiv:astro-ph/0508325. Bibcode:2005ApJ...630L.149B. doi:10.1086/491785. S2CID 14782284.
^Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue". Monthly Notices of the Royal Astronomical Society. 278 (4): 1025–1048. arXiv:astro-ph/9603040. Bibcode:1996MNRAS.278.1025L. doi:10.1093/mnras/278.4.1025.
^Dressler, A. (March 1980). "Galaxy morphology in rich clusters — Implications for the formation and evolution of galaxies". The Astrophysical Journal. 236: 351–365. Bibcode:1980ApJ...236..351D. doi:10.1086/157753.
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