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Galaxy ○꠹｜Definition｜1st｜20251119205401-00-⌔

Galaxy - Wikipedia

Galaxy

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NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.12 The word is derived from the Greek galaxias (γαλαξίας), meaning ‘milky’, a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars,3 range in size from dwarfs with less than a thousand stars4 to the largest galaxies known — supergiants with one hundred trillion stars, each orbiting its galaxy’s center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centers of galaxies.

Galaxies are categorised according to their visual characteristics such as elliptical,5 spiral, or irregular.6 The Milky Way is an example of a spiral galaxy. In addition to shape, galaxies may be notable due to special properties, such as interacting with another galaxy, producing stars at an unusual rate, or having an active galactic nucleus. It is estimated that there are between 200 billion7 (2 × 10) and 2 trillion8 galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances in the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly).910 Its nearest large neighbour, the Andromeda Galaxy, is just over 750,000 parsecs (2.4 million ly) away.11

Most galaxies are gravitationally organised into groups, clusters and superclusters. The Milky Way is part of the Local Group, which it dominates along with the Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.12 While there are good models describing the formation of stars from gravitational condensation of dense clouds of gas, galaxy formation is less well understood. The process operates on the scale of a billion years. Galaxies occasionally collide during their lifetime.

Printed 2026-06-28.

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Footnotes

  1. Sparke & Gallagher 2000, p. i ↩

  2. Hupp, Erica; Roy, Steve; Watzke, Megan (August 12, 2006). “NASA Finds Direct Proof of Dark Matter”. NASA. Archived from the original on March 28, 2020. Retrieved April 17, 2007. ↩

  3. Howell, Elizabeth; Harvey, Ailsa (February 11, 2022). “How many stars are in the universe? - Can we estimate the total number of stars?”. Space.com. Retrieved July 16, 2022. ↩

  4. Strigari, Louis E.; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth; Walker, Matthew G. (2008). “A common mass scale for satellite galaxies of the Milky Way”. Nature. 454 (7208): 1096–1097. arXiv:0808.3772. Bibcode:2008Natur.454.1096S. doi:10.1038/nature07222. ISSN 0028-0836. PMID 18756252. ↩

  5. Hoover, Aaron (June 16, 2003). “UF Astronomers: Universe Slightly Simpler Than Expected” (Press release). University of Florida. Archived from the original on July 20, 2011. Retrieved March 4, 2011. Based on: Graham, Alister W.; Guzmán, Rafael (2003). “HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies”. The Astronomical Journal. 125 (6): 2936–2950. arXiv:astro-ph/0303391. Bibcode:2003AJ…125.2936G. doi:10.1086/374992. S2CID 13284968. ↩

  6. Jarrett, T. H. “Near-Infrared Galaxy Morphology Atlas”. Infrared Processing and Analysis Center. California Institute of Technology. Archived from the original on November 20, 2023. Retrieved January 9, 2007. ↩

  7. Chemla, Sarah (January 14, 2021). “Astronomers were wrong about the number of galaxies in universe”. The Jerusalem Post. Archived from the original on January 14, 2021. Retrieved January 14, 2021. ↩

  8. Saunders, Toby (July 25, 2023). “How many galaxies are in the Universe? A lot more than you’d think”. BBC Science Focus. Retrieved January 9, 2024. ↩

  9. Goodwin, Simon P.; Gribbin, John; Hendry, Martin A. (1998). “The relative size of the Milky Way” (PDF). The Observatory. 118: 201–208. Bibcode:1998Obs…118..201G. ↩

  10. This is the diameter measured using the D standard. A 2018 study suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.^{[10]}$$^{[11]} ↩

  11. Riess, Adam G.; Fliri, Jürgen; Valls-Gabaud, David (2012). “Cepheid Period-Luminosity Relations in the Near-Infrared and the Distance to M31 from the Hubble Space Telescope Wide Field Camera 3”. The Astrophysical Journal. 745 (2): 156. arXiv:1110.3769. Bibcode:2012ApJ…745..156R. doi:10.1088/0004-637X/745/2/156. S2CID 119113794. ↩

  12. “Galaxy Clusters and Large-Scale Structure”. University of Cambridge. Archived from the original on April 26, 2012. Retrieved January 15, 2007. ↩

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