Spiral galaxy

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An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
A barred spiral galaxy

A spiral galaxy is a kind of galaxy that looks like a flat, slowly rotating disk with a bulge in the centre. It contains stars, gas, dust, dark matter and a supermassive black hole at its centre.[1]

Galaxies were first listed in the 18th century by William Herschel. The spiral galaxy as a type was first described by Edwin Hubble in his 1936 work The Realm of the Nebulae.[2] They are named after their shape. The spiral arms are where young, hot stars are made, which is why they are brighter than the center of the galaxy.[3] Some spiral galaxies have yellow, 'fossil' arms of older stars.[4]

The barred spiral galaxy is an important and common type of spiral galaxy. There are three other kinds of spiral galaxies. Grand-design spiral galaxies have two well-shaped arms. Multiple-arm spiral galaxies have more arms. In flocculent spiral galaxies, it is hard to see the arms at all.[5]

About 60% of galaxies in the universe near us (the 'local universe') are spiral and irregular galaxies.[6] They are mostly found in low-density parts of the universe,[7] and are rare in the centers of galaxy clusters.[8]

Milky Way[change | change source]

Our own Milky Way was found to be a spiral galaxy in the early 20th century. In the 1990s been confirmed to be a barred spiral galaxy, although the bar is difficult to observe from our position in the Galactic disk.[9] The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center.[3]

References[change | change source]

  1. 'Supermassive' means not just large size, but huge gravitational effect.
  2. Hubble, E.P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 0300025009.
  3. 3.0 3.1 Benjamin, R. A. et al.; Churchwell, E.; Babler, B. L.; Indebetouw, R.; Meade, M. R.; Whitney, B. A.; Watson, C.; Wolfire, M. G. et al. (September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy.". The Astrophysical Journal Letters 630 (2): L149–L152. doi:10.1086/491785. http://www.journals.uchicago.edu/doi/full/10.1086/491785. Retrieved 2007-09-21. 
  4. "BBC - Science & Nature - Space - Galaxy Guide". bbc.co.uk. Retrieved 2 October 2010.
  5. "Spiral galaxies". burro.cwru.edu. Retrieved 2 October 2010.
  6. Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue.". Monthly Notices of the Royal Astronomical Society 278 (4): 1025–1048. http://articles.adsabs.harvard.edu/full/1996MNRAS.278.1025L. Retrieved 2007-09-15. 
  7. Galaxies are not spread randomly in the universe.
  8. Dressler, A. (March 1980). accessdate= 2007-09-15 "Galaxy morphology in rich clusters — Implications for the formation and evolution of galaxies.". The Astrophysical Journal 236: 351–365. doi:10.1086/157753. http://articles.adsabs.harvard.edu/full/1980ApJ...236..351D accessdate= 2007-09-15. 
  9. Ripples in a galactic pond, Scientific American, October 2005

Other websites[change | change source]