A spiral galaxy is a kind of galaxy that looks like a flat, slowly rotating disk with a bulge in the centre, and spiral patterns extending outward from the bulge. It contains stars, gas, dust, dark matter and a supermassive black hole at its centre.
Galaxies were long thought to be nebulae. The spiral nebula as a type was first described by Edwin Hubble in his 1936 work The Realm of the Nebulae. They are now listed as galaxies, and are named after their shape.
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 and well-defined arms. Multiple-arm spiral galaxies have more arms. In flocculent spiral galaxies, it is hard to see the arms at all, as they are choppy and are ill-defined.
About 60% of galaxies in the universe near us (the 'local universe') are spiral and irregular galaxies. They are mostly found in low-density parts of the universe, and are rare in the centers of galaxy clusters.
Spiral arms are regions in spiral galaxies. They often contain dust,gas and star clusters of young ,hot and massive stars. They extend from the central bulge area of spiral and barred spiral galaxies. The young, hot and massive stars are why the arms are brighter than the center of the galaxy. Some spiral galaxies have yellow, 'fossil' arms of older stars.
Our own Milky Way was found to be a spiral galaxy in the early 20th century. In the 1990s it was found to be a barred spiral galaxy. The bar is difficult to see from our position in the Galactic disk. The most convincing evidence for its existence comes from a survey,  performed by the Spitzer Space Telescope, of stars in the Galactic center.[dead link]
- ↑ 'Supermassive' means not just large size and huge mass, but huge gravitational effect.
- ↑ Hubble, E.P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 0300025009.
- ↑ "Spiral galaxies". burro.cwru.edu. Archived from the original on 8 June 2019. Retrieved 2 October 2010.
- ↑ 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. Archived from the original on 2021-01-11. Retrieved 2007-09-15.
- ↑ Galaxies are not spread randomly in the universe.
- ↑ 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. Archived from the original on 2021-10-26. Retrieved 2007-09-15.
- ↑ 7.0 7.1 Benjamin, R. A.; Churchwell, E.; Babler, B. L.; Indebetouw, R.; Meade, M. R.; Whitney, B. A.; Watson, C.; Wolfire, M. G.; Wolff, M. J.; et al. (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. Retrieved 2007-09-21.[permanent dead link]
- ↑ "BBC - Science & Nature - Space - Galaxy Guide". bbc.co.uk. Archived from the original on 6 October 2010. Retrieved 2 October 2010.
- ↑ Ripples in a galactic pond Archived 2013-09-06 at the Wayback Machine, Scientific American, October 2005
- ↑ "GLIMPSE: the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire". www.astro.wisc.edu. Archived from the original on 2021-05-08. Retrieved 2020-04-18.
- Media related to Spiral galaxies at Wikimedia Commons