A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar medium, cosmic dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the birth of the universe.
Recent estimates of the number of galaxies in the observable universe range from 200 billion (200,000,000,000) to 2 trillion (2,000,000,000,000) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs (or megaparsecs).
The intergalactic space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. At the largest scale, these associations are generally arranged into galaxy filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea.
- ↑ Sparke & Gallagher III 2000, p. i
- ↑ Hupp, E.; Roy, S.; Watzke, M. (August 12, 2006). "NASA Finds Direct Proof of Dark Matter". NASA. http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html. Retrieved April 17, 2007.
- ↑ Uson, J. M.; Boughn, S. P.; Kuhn, J. R. (1990). "The central galaxy in Abell 2029 – An old supergiant". Science 250 (4980): 539–540. doi:10.1126/science.250.4980.539. PMID 17751483. Bibcode: 1990Sci...250..539U.
- ↑ Hoover, A. (June 16, 2003). "UF Astronomers: Universe Slightly Simpler Than Expected". Hubble News Desk. Archived from the original on July 20, 2011. https://web.archive.org/web/20110720083835/http://news.ufl.edu/2003/06/16/galaxies/. Retrieved March 4, 2011.
- ↑ Jarrett, T. H.. "Near-Infrared Galaxy Morphology Atlas". California Institute of Technology. http://www.ipac.caltech.edu/2mass/gallery/galmorph/. Retrieved January 9, 2007.
- ↑ Finley, D.; Aguilar, D. (November 2, 2005). "Astronomers Get Closest Look Yet At Milky Way's Mysterious Core". National Radio Astronomy Observatory. http://www.nrao.edu/pr/2005/sagastar/. Retrieved August 10, 2006.
- ↑ Gott III, J. R. (2005). "A Map of the Universe". The Astrophysical Journal 624 (2): 463–484. doi:10.1086/428890. Bibcode: 2005ApJ...624..463G.
- ↑ Christopher J. Conselice et al (2016). "The Evolution of Galaxy Number Density at z < 8 and its Implications". The Astrophysical Journal 830 (2): 83. doi:10.3847/0004-637X/830/2/83. Bibcode: 2016ApJ...830...83C.
- ↑ Fountain, Henry (17 October 2016). "Two Trillion Galaxies, at the Very Least". The New York Times. https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html. Retrieved 17 October 2016.
- ↑ Mackie, Glen (1 February 2002). "To see the Universe in a Grain of Taranaki Sand". Centre for Astrophysics and Supercomputing. http://astronomy.swin.edu.au/~gmackie/billions.html. Retrieved 28 January 2017.
- ↑ "Galaxy Clusters and Large-Scale Structure". University of Cambridge. http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html. Retrieved January 15, 2007.
- ↑ Gibney, Elizabeth (2014). "Earth's new address: 'Solar System, Milky Way, Laniakea'". Nature. doi:10.1038/nature.2014.15819.