Astronomy:Galaxy cluster

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Short description: Structure made up of a gravitationally-bound aggregation of hundreds of galaxies
Composite image of five galaxies clustered together just 600 million years after the Universe's birth[1]

A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity,[1] with typical masses ranging from 1014 to 1015 solar masses. They are the second-largest known gravitationally bound structures in the universe after some superclusters (of which only one, the Shapley Supercluster, is known to be bound). They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.[2] One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with galactic clusters (also known as open clusters), which are star clusters within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.

Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.

Basic properties

Galaxy cluster IDCS J1426 is located 10 billion light-years from Earth and has the mass of almost 500 trillion suns (multi-wavelength image: X-rays in blue, visible light in green, and infrared light in red).[3]

Galaxy clusters typically have the following properties:

  • They contain 100 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter.[4] Details are described in the "Composition" section.
  • The distribution of the three components is approximately the same in the cluster.[citation needed]
  • They have total masses of 1014 to 1015 solar masses.
  • They typically have a diameter from 1 to 5 Mpc (see 1023 m for distance comparisons).
  • The spread of velocities for the individual galaxies is about 800–1000 km/s.


There are three main components of a galaxy cluster. They are tabulated below:[2]

Name of the components Mass fraction Description
Galaxies 1% In optical observations, only galaxies are visible
Intergalactic gas in intracluster medium 9% Plasma between the galaxies at high temperature and emit x-ray radiation by thermal bremsstrahlung
Dark matter 90% Most massive component but cannot be detected optically and is inferred through gravitational interactions


Galaxy clusters are categorized as type I, II, or III based on morphology.[5][6]

Galaxy clusters as measuring instruments

Gravitational redshift

Galaxy clusters have been used by Radek Wojtak from the Niels Bohr Institute at the University of Copenhagen to test predictions of general relativity: energy loss from light escaping a gravitational field. Photons emitted from the center of a galaxy cluster should lose more energy than photons coming from the edge of the cluster because gravity is stronger in the center. Light emitted from the center of a cluster has a longer wavelength than light coming from the edge. This effect is known as gravitational redshift. Using the data collected from 8000 galaxy clusters, Wojtak was able to study the properties of gravitational redshift for the distribution of galaxies in clusters. He found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster as predicted by general relativity. The result also strongly supports the Lambda-Cold Dark Matter model of the Universe, according to which most of the cosmos is made up of Dark Matter that does not interact with matter.[7]

Gravitational lensing

Galaxy clusters are also used for their strong gravitational potential as gravitational lenses to boost the reach of their telescopes. The gravitational distortion of space-time occurs near massive galaxy clusters and bends the path of photons to create a cosmic magnifying glass. This can be done with photons of any wavelength from the optical to the X-ray band. The latter is more difficult, because galaxy clusters emit a lot of X-rays. However, X-ray emission may still be detected when combining X-ray data to optical data. One particular case is the use of the Phoenix galaxy cluster to observe a dwarf galaxy in its early high energy stages of star formation.[8]


The Laniakea supercluster with many galaxy clusters
Main page: Astronomy:List of galaxy groups and clusters
Notable clusters
Cluster Notes
Virgo Cluster The nearest massive galaxy cluster
Norma Cluster The cluster at the heart of the Great Attractor
Bullet Cluster A cluster merger with the first observed separation between dark matter and normal matter
This lists some of the most notable clusters; for more clusters, see the list article.


Left: Image by the Hubble Space Telescope (2017) Right: Image by the James Webb Space Telescope (2022)[9]
Deep Field – Galaxy cluster SMACS J0723.3-7327.[10][11][12][13][14][15]
Abell 2744 galaxy cluster – extremely distant galaxies revealed by gravitational lensing (16 October 2014).[16][17]



See also


  1. 1.0 1.1 "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. 
  2. 2.0 2.1 Kravtsov, A. V.; Borgani, S. (2012). "Formation of Galaxy Clusters". Annual Review of Astronomy and Astrophysics 50: 353–409. doi:10.1146/annurev-astro-081811-125502. Bibcode2012ARA&A..50..353K. 
  3. "Galaxy cluster IDCS J1426". 
  4. "Chandra :: Field Guide to X-ray Sources :: Groups & Clusters of Galaxies". 
  5. Bautz; Morgan, W. W. (December 1970). "On the Classification of the Forms of Clusters of Galaxies". Astrophysical Journal 162: L149. doi:10.1086/180643. A&AA ID. AAA004.160.015. Bibcode1970ApJ...162L.149B. Retrieved March 10, 2012. 
  6. Bautz; Morgan, W. W. (September 1970). "Preliminary Classification of Clusters of Galaxies". Bulletin of the American Astronomical Society 2: 294. A&AA ID. AAA004.160.006. Bibcode1970BAAS....2R.294B. Retrieved March 10, 2012. 
  7. Yudhijit, Bhattacharjee. "Galaxy Clusters Back Up Einstein's Theory of Relativity". Wired. Retrieved 2022-04-04. 
  8. Chu, Jennifer (15 October 2019). "Astronomers use giant galaxy cluster as X-ray magnifying lens". 
  9. Chow, Denise; Wu, Jiachuan (12 July 2022). "Photos: How pictures from the Webb telescope compare to Hubble's - NASA's $10 billion telescope peers deeper into space than ever, revealing previously undetectable details in the cosmos.". NBC News. Retrieved 16 July 2022. 
  10. Garner, Rob (11 July 2022). "NASA's Webb Delivers Deepest Infrared Image of Universe Yet". NASA. Retrieved 12 July 2022. 
  11. Overbye, Dennis; Chang, Kenneth; Tankersley, Jim (11 July 2022). "Biden and NASA Share First Webb Space Telescope Image – From the White House on Monday, humanity got its first glimpse of what the observatory in space has been seeing: a cluster of early galaxies.". The New York Times. Retrieved 12 July 2022. 
  12. Pacucci, Fabio (15 July 2022). "How Taking Pictures of 'Nothing' Changed Astronomy - Deep-field images of "empty" regions of the sky from Webb and other space telescopes are revealing more of the universe than we ever thought possible". Scientific American. Retrieved 16 July 2022. 
  13. Deliso, Meredith; Longo, Meredith; Rothenberg, Nicolas (14 July 2022). "Hubble vs. James Webb telescope images: See the difference". ABC News. Retrieved 15 July 2022. 
  14. Kooser, Amanda (13 July 2012). "Hubble and James Webb Space Telescope Images Compared: See the Difference - The James Webb Space Telescope builds on Hubble's legacy with stunning new views of the cosmos.". CNET. Retrieved 16 July 2022. 
  15. Atkinson, Nancy (2 May 2022). "Now, We can Finally Compare Webb to Other Infrared Observatories". Universe Today. Retrieved 12 May 2022. 
  16. 16.0 16.1 Clavin, Whitney; Jenkins, Ann; Villard, Ray (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". NASA. 
  17. Chou, Felecia; Weaver, Donna (16 October 2014). "RELEASE 14-283 – NASA's Hubble Finds Extremely Distant Galaxy through Cosmic Magnifying Glass". NASA. 
  18. "Distant and ancient" (in en). 
  19. "Strings of homeless stars". 
  20. "From toddlers to babies". 
  21. "Approaching the Universe's origins". 
  22. "HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns". 
  23. "Streaks and stripes". 
  24. "Cosmic RELICS". 
  25. "Cosmic archaeology". 
  26. "Hubble pushed beyond limits to spot clumps of new stars in distant galaxy". 
  27. Loff, Sarah; Dunbar, Brian (10 February 2015). "Hubble Sees A Smiling Lens". NASA. 
  28. "Image of the galaxy cluster SpARCS1049". 
  29. "Magnifying the distant Universe". ESA/Hubble Picture of the Week. 
  30. "Seeing Triple". October 18, 2023.