A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...

that are bound together by

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

, with typical masses ranging from 10¹⁴ to 10¹⁵

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es. Clusters consist of galaxies, heated gas, and dark matter. They are the second-largest known

gravitationally bound The gravitational binding energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. A gravitationally bound system has a lower (''i.e.'', more negative) gravitatio ...

structures in the

universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

after

supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; they are among the largest known structures in the universe. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in tu ...

s. They were believed to be the largest known structures in the universe until the 1980s, when

s were discovered. Small aggregates of galaxies are referred to as

galaxy group A galaxy group or group of galaxies (GrG) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxi ...

s rather than clusters of galaxies. Together,

galaxy groups and clusters Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the gra ...

form superclusters.

Basic properties

Galaxy clusters typically have the following properties: * They contain 100 to 1,000 galaxies, hot

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

emitting gas and large amounts of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. Details are described in the "Composition" section. * They have total masses of 10¹⁴ to 10¹⁵

solar masses The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies ...

. * They typically have diameters 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.

Composition

Galaxy clusters have three main components. Galaxies themselves only make up a small fraction of clusters, although they are the only component we can detect in the

visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...

. The heated gas of the intracluster medium (ICM) has a peak temperature between 30 and 100 million degrees Celsius. Dark matter makes up the majority of the mass of galaxy clusters, but cannot be detected optically.

Cluster formation and evolution

As galaxy clusters form, massive amounts of energy are released due to shock waves, the heating of gas, and galaxy interactions. Gas collides with existing material which generates shock waves, heating it to tens of millions of degrees and producing X-ray emissions. Galaxy evolution within the cluster is governed by interactions between galaxies, such as galaxy mergers, and gas stripping.

Classification

There are many classification systems for galaxy clusters, based on characteristics such as shape symmetry, X-ray

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

, and dominant galaxy type. The Bautz-Morgan classification sorts clusters into types I, II, and III based on the relative brightness of their galaxies–type I with greatest contrast and type III with the least.

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 General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

: 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 In physics and general relativity, gravitational redshift (known as Einstein shift in older literature) is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well lose energy. This loss of energy correspo ...

. 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 In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

that does not interact with matter.

Gravitational lensing

Galaxy clusters are also used for their strong gravitational potential as

gravitational lens A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

es to boost the reach of 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.

Notable galaxy clusters

Notable galaxy clusters in the relatively nearby universe include the

Virgo Cluster The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the Virgo constellation. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the larger ...

Fornax Cluster The Fornax Cluster is a cluster of galaxies lying at a distance of 19 megaparsecs (62 million light-years). It has an estimated mass of solar masses, making it the second richest galaxy cluster within 100 million light-years, after the consider ...

, Hercules Cluster, and the

Coma Cluster The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and tak ...

. A very large aggregation of galaxies known as the

Great Attractor The Great Attractor is a region of gravitational attraction in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster of galaxies that includes the Milky Way galaxy, as well as about 100,000 other galaxi ...

, dominated by the

Norma Cluster The Norma Cluster (ACO 3627 or Abell 3627) is a rich cluster of galaxies located near the center of the Great Attractor; it is about distant. Although it is both nearby and bright, it is difficult to observe because it is located in the Zone of ...

, 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 The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...

, structures such as cold fronts and

shock waves In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

have also been found in many galaxy clusters.

Gallery

Images

File:Distant_and_ancient_SPT0615-JD.jpg, Galaxy cluster SPT-CL J0615-5746. File:Strings of homeless stars RXC J0232.2-4420.jpg, Galaxy cluster RXC J0232.2-4420. File:From toddlers to babies RXC J0032.1+1808.jpg, Galaxy cluster RXC J0032.1+1808 as part of th
RELICS
program. File:Approaching the Universe's origins PSZ2 G138.61-10.84.jpg, Massive galaxy cluster PSZ2 G138.61-10.84 is about six billion light-years away. File:HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns.jpg, HAWK-I and Hubble explore RCS2 J2327 cluster with the mass of two quadrillion Suns. File:Streaks and stripes Abell 2537.jpg, Abell 2537 is useful in probing cosmic phenomena like dark matter and dark energy. File:Cosmic RELICS Abell 1300.jpg, Abell 1300 acts like a lens, bending the very fabric of space around it. File:Cosmic archaeology WHL J24.3324-8.477.jpg, Galaxy cluster WHL J24.3324-8.477. File:Hubble pushed beyond limits to spot clumps of new stars in distant galaxy.jpg, Background galaxy has been gravitationally lensed by the intervening galaxy cluster. File:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg, "

Smiley A smiley, sometimes called a smiley face, is a basic ideogram representing a Smile, smiling face. Since the 1950s, it has become part of popular culture worldwide, used either as a standalone ideogram or as a form of communication, such as em ...

" image – galaxy cluster (SDSS J1038+4849) &

gravitational lensing A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

(an Einstein ring) ( HST). File:Image of the galaxy cluster SpARCS1049.jpg, Galaxy cluster SpARCS1049 taken by Spitzer and the Hubble Space Telescope. File:PIA20052-GalaxyCluster-MOO-J1142+1527-20151103.jpg, Galaxy cluster MOO J1142+1527 discovered by the MaDCoWS survey File:Heic1401a-Abell2744-20140107.jpg, Abell 2744 galaxy cluster ( HST). File:Magnifying the distant Universe.jpg, Magnifying the distant universe through MACS J0454.1-0300. File:14-296-GalaxyClusters-PerseusVirgo-ChandraXRay-20141027.jpg,

Turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

may prevent galaxy clusters from cooling; illustrated:

Perseus Cluster The Perseus cluster (Abell 426) is a cluster of galaxies in the constellation Perseus. It has a recession speed of 5,366 km/ s and a diameter of 863. It is one of the most massive objects in the known universe, containing thousands of galaxi ...

and

( Chandra X-ray). File:Color image of galaxy cluster MCS J0416.1–2403.jpg, MACS0416.1-2403 imaged by the HST File:Light_Bends_from_the_Beyond.jpg, The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

File:A_Menagerie_of_Galaxies.jpg, A menagerie of galaxies — the galaxy cluster ACO S 295 File:Cosmic_Lens_Flare.jpg, Cosmic lens flare File:Hubble spots three images of a distant supernova.jpg, Hubble spots three images of a distant supernova File:Galaxy cluster WHL0137-08 (sunrisearc1).jpg, A massive galaxy cluster called WHL0137-08 File:El Gordo (NIRCam Image) (2023-119).png, Galaxy cluster known as "El Gordo" File:Seeing Triple (potm2302a).jpeg, Observation from the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

the massive galaxy cluster RX J2129.

Videos

File:Artist’s impression of a protocluster forming in the early Universe.ogv, Video: Formation of galaxy cluster MRC 1138-262 (artist's concept).

References

{{Portal bar, Astronomy, Stars, Outer space Cluster Articles containing video clips Types of groupings