The galaxies of HCG 87, about four hundred million light-years distant. The large edge-on spiral, the fuzzy elliptical galaxy immediately to its right, and the spiral near the top of the image are members of the group, while the small spiral galaxy exactly in the middle is a more distant background galaxy.

Galaxy groups and clusters are the largest gravitationally-bound objects. They form the densest part of the large scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters. The galaxies of HCG (Hickson Compact Group) 87, about four hundred million light-years distant toward the constellation Capricornus. ... The galaxies of HCG (Hickson Compact Group) 87, about four hundred million light-years distant toward the constellation Capricornus. ... Categories: Stub | Hickson Compact Groups | Galaxy compact groups ... Gravity is a force of attraction that acts between bodies that have mass. ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... Cold dark matter (or CDM) is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material which cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up... Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ...

Groups of galaxies

Groups of galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of 1 to 2 Megaparsecs (Mpc) (see 1 E22 m for distance comparisons). Their mass are approximately 10¹³ solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... The megaparsec (abbreviated Mpc) is a unit of distance used in astronomy, equal to one million parsecs. ... (Redirected from 1 E22 m) Categories: Orders of magnitude (length) ...

The group which contains our own galaxy, the Milky Way, is called the Local Group, and contains more than 40 galaxies. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ... A member of the Local Group of galaxies, irregular galaxy Sextans A is 10 million light years distant. ...

Clusters of galaxies

Clusters are larger than groups, although there is no sharp dividing line between a group and a cluster. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally-bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, between 10⁷K and 10⁸K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter. In astronomy, the intracluster medium (or ICM) is the superheated gas present at the center of a galaxy cluster. ... (help· info), (from the German bremsen, to brake and Strahlung, radiation, thus, braking radiation), is electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. ... In physics, atomic spectral lines are of two types: An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ... In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. ...

Clusters typically have the following properties.

• They contain 50 to 1000 galaxies, hot X-ray emitting gas and large amounts of dark matter
• The distribution of these three components is approximately the same in the cluster.
• They have total masses of 10¹⁴ to 10¹⁵ solar masses.
• They typically have a diameter from 2 to 10 Mpc (see 1 E23 m for distance comparisons).
• The spread of velocities for the individual galaxies is about 800-1000 km/s.

Notable galaxy clusters in the relatively nearby universe include the Virgo 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 (Hubble flow). In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. ... (Redirected from 1 E23 m) Categories: Orders of magnitude (length) ... A sky field near some of the brighter galaxies in the Virgo cluster. ... The Coma Cluster (Abell 1656) is a huge galaxy cluster and the prototypical rich cluster with over a thousand member galaxies known. ... An image taken by the European Southern Observatory looking in the direction of the Great Attractor. ... The Norma Cluster (ACO 3627 or Abell 3627) (galactic coordinates: (325°,−7°)) is a rich cluster of galaxies, lying near the center of the Great Attractor. ... In cosmology, the Hubble expansion is the motion of galaxies away from each other, due to the expansion of the universe. ...

Note: clusters of galaxies should not be confused with star clusters such as galactic clusters and open clusters, which are structures within galaxies, as well as globular clusters, which typically orbit galaxies. Globular Cluster M92 in the Hercules constellation. ... An open cluster is a group of stars (star cluster) that were born at the same time from a molecular cloud, and are still near to each other. ... An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ...

Superclusters

Main article: Supercluster Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ...

Groups, clusters and some isolated galaxies form even larger structures, the superclusters. At the very largest scales of the visible universe, matter is gathered into filaments and walls surrounding vast voids. This structure resembles a foam. Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ... In astronomy, filaments are one of the largest known structures in the Universe, thread-like structures with a typical length of 70 to 150 megaparsec that form the boundaries between large voids in the universe. ... The Great Wall is the largest known super-structure in the Universe. ... In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe that contain very few, or no, galaxies. ... Foam The most general definition of foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. ...

Observational methods

Clusters of galaxies have been found in surveys by a number of observational techniques and have been studied in detail using many methods: An astronomical catalog or catalogue is a list or tabulation of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery. ...

• Optical or infrared: The individual galaxies of clusters can be studied through optical or infrared imaging and spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar redshift. Infrared searches are more useful for finding more distant (higher redshift) clusters.
• X-ray: The hot plasma emits X-rays which can be detected by X-ray telescopes. The cluster gas can be studied using both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along with AGN are the brightest X-ray emitting extragalactic objects.
• Radio: A number of diffuse structures emitting at radio frequencies have been found in clusters. Groups of radio sources (which may include diffuse structures or AGN have been used as tracers of cluster location. At high redshift imaging around individual radio sources (in this case AGN) has been used to detect proto-clusters (clusters in the process of forming).
• Sunyaev-Zel'dovich effect: The hot electrons in the intracluster medium scatter radiation from the cosmic microwave background through Compton scattering. This produces a "shadow" in the observed cosmic microwave background at some radio frequencies.
• Gravitational Lensing: Clusters of galaxies contain enough matter to distort the observed orientations of galaxies behind them. The observed distortions can be used to model the distribution of dark matter in the cluster.

The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left). ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left). ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... 50 cm refracting telescope at Nice Observatory. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... Radio halos are large-scale areas of radio emission found in clusters of galaxies. ... Radio halos are large-scale areas of radio emission found in clusters of galaxies. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left). ... The Sunyaev-Zeldovich effect (SZ effect or Sunyaev-Zeldovich theory) is due to high energy electrons distorting the cosmic microwave background radiation (CMB) through the inverse Compton effect, in which some of the high energy of the electrons is transferred to the low energy photons. ... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... In physics, Compton scattering or the Compton effect, is the decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, when it interacts with matter. ... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... This article is in need of attention from an expert on the subject. ...

Instruments and surveys used for finding clusters of galaxies

• The AMI array, a radio telescope at Mullard Radio Astronomy Observatory
• The Sunyaev-Zel'dovic Array, a radio telescope in California
• XMM-Newton, an X-ray telescope
• Chandra X-ray telescope.

The long baseline array of AMI, utilizing receivers from the Ryle Telescope. ... The 64 metre radio telescope at Parkes Observatory The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes. ... Mullard Radio Astronomy Observatory is home to a number of large aperture synthesis radio telescopes, including the One-Mile Telescope, 5km Ryle Telescope, and the Arcminute Microkelvin Imager. ... The Sunyaev-Zeldovic Array (SZA) in California is surveying the Cosmic Microwave Background to find clusters of galaxies. ... The 64 metre radio telescope at Parkes Observatory The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes. ... This article is about XMM-Newton. ... ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon. ... For other uses, see Chandra (disambiguation). ... ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon. ...

See also

• Fossil group
• List of galaxy clusters
• Large-scale structure of the cosmos
• Timeline of galaxies, clusters of galaxies, and large-scale structure
• Intracluster medium