Galaxy group

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Four of the seven members of galaxy group HCG 16. Hubble views bizarre cosmic quartet HCG 16.jpg
Four of the seven members of galaxy group HCG 16.

A galaxy group [2] or group of galaxies [3] (GrG [4] ) 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 galaxies larger than groups that are first-order clustering are called galaxy clusters. [5] The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.


The Milky Way galaxy is part of a group of galaxies called the Local Group. [6]


Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc). [NB 1] Their mass is 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. [7]

Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies. [8] In the local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media. Those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10-50% of the groups' virial radius, generally 50-500 kpc. [9]


There are several subtypes of groups.

Compact Groups

A compact group consists of a small number of galaxies, typically around five, in close proximity and relatively isolated from other galaxies and formations. [10] The first compact group to be discovered was Stephan's Quintet, found in 1877. [11] Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy. [10] Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups. [12]

Compact groups of galaxies readily show the effect of dark matter, as the visible mass is greatly less than that needed to gravitationally hold the galaxies together in a bound group. Compact galaxy groups are also not dynamically stable over Hubble time, thus showing that galaxies evolve by merger, over the timescale of the age of the universe. [10]

Fossil Groups

Fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous (or L*) galaxies suggest that fossil groups are old, undisturbed systems that have seen little infall of L* galaxies since their initial collapse. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies and the intragroup medium in an isolated system. Fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy. [9] [10] This hypothesis is supported by studies of computer simulations of cosmological volumes. [13]

The closest fossil group to the Milky Way is NGC 6482, an elliptical galaxy at a distance of approximately 180 million light-years located in the constellation of Hercules. [14]


Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters. [15] These contain galaxies and protogalaxies embedded in dark matter haloes that are in the process of fusing into group-formations of singular dark matter halos. [16]


Notable groups
Local Group The group where the Milky Way, including the Earth, is located
Stephan's Quintet One of the most photogenic groups
Robert's Quartet Another very notable group
Bullet Group The merging group exhibits separation of dark matter from normal matter
This lists some of the most notable groups; for more groups, see the list article.

See also


  1. see 1022 m for distance comparisons

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The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the Ethiopian princess who was the wife of Perseus in Greek mythology.

Virgo Supercluster Galactic supercluster containing the Virgo Cluster

The Virgo Supercluster or the Local Supercluster is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs. The Virgo SC is one of about 10 million superclusters in the observable universe and is in the Pisces–Cetus Supercluster Complex, a galaxy filament.

Messier 87 Elliptical galaxy in the Virgo Galaxy Cluster

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Stephans Quintet

Stephan's Quintet is a visual grouping of five galaxies of which four form the first compact galaxy group ever discovered. The group, visible in the constellation Pegasus, was discovered by Édouard Stephan in 1877 at the Marseille Observatory. The group is the most studied of all the compact galaxy groups. The brightest member of the visual grouping is NGC 7320, which has extensive H II regions, identified as red blobs, where active star formation is occurring.

Sombrero Galaxy Spiral galaxy in the constellation Virgo

The Sombrero Galaxy is a spiral galaxy in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs from our galaxy, within the local supercluster. It has a diameter of approximately 15 kiloparsecs, 0.3x times the size of the Milky Way. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its outer disk, which is viewed almost edge-on. The dark dust lane and the bulge give this galaxy the appearance of a sombrero hat. Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy, but the Spitzer Space Telescope found that the dust ring around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy. The galaxy has an apparent magnitude of +8.0, making it easily visible with amateur telescopes, and it is considered by some authors to be the galaxy with the highest absolute magnitude within a radius of 10 megaparsecs of the Milky Way. Its large bulge, its central supermassive black hole, and its dust lane all attract the attention of professional astronomers.

Dwarf galaxy Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

NGC 1569 Dwarf irregular galaxy in the constellation Camelopardalis

NGC 1569 is a dwarf irregular galaxy in Camelopardalis. The galaxy is relatively nearby. Consequently, the Hubble Space Telescope can easily resolve the stars within the galaxy. The distance to the galaxy was previously believed to be only 2.4 Mpc. However, in 2008 scientists studying images from Hubble calculated the galaxy's distance at nearly 11 million light-years away, about 4 million light-years farther than previous thought, meaning it is a member of the IC 342 group of galaxies.

A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster. These clusters are referred to as "super" due to the fact that they are relatively more luminous and contain more mass than other young star clusters. The SSC, however, does not have to physically be larger than other clusters of lower mass and luminosity. They typically contain a very large number of young, massive stars that ionize a surrounding HII region or a so-called "Ultra dense HII regions (UDHIIs)" in the Milky Way Galaxy as well as in other galaxies. An SSC's HII region is in turn surrounded by a cocoon of dust. In many cases, the stars and the HII regions will be invisible to observations in certain wavelengths of light, such as the visible spectrum, due to high levels of extinction. As a result, the youngest SSCs are best observed and photographed in radio and infrared. SSCs, such as Westerlund 1 (Wd1), have been found in the Milky Way Galaxy. However, most have been observed in farther regions of the universe. In the galaxy M82 alone, 197 young SSCs have been observed and identified using the Hubble Space Telescope.

A Hickson Compact Group is a collection of galaxies designated as published by Paul Hickson in 1982.

NGC 7319 Highly distorted spiral galaxy in the constellation Pegasus

NGC 7319 is a highly distorted barred spiral galaxy that is a member of the compact Stephan's Quintet group located in the constellation Pegasus, some 311 megalight-years distant from the Milky Way. The galaxy's arms, dust and gas have been highly disturbed as a result of the interaction with the other members of the Quintet. Nearly all of the neutral hydrogen has been stripped from this galaxy, most likely as a result of a collision with NGC 7320c some 100 million years ago. A pair of long, parallel tidal tails extend southward from NGC 7319 in the direction of NGC 7320c, and is undergoing star formation.

NGC 4449 Irregular magellanic type galaxy in the constellation Canes Venatici

NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 12 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.

NGC 3311 Elliptical galaxy in the constellation Hydra

NGC 3311 is a supergiant elliptical galaxy located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.

NGC 720 Elliptical galaxy in the constellation Cetus

NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of circa 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.

NGC 536 Barred spiral galaxy in the constellation Andromeda

NGC 536 is a barred spiral galaxy located in the constellation Andromeda. It is located at a distance of circa 200 million light years from Earth, which, given its apparent dimensions, means that NGC 536 is about 180,000 light years across. It was discovered by William Herschel on September 13, 1784. It is a member of Hickson Compact Group 10, which also includes the galaxies NGC 529, NGC 531, and NGC 542. It belongs to the Perseus-Pisces Supercluster.

NGC 4636 Elliptical galaxy in the constellation Virgo

NGC 4636 is an elliptical galaxy located in the constellation Virgo. It is located at a distance of circa 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4636 is about 105,000 light years across. It was discovered by William Herschel on February 23, 1784. NGC 4636 lies one and a half degrees southwest of Delta Virginis. It can be viewed through a telescope at a ×23 magnification as a bright oval glow. It is part of the Herschel 400 Catalogue.

NGC 1132 Elliptical galaxy in the constellation Eridanus

NGC 1132 is an elliptical galaxy located in the constellation Eridanus. The galaxy was discovered by John Herschel on November 23, 1827. It is located at a distance of about 318 million light-years away from Earth.


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