Galaxy groups and clusters are super-structures in the spread of galaxies of the cosmos. Matter throughout the visible universe has, over the course of the universe's history, aggregated into a range of large-scale structures under the influence of gravity. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are involved in larger groups called superclusters. Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... Gravity is the force of attraction between massive particles. ... 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 about 2 megaparsec (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. 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 parsec (symbol pc) is a unit of length used in astronomy. ... (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. A NASA artists conception of what the Milky Way would look like if seen off-axis. ... Map of the local group The Local Group is the group of galaxies that includes our galaxy, the Milky Way. ...

Clusters of galaxies

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.

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. This gas is very hot, around 10⁸K, hence emits X-rays. 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 equilibrium with the overall cluster gravitational field, its distribution in the cluster traces out the overall cluster gravitational field, and therefore allows calculation of the total mass distribution in the cluster. It turns out the total mass deduced from this measurement is much 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. 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 ... In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. ...

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 of 8Mpc (see 1 E23 m for distance comparisons).
• The spread of velocities for the individual galaxies is about 800-1000 km/s.
• The average distance between clusters is approximately 10 Mpc.

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 cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. ... (Redirected from 1 E23 m) Categories: Orders of magnitude (length) ... A sky field near some of the brighter galaxies in the Virgo cluster. ... // Coma galaxy cluster The Coma Cluster is an huge galaxy cluster and the prototypical rich cluster with over a thousand member galaxies known. ... The Great Attractor is a gravitational anomaly in intergalactic space, at the centre of the local supercluster, which is drawing in galaxies over a region hundreds of millions of light years across. ... The Norma Cluster (ACO 3627) is a partly obscured cluster of galaxies, also named Great Attractor, located 7 degrees from the plane of the Milky Way. ... 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 gravitationally bound to each other. ... A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ...

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. ... In astronomy, voids are the empty spaces between filaments, some of the largest-scale structures in the Universe that contain hardly any, 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. ...

Finding galaxy clusters

Galaxy clusters are very difficult to find for three main reasons:

• They are faint
• There are only a small number of them
• There are about 100 billion stars in our own galaxy, which get in the way of searches for fainter, more distant objects

Traditionally clusters of galaxies were found by looking through telescopes for groups of galaxies on the sky. This was quite inefficient and limited to nearby, bright clusters, so a number of new techniques have been developed:

• Infrared surveys -- distant clusters of galaxies tend to be redder than nearby stars, so it is slightly easier to spot them in the infrared
• Radio surveys -- very few objects in our own galaxy emit strong radio emission, so there is less of a problem with nearby sources getting in the way
• Sunyaev-Zeldovich effect surveys -- clusters of galaxies contain hot plasma (gas) which interacts with the cosmic microwave background radiation creating a sort of shadow against the background radiation at radio frequencies.
• X-ray surveys -- the hot plasma also emits X-rays which can be detected by X-ray telescopes.

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 microwave radiation. ... An astronomical catalog 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. ... An astronomical catalog 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. ... 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. ... An astronomical catalog 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. ... In cosmology, the cosmic microwave background radiation (CMB) is a form of electromagnetic radiation discovered in 1964 that radiates throughout the universe in the microwave range. ... 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... An astronomical catalog 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. ... 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. ...

Instruments and surveys for clusters of galaxies

• The AMI array at Mullard Radio Astronomy Observatory

The long baseline array of AMI, utilizing receivers from the Ryle Telescope. ... 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. ...

See also

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