A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of most galaxies. NSCs are the

central massive object A central massive object (CMO) is a high mass object or cluster of objects at the centre of a large star system, such as a galaxy or globular cluster. In the case of the former, the CMO may be a supermassive black hole, a nuclear star cluster, or ...

s of fainter, low-mass galaxies where

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical ob ...

s (SMBHs) are not present or are of negligible mass. In the most massive galaxies, NSCs are entirely absent. Some galaxies, including the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

, are known to contain both a NSC and a SMBH of comparable mass.

Properties

Nuclear star clusters are found in most galaxies that can be resolved sufficiently: * at least 50% of all early

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has ...

al galaxies (types S0 and E). NSCs are the densest known star clusters in the

Universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...

. With

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

s between -14 and -10 mag in the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

, they are on average 40 times brighter than

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

s, although their effective radii are not larger than 2 to 5

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

s. With a dynamic mass of 10⁶ to 10⁸

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

, they are at the upper end of the values reached by globular clusters. The majority of nuclear star clusters contain a mix of old (at least one billion years old) and young

stellar population During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926: Baade noticed ...

s and show signs of star formation within the last 100 million years.

Formation

Although the mechanisms behind their formation are not entirely known, hypotheses provide four possibilities: * Nuclear star clusters originate somewhere else and are captured by a central black hole. * Nuclear star clusters are due to an incidence of gas at some distance from the center of the galaxy. * A combination of the above possibilities whereby the

gravitational potential In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move an object to that location from a fixed reference location. It is analogous to the electric ...

of a trapped object, such as the nucleus of a

dwarf galaxy 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 so ...

, triggers new star formation by incident gas near the galactic center. * Nuclear star clusters are created by merging star clusters with subsequent migration to the galactic center due to

dynamical friction In astrophysics, dynamical friction or Chandrasekhar friction, sometimes called ''gravitational drag'', is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first d ...

with background stars.

Relationship with globular clusters

Because nuclear star clusters occur in most galaxy species, they should still be present in the

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...

of the resulting galaxy after the fusion of galaxies. This is a hypothesis for the formation of globular clusters. Thus, globular clusters could be the remains of nuclear star clusters excluded from gas incidence, in which no new star formation occurs. According to other hypotheses, however, the nuclear star clusters could be the result of a fusion of globular clusters captured by a supermassive black hole in the center of the galaxy and dynamically destroyed.{{cite journal, author=Capuzzo-Dolcetta, R., title=Galactic Nuclear Cluster Formation Via Globular Cluster Mergers, year=2013, journal= Memorie della Societa Astronomica Italiana, volume=84, pages=167, arxiv=1301.2899, bibcode=2013MmSAI..84..167C

References

Star clusters