Dwarf galaxy
Encyclopedia
A dwarf galaxy is a small galaxy
composed of up to several billion star
s, a small number compared to our own Milky Way
's 200-400 billion stars. The Large Magellanic Cloud
, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy orbiting the Milky Way galaxy.
or out of gas containing metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo
.
: these small galaxies frequently orbit around larger galaxies, such as the Milky Way
, the Andromeda Galaxy
and the Triangulum Galaxy
. A 2007 paper has suggested that many dwarf galaxies were created by tidal forces during the early evolution of the Milky Way
and Andromeda
. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galactic material are pulled out away from the parent galaxies and the halos of dark matter that surround them.
The Milky Way has 14 known dwarf galaxies orbiting it, and recent observations have also led astronomers to believe the largest globular cluster
in the Milky Way, Omega Centauri
, is in fact the core of a dwarf galaxy with a black hole in its center, which was at some time absorbed by the Milky Way.
, Fornax Cluster
, Abell 1689
, Coma Cluster, amongst other clusters.
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
composed of up to several billion star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, a small number compared to our own Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
's 200-400 billion stars. The Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy orbiting the Milky Way galaxy.
Formation of dwarf galaxies
Current theory states that most galaxies, including dwarf galaxies, form in association with dark matterDark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
or out of gas containing metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo
Leo (constellation)
Leo is one of the constellations of the zodiac. Its name is Latin for lion. Its symbol is . Leo lies between dim Cancer to the west and Virgo to the east.-Stars:...
.
Local dwarfs
There are many dwarf galaxies in the Local GroupLocal Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
: these small galaxies frequently orbit around larger galaxies, such as the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
and the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
. A 2007 paper has suggested that many dwarf galaxies were created by tidal forces during the early evolution of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
and Andromeda
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galactic material are pulled out away from the parent galaxies and the halos of dark matter that surround them.
The Milky Way has 14 known dwarf galaxies orbiting it, and recent observations have also led astronomers to believe the largest globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
in the Milky Way, Omega Centauri
Omega Centauri
Omega Centauri or NGC 5139 is a globular clusterin the constellation of Centaurus, discovered by Edmond Halley in 1677 who listed it as a nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star. Lacaille included it in his catalog as number I.5...
, is in fact the core of a dwarf galaxy with a black hole in its center, which was at some time absorbed by the Milky Way.
Dwarf galaxies
Dwarf galaxies come in many different morphologies:- Elliptical galaxyElliptical galaxyAn elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
: dwarf elliptical galaxyDwarf elliptical galaxyDwarf elliptical galaxies, or dEs, are elliptical galaxies that are much smaller than others. They are classified as dE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.- Examples :...
(dE) and its subtype dwarf spheroidal galaxyDwarf spheroidal galaxyDwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
(dSph) - Irregular galaxyIrregular galaxyAn irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
: dwarf irregular galaxy (dI) - Spiral galaxySpiral galaxyA spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
: dwarf spiral galaxy
Hobbit galaxies
The recently coined term, hobbit galaxy has been used to describe galaxies smaller and dimmer than dwarf galaxies.Ultra Compact Dwarfs
Ultra Compact Dwarf galaxies (UCD) are a recently discovered class of very compact galaxies with very high stellar population counts. They are thought to be on the order of 200 light years across, with a hundred million stars. It is theorized that these are the cores of nucleated dwarf elliptical galaxies, that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters. UCDs have been found in the Virgo ClusterVirgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...
, Fornax Cluster
Fornax Cluster
At a distance of approximately 62.0 Mly , the Fornax Cluster is the second richest cluster of galaxies within 100 million light-years, although it is much smaller than the Virgo Cluster. It lies primarily in the constellation Fornax, and may be associated with the nearby Eridanus Group...
, Abell 1689
Abell 1689
Abell 1689 is a galaxy cluster in the constellation Virgo. It is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it...
, Coma Cluster, amongst other clusters.
Partial list of dwarf galaxies
- Aquarius DwarfAquarius DwarfThe Aquarius Dwarf galaxy is a dwarf galaxy and an irregular galaxy, that was first catalogued in 1959 by the DDO survey. Its most distinctive characteristic is that it is one of the few galaxies known to display a blueshift, as it is traveling towards the Milky Way at 137 km/s. Aquarius...
- Canis Major Dwarf GalaxyCanis Major Dwarf GalaxyThe Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation Canis Major. The galaxy contains a relatively high percentage of red giant stars, and is thought to contain an estimated one billion stars in all....
- Henize 2-10Henize 2-10Henize 2-10 is a dwarf galaxy located 34 million light years away in the constellation of Pyxis. It is the first dwarf galaxy ever discovered that contains, at its center, a supermassive black hole...
- I Zwicky 18I Zwicky 18I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted...
- Irregular Galaxy IC 10
- Large Magellanic CloudLarge Magellanic CloudThe Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
- NGC 1569NGC 1569NGC 1569 is a dwarf irregular galaxy in Camelopardalis. While this faint galaxy is not a popular amateur astronomy target, it is well studied by professional astronomers, who are interested in the history of star formation within the galaxy. The galaxy is relatively nearby. Consequently, the...
- NGC 1705NGC 1705NGC 1705 is a peculiar lenticular galaxy in the constellation Pictor. It is estimated to be approximately 17 million light-years from the Earth. It is a member of the Dorado Group.-External links:*...
- Pegasus Dwarf Irregular GalaxyPegasus Dwarf Irregular GalaxyThe Pegasus Dwarf Irregular Galaxy is an irregular galaxy and a dwarf galaxy in the direction of the constellation Pegasus. It was discovered by A.G. Wilson in the 1950s...
- Phoenix DwarfPhoenix DwarfThe Phoenix Dwarf is a dwarf galaxy and an irregular galaxy that was discovered in 1976 by Hans-Emil Schuster and Richard Martin West and mistaken for a globular cluster. It is currently 1.44 Mly away from Earth...
- Sagittarius Dwarf Elliptical GalaxySagittarius Dwarf Elliptical GalaxyThe Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...
- Sagittarius Dwarf Irregular GalaxySagittarius Dwarf Irregular GalaxyThe Sagittarius Dwarf Irregular Galaxy or SagDIG is a dwarf galaxy in the constellation of Sagittarius. It lies about 3.4 million light-years away. SagDIG should not be confused with the Sagittarius Dwarf Elliptical Galaxy or SagDEG, a satellite galaxy of the Milky Way. It was discovered by...
- Sculptor Dwarf GalaxySculptor Dwarf GalaxyThe Sculptor Dwarf Galaxy is a dwarf spheroidal galaxy that is a satellite of the Milky Way. The galaxy lies within the constellation Sculptor. It was discovered in 1937 by Harlow Shapley. The galaxy is located about 290,000 light-years away from the solar system...
- Sculptor Dwarf Irregular GalaxySculptor Dwarf Irregular GalaxyThe Sculptor Dwarf Irregular Galaxy is an irregular galaxy in the constellation Sculptor. The galaxy was discovered in 1976.-Nearby galaxies and galaxy group information:...
- Sextans ASextans ASextans A , is a tiny dwarf irregular galaxy. It spans about 5000 light-years across, and is located within the Local Group of galaxies, which includes our Milky Way galaxy. At 4.3 million light-years away from Earth, Sextans A is one of most distant members of the Local Group, and is notable for...
- Sextans Dwarf
- Small Magellanic CloudSmall Magellanic CloudThe Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
- Tucana Dwarf
- Ursa Minor DwarfUrsa Minor DwarfThe Ursa Minor Dwarf dwarf elliptical galaxy was discovered by A.G. Wilson of the Lowell Observatory in 1954. It is part of the Ursa Minor constellation, and a satellite galaxy to the Milky Way...
- Willman 1Willman 1Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster. It is named after Beth Willman of Haverford College, the lead author of a study based on the Sloan Digital Sky Survey data. The object is a satellite of the Milky Way; ~120,000 light-years away. Willman 1 has an elliptical shape...
- Carina DwarfCarina DwarfThe Carina Dwarf Spheroidal is a dwarf galaxy in the Carina constellation. It was discovered in 1977 with the UK Schmidt Telescope by Cannon et al. The Carina Dwarf is receding from the Milky Way at 230 km/s and is a satellite galaxy to the Milky Way. The galaxy may also be referred to as...
- Draco DwarfDraco DwarfThe Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey . It is part of the local group and a satellite galaxy of the Milky Way galaxy...
- Fornax DwarfFornax DwarfThe Fornax Dwarf Spheroidal is an elliptical dwarf galaxy in the constellation Fornax that was discovered in 1938 by Harlow Shapley. He discovered it while he was in South Africa on photographic plates taken by a 24 inch reflecting telescope at Boyden Observatory, shortly after he discovered...
- Leo II (dwarf galaxy)Leo II (dwarf galaxy)Leo II is an dwarf spheroidal galaxy about 690,000 light-years away in the constellation Leo. As of October 2008 it is one of 24 known satellite galaxies of the Milky Way....