galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 ...

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

Galaxy Evolution Explorer Galaxy Evolution Explorer (GALEX or Explorer 83 or SMEX-7) was a NASA orbiting space telescope designed to observe the universe in ultraviolet wavelengths to measure the history of star formation in the universe. In addition to paving the way ...

space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ent ...

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

Leo Ring The Leo Ring is an immense intergalactic cloud of hydrogen and helium gas some in diameter, in orbit of two galaxies, in the center of the Leo Group of galaxies, within the constellation of Leo. Observation history Radio astronomers discover ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

ripped Ripped may refer to: * Ripped, a slang term for having achieved muscle hypertrophy * '' Ripped: How the Wired Generation Revolutionized Music'', a book by Greg Kot * ''Ripped,'' a series of books and DVDs by Clarence Bass * "Ripped", an episode of ...

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''stellar streams and eventually

galaxy merger Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies ...

.

Local dwarf galaxies

There are many dwarf galaxies in the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

; these small galaxies frequently orbit larger galaxies, such as 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 ...

, the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

and the

Triangulum Galaxy The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (''New General Catalogue)'' 598. With the D25 isophotal diameter of , the Triangulum Ga ...

. A 2007 paper has suggested that many dwarf galaxies were created by

galactic tide A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satelli ...

s during the early evolutions 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 away from the parent galaxies and the halos of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

that surround them. A 2018 study suggests that some local dwarf galaxies formed extremely early, during the Dark Ages within the first billion years after the

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

. More than 20 known dwarf galaxies orbit the Milky Way, and recent observations have also led astronomers to believe the largest

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 ...

in the Milky Way,

Omega Centauri Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of , it is the largest-known globular c ...

, is in fact the core of a dwarf galaxy with a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

at its centre, which was at some time absorbed by the Milky Way.

Common types

*

Elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

: dwarf elliptical galaxy (dE) *

Dwarf spheroidal galaxy A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are com ...

(dSph): Once a subtype of dwarf ellipticals, now regarded as a distinct type *

Irregular galaxy An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...

: dwarf irregular galaxy (dIrr) * Spiral galaxy:

dwarf spiral galaxy A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters (less than 5 kpc), low surface brightnesses, and low hydrogen masses. The galaxies may be considered a s ...

(dS) * Magellanic type dwarfs * Blue compact dwarf galaxies (see section below) * Ultra-compact dwarf galaxies (see section below)

Blue compact dwarf galaxies

An intriguing young-looking dwarf galaxy

In

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s. These stars, the brightest of which are blue, cause the galaxy itself to appear

blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...

in colour. Most BCD galaxies are also classified as dwarf

irregular galaxies An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...

or as dwarf

lenticular galaxies A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. ...

. Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming. BCD galaxies cool in the process of forming new stars. The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include

NGC 1705 NGC 1705 is a peculiar lenticular galaxy and a blue compact dwarf galaxy (BCD) in the southern constellation of Pictor, positioned less than a degree to the east of Iota Pictoris, and is undergoing a starburst. With an apparent visual magnit ...

,

NGC 2915 NGC 2915 is a blue dwarf galaxy located 12 million light-years away in the southern constellation Chamaeleon, right on the edge of the Local Group. The optical galaxy corresponds to the core of a much larger spiral galaxy traced by radio observ ...

,

NGC 3353 NGC commonly refers to: * New General Catalogue of Nebulae and Clusters of Stars, a catalogue of deep sky objects in astronomy NGC may also refer to: Companies * NGC Corporation, name of US electric company Dynegy, Inc. from 1995 to 1998 * Na ...

and UGCA 281.

Ultra-faint dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are a class of galaxies that contain from a few hundred to one hundred thousand

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s, making them the faintest galaxies 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. Acc ...

. UFDs resemble

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 (GCs) in appearance but have very different properties. Unlike GCs, UFDs contain a significant amount of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

and are more extended. UFDs were first discovered with the advent of digital sky surveys in 2005, in particular with the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

(SDSS). UFDs are the most

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

-dominated systems known.

Astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

s believe that UFDs encode valuable information about the early

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. Acc ...

, as all UFDs discovered so far are ancient systems that have likely formed very early on, only a few million years after the

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

and before the epoch of

reionization In the fields of Big Bang theory and cosmology, reionization is the process that caused matter in the universe to reionize after the lapse of the " dark ages". Reionization is the second of two major phase transitions of gas in the universe (t ...

. Recent theoretical work has hypothesised the existence of a population of young UFDs that form at a much later time than the ancient UFDs. These galaxies have not been observed in our

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. Acc ...

so far.

Ultra-compact dwarfs

Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities, discovered in the 2000s. They are thought to be on the order of 200 light years across, containing about 100 million stars. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by

tidal interactions The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

, travelling through the hearts of rich clusters. UCDs have been found in the

Virgo Cluster The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation Virgo. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the lar ...

,

Fornax Cluster The Fornax Cluster is a cluster of galaxies lying at a distance of 19 megaparsecs (62 million light-years). It has an estimated mass of solar masses, making it the second richest galaxy cluster within 100 million light-years, after the consider ...

,

Abell 1689 Abell 1689 is a galaxy cluster in the constellation Virgo (constellation), Virgo over 2.3 billion light-years away. Details Abell 1689 is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the i ...

, and the

Coma Cluster The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and tak ...

, amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team. The first ever relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; its central region packs in stars about 25 times closer together than stars in Earth's region in the Milky Way.

M59-UCD3 M59-UCD3 is an Dwarf galaxy#Ultra-compact dwarfs, ultra-compact dwarf galaxy located near the Messier 59 galaxy. , it is the second-densest galaxy currently observed, second to M85-HCC1. References See also

* M60-UCD1 (densest galaxy known, ...

is approximately the same size as M60-UCD1 with a

half-light radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vie ...

, r_h, of approximately 20

parsecs 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 ...

but is 40% more luminous with an

absolute visual magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

of approximately −14.6. This makes M59-UCD3 the densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have

supermassive black holes 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 ...

weighing 13% and 18% of the galaxies' masses.

Partial list

*

Aquarius Dwarf The Aquarius Dwarf is a dwarf irregular galaxy, first catalogued in 1959 by the DDO survey. It is located within the boundaries of the constellation of Aquarius. It is a member of the Local Group of galaxies, albeit an extremely isolated one; ...

*

Canis Major Dwarf Galaxy The Canis Major Overdensity (''CMa Overdensity'') or Canis Major Dwarf Galaxy (''CMa Dwarf'') is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major. The supposed small gal ...

* Crater 2 dwarf *

Eridanus II The Eridanus II Dwarf is a low-surface brightness dwarf galaxy in the constellation Eridanus. Eridanus II was independently discovered by two groups in 2015, using data from the Dark Energy Survey ( Bechtol et al., 2015; Koposov et al. 2015) ...

*

Henize 2-10 Hen 2-10, also known as He 2-10 and Henize 2-10, is a dwarf starburst galaxy located 34 million light years away in the constellation of Pyxis. The galaxy is believed to be an early stage starburst galaxy. A black hole was later discovered near t ...

*

I Zwicky 18 I Zwicky 18 is a blue compact dwarf galaxy located about 59 million light years away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. Age Studies at t ...

*

IC 10 IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by Lewis Swift in 1887 and in 1935 Nicholas Mayall became the first to suggest that the object is extragalactic. Edwin Hubble suspected it might belong to the Loc ...

* Large Magellanic Cloud *

NGC 1569 NGC 1569 is a dwarf irregular galaxy in Camelopardalis. The galaxy is relatively nearby and 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. ...

*

NGC 1705 NGC 1705 is a peculiar lenticular galaxy and a blue compact dwarf galaxy (BCD) in the southern constellation of Pictor, positioned less than a degree to the east of Iota Pictoris, and is undergoing a starburst. With an apparent visual magnit ...

*

NGC 2915 NGC 2915 is a blue dwarf galaxy located 12 million light-years away in the southern constellation Chamaeleon, right on the edge of the Local Group. The optical galaxy corresponds to the core of a much larger spiral galaxy traced by radio observ ...

* *

Pegasus Dwarf Irregular Galaxy The Pegasus Dwarf Irregular Galaxy (also known as Peg DIG or the Pegasus Dwarf) is a dwarf irregular galaxy in the direction of the constellation Pegasus. It was discovered by A. G. Wilson in the 1950s. The Pegasus Dwarf is a companion of the A ...

*

PHL 293B PHL 293B, also known as Kinman's dwarf, is a low- metallicity blue compact dwarf galaxy about 22.6 Mpc from the Earth in the constellation Aquarius. It had a very likely associated, notable, blue-light, long-lived star with constant ...

*

Phoenix Dwarf The Phoenix Dwarf is a dwarf irregular galaxy 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 Earth is the third planet from the Sun ...

*

Sagittarius Dwarf Spheroidal Galaxy The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in ...

*

Sagittarius Dwarf Irregular Galaxy The Sagittarius Dwarf Irregular Galaxy (SagDIG) is a dwarf galaxy in the constellation of Sagittarius. (SagDIG should not be confused with the Sagittarius Dwarf Spheroidal Galaxy, SagDEG, a satellite galaxy of the Milky Way discovered decades ...

*

Sculptor Dwarf Galaxy The Sculptor Dwarf Galaxy (also known as Sculptor Dwarf Elliptical Galaxy or the Sculptor Dwarf Spheroidal Galaxy, and formerly as the Sculptor System) is a dwarf spheroidal galaxy that is a satellite of the Milky Way. The galaxy lies within the ...

*

Sculptor Dwarf Irregular Galaxy The Sculptor Dwarf Irregular Galaxy (SDIG) is an irregular galaxy in the constellation Sculptor. It is a member of the NGC 7793 subgroup of the Sculptor Group. The galaxy was discovered in 1976. __NOTOC__ Nearby galaxies and galaxy group info ...

*

Sextans A Sextans A (also known as UGCA 205) is a small dwarf irregular galaxy. It spans about 5000 light-years across, and is located at 4.3 million light-years away, in the outskirts of the Local Group of galaxies, which includes the Milky Way galaxy, an ...

* Sextans Dwarf Spheroidal * Small Magellanic Cloud *

Tucana Dwarf The Tucana Dwarf Galaxy is a dwarf galaxy in the constellation Tucana. It was discovered in 1990 by R.J. Lavery of Mount Stromlo Observatory. It is composed of very old stars and is very isolated from other galaxies. Its location on the opposite ...

*

Ursa Major I Dwarf Ursa Major I Dwarf (UMa I dSph) is a dwarf spheroidal galaxy that orbits the Milky Way galaxy. It was discovered in 2005 within the Ursa Major constellation and is the third least luminous known galaxy. Discovery It was discovered by Beth Willm ...

*

Ursa Major II Dwarf Ursa Major II Dwarf (UMa II dSph) is a dwarf spheroidal galaxy situated in the Ursa Major constellation and discovered in 2006 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located approximately 30 kpc from the Sun and ...

*

Ursa Minor Dwarf The Ursa Minor Dwarf is a dwarf spheroidal galaxy, discovered by A.G. Wilson of the Lowell Observatory, in the United States, during the Palomar Sky Survey in 1955. It appears in the Ursa Minor constellation, and is a satellite galaxy of the Mi ...

*

Willman 1 Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster. Willman 1 was discovered in 2004. It is Galaxies named after people, named after Beth Willman of Haverford College, the lead author of a study based on the Sloan Digital Sky Su ...

*

Carina Dwarf The Carina Dwarf Spheroidal Galaxy is a dwarf galaxy in the Carina constellation. It was discovered in 1977 with the UK Schmidt Telescope The UK Schmidt Telescope (UKST) is a 1.24 metre Schmidt telescope operated by the Australian Ast ...

* Draco Dwarf *

Fornax Dwarf The Fornax Dwarf Spheroidal (formerly known as the Fornax System) 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 take ...

*

Leo I The LEO I (Lyons Electronic Office I) was the first computer used for commercial business applications. The prototype LEO I was modelled closely on the Cambridge EDSAC. Its construction was overseen by Oliver Standingford, Raymond Thompson and ...

* Leo II

Gallery

Hubble's Legacy UGC 685.jpg, Dwarf galaxy

UGC 685 UGC may refer to: Science and technology * Universal gravitational constant G, in physics * Uppsala General Catalogue, an astronomical catalogue of galaxies * UGC, a codon for cysteine * Unique games conjecture, a conjecture in computational com ...

taken by Hubble. *

Galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

References

External links

Milky Way Satellite Galaxies

''Science'' article on "hobbit galaxies"
{{Authority control Galaxy morphological types

Formation

Local dwarf galaxies

Common types

Blue compact dwarf galaxies

Ultra-faint dwarf galaxies

Ultra-compact dwarfs

Partial list

Gallery

See also

References

External links