A dwarf galaxy is a small

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 ...

composed of about 1000 up to several billion

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, as compared to 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. ...

's 200–400 billion stars. The

Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...

, 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.

Formation

One theory states that most galaxies, including dwarf galaxies, form in association with

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 ...

, or from gas that contains metals. However,

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, succeedin ...

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; o ...

identified new dwarf galaxies forming out of gases with low

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 ...

. These galaxies were located in the Leo Ring, a cloud of

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-to ...

and

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 ta ...

around two massive galaxies in the

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

Leo. Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in

stellar streams This is a list of stellar streams. A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces. An exception in the ...

and eventually galaxy merger.

Local dwarf galaxies

There are many dwarf galaxies in the Local Group; these small galaxies frequently orbit larger galaxies, such as the

, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper has suggested that many dwarf galaxies were created by galactic tides 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

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 in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...

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

Common types

* Elliptical galaxy:

dwarf elliptical galaxy Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. Examples "Dwarf elliptical" galaxie ...

(dE) * Dwarf spheroidal galaxy (dSph): Once a subtype of dwarf ellipticals, now regarded as a distinct type * Irregular galaxy:

dwarf 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, ...

(dIrr) *

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

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

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

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 ...

in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. 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 2915, NGC 3353 and

UGCA 281 __NOTOC__ Year 281 ( CCLXXXI) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Probus and Tiberianus (or, less frequently, year 1034 ...

Ultra-faint dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are a class of

galaxies 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 ...

that contain from a few hundred to one hundred thousand

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 univers ...

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

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 ...

(SDSS). UFDs are the most

-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, moons, comets and galaxies – in either ...

s believe that UFDs encode valuable information about the early

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

and before the epoch of reionization. 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

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

Abell 1689 Abell 1689 is a galaxy cluster in the 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 images of galaxies that ...

, 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 M60-UCD1 is an ultracompact dwarf galaxy. It is 54 million light years from Earth, close to Messier 60 (M60, NGC 4649) in the Virgo Cluster. Half of its stellar mass is in the central sphere 160 light years in diameter. Characteristics M60-U ...

, 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 is approximately the same size as M60-UCD1 with a half-light radius, r_h, of approximately 20 parsecs 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 w ...

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

Partial list

* Aquarius Dwarf *

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 *

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 th ...

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 Lo ...

* NGC 1569 * NGC 1705 * NGC 2915 * * Pegasus Dwarf Irregular Galaxy * PHL 293B *

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. Its name comes from the fact that it is part of ...

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 th ...

* Sculptor Dwarf Irregular Galaxy * Sextans A *

Sextans Dwarf Spheroidal The Sextans Dwarf Spheroidal is a dwarf spheroidal galaxy that was discovered in 1990 by Mike Irwin as the 8th satellite of the Milky Way, located in the constellation of Sextans. It is also an elliptical galaxy, and displays a redshift becau ...

Small Magellanic Cloud The Small Magellanic Cloud (SMC), or Nubecula Minor, is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of ...

* Tucana Dwarf *

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 Will ...

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 an ...

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 Ursa Minor (Latin: 'Lesser Bear', contrasting wi ...

* Willman 1 * Carina Dwarf * Draco Dwarf * Fornax Dwarf *

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 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