The Local Group is the

galaxy group A galaxy group or group of galaxies (GrG) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxi ...

that includes the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, where

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

is located. 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 The dumbbell, a type of free weight, is a piece of equipment used in weight training. It is usually used individually and/or in pairs, with one in each hand. History The forerunner of the dumbbell, halteres, were used in ancient Greece as li ...

" shape; the Milky Way and its satellites form one lobe, and the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

and its satellites constitute the other. The two collections are separated by about and are moving toward one another with a velocity of . The group itself is a part of the larger

Virgo Supercluster The Local Supercluster (LSC or LS), or Virgo Supercluster is a formerly defined supercluster containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least 100 galaxy group ...

, which may be a part of the

Laniakea Supercluster The Laniakea Supercluster (; Hawaiian language, Hawaiian for "open skies" or "immense heaven") or the Local Supercluster (LSC or LS) is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies. It w ...

. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are

dwarf galaxies 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 ...

. The two largest members, the Andromeda and the Milky Way galaxies, are both

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

with masses of about solar masses each. Each has its own system of

satellite galaxies A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and Luminosity, luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents ...

: * The Andromeda Galaxy's satellite system consists of

Messier 32 Messier 32 (also known as M32 and NGC 221) is a dwarf "early-type" galaxy about from the Solar System, appearing in the constellation Andromeda (constellation), Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discover ...

(M32),

Messier 110 Messier 110, or M110, also known as NGC 205, is a dwarf elliptical galaxy that is a satellite of the Andromeda Galaxy in the Local Group. Early observational history Charles Messier never included the galaxy in his list, but it was depicte ...

(M110),

NGC 147 NGC 147 (also known as DDO3 or Caldwell 17) is a dwarf spheroidal galaxy about 2.58 Mly away in the constellation Cassiopeia. NGC 147 is a member of the Local Group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It forms a p ...

NGC 185 NGC 185 (also known as Caldwell 18) is a dwarf spheroidal galaxy located 2.08 million light-years from Earth, appearing in the constellation Cassiopeia. It is a member of the Local Group, and is a satellite of the Andromeda Galaxy (M31). NGC 18 ...

, Andromeda I (And I), And II, And III,

And V Andromeda V is a dwarf spheroidal galaxy about 2.52 Mly away in the constellation Andromeda. Andromeda V was discovered by Armandroff et al. and published in 1998 after their analysis of the digitized version of the second Palomar Sky Sur ...

And VI The Pegasus Dwarf Spheroidal (also known as ''Andromeda VI'' or ''Peg dSph'' for short) is a dwarf spheroidal galaxy about 2.7 million light-years away in the constellation Pegasus. The Pegasus Dwarf is a member of the Local Group and a satell ...

(also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph),

And VII The Cassiopeia Dwarf (also known as Andromeda VII) is a dwarf spheroidal galaxy about 2.45 Mly away in the constellation Cassiopeia. The Cassiopeia Dwarf is part of the Local Group and a satellite galaxy of the Andromeda Galaxy (M31). In the s ...

(a.k.a. the Cassiopeia Dwarf Galaxy),

And VIII Andromeda VIII (And VIII / 8) is a galaxy discovered in August 2003. It is a companion galaxy to the Andromeda Galaxy, M31, and evaded detection for so long due to its diffuse nature. The galaxy was finally discovered by measuring the redshift ...

And IX Andromeda IX (And 9) is a dwarf spheroidal satellite of the Andromeda Galaxy. It was discovered in 2004 by resolved stellar photometry from the Sloan Digital Sky Survey (SDSS), by Zucker et al. (2004). At the time of its discovery, it was the g ...

And X Andromeda X (And 10) is a dwarf spheroidal galaxy about 2.9 million light-years away from the Sun in the constellation Andromeda. Discovered in 2005 by Zucker et al., And X is a satellite galaxy of the Andromeda Galaxy (M31). Aided by the appl ...

, And XI,

And XIX Andromeda XIX is a satellite galaxy of the Andromeda Galaxy (M31), a member of the Local Group, like the Milky Way Galaxy. Andromeda XIX is considered "the most extended dwarf galaxy known in the Local Group", and has been shown to have a half-l ...

And XXI Andromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy about away from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy. The discovery arose from the first year data o ...

and

And XXII Andromeda XXII (Pisces VI, Triangulum I) is a low surface brightness dwarf spheroidal galaxy about away from the Sun in the constellation Pisces, of the Local Group. Andromeda XXII is located much closer in projection to M33 than M31 vs. This ...

, plus several additional ultra-faint

dwarf spheroidal galaxies 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 as systems that are c ...

. * The Milky Way's satellite galaxies system comprises the Sagittarius Dwarf Galaxy,

Large Magellanic Cloud The Large Magellanic Cloud (LMC) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...

Small Magellanic Cloud The Small Magellanic Cloud (SMC) 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 approximately 7 bill ...

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 ...

(disputed, considered by some not a galaxy),

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 Mil ...

Galaxy,

Draco Dwarf The 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 (POSS). It is part of the Local Gr ...

Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy,

Fornax Dwarf The Fornax Dwarf Spheroidal (formerly known as the Fornax System) is a dwarf elliptical 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 ...

Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy),

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 Wi ...

Galaxy and

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 ...

Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies. Local Group Galaxies Comparison

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 598. With the D25 isophotal diameter of , the Triangulum Galaxy is the third-largest me ...

(M33) is the third-largest member of the Local Group, with a mass of approximately , and is the third spiral galaxy. It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart, and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk. The

Pisces Dwarf The Pisces Dwarf, also known as Pisces I, is an irregular dwarf galaxy that is part of the Local Group. The galaxy, taking its name from the constellation Pisces where it appears, is suspected of being a satellite galaxy of the Triangulum Gala ...

Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either. The other members of the group are likely gravitationally secluded from these large subgroups:

IC 10 IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by American astronomer Lewis Swift on 8 October 1887. In 1935 Nicholas Mayall became the first to suggest that the object is extragalactic. Edwin Hubble suspected i ...

, IC 1613,

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 ...

Galaxy,

Leo A Leo A (also known as Leo III) is an irregular galaxy that is part of the Local Group. It lies 2.6 million light-years from Earth, and was discovered by Fritz Zwicky in 1942. The estimated mass of this galaxy is solar masses, with at least ...

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 opposi ...

Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte,

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; i ...

Galaxy, and

Sagittarius Dwarf Irregular Galaxy The Sagittarius Dwarf Irregular Galaxy (SagDIG) is a dwarf galaxy in the constellation of Sagittarius. It lies about 3.4 million light-years away. It was discovered by Cesarsky ''et al.'' on a photographic plate taken for the ESO&nbs ...

. The membership of NGC 3109, with its companions Sextans A and the

Antlia Dwarf The Antlia Dwarf is a dwarf spheroidal/irregular galaxy. It lies about 1.3 Mpc (4.3 million light-years) from Earth in the constellation Antlia. It is the fourth and faintest member of the nearby Antlia-Sextans Group of galaxies. The gal ...

Galaxy as well as Sextans B, Leo P, Antlia B and possibly

, is uncertain due to extreme distances from the center of the Local Group. The Antlia-Sextans Group is unlikely to be gravitationally bound to the Local Group due to probably lying outside the Local Group's

Zero-velocity surface A zero-velocity surface is a concept that relates to the N-body problem of gravity. It represents a surface a body of given energy cannot cross, since it would have zero velocity on the surface. It was first introduced by George William Hill. The z ...

—which would make it a true galaxy group of its own rather than a subgroup within the Local Group. This possible independence may, however, disappear as the Milky Way continues coalescing with Andromeda due to the increased mass, and density thereof, plausibly widening the radius of the zero-velocity surface of the Local Group.

History

The term "The Local Group" was introduced by

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects previously ...

in Chapter VI of his 1936 book ''The Realm of the Nebulae''.Alt URL
pp. 124–151) There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31,

, M33,

, M32, NGC 205, NGC 6822,

, IC 1613 and

. He also identified

as a possible part of the Local Group.

Component galaxies

Clickable map

Image:Local_Group.svg, frame, center, Local Group (clickable map) rect 392 53 405 72 ly circle 318 239 20

rect 345 256 375 284

circle 383 310 5 NGC 6822 rect 348 292 380 322 NGC 6822 circle 177 27 10 Sextans B rect 155 17 215 35 Sextans B circle 130 37 10 Sextans A rect 95 25 155 45 Sextans A circle 100 144 5 NGC 3109 rect 65 130 100 160 NGC 3109 circle 109 149 5

rect 109 146 145 175

circle 314 119 5

rect 280 97 340 125

circle 315 210 5

Leo I (dwarf galaxy) Leo I is a dwarf spheroidal galaxy in the constellation Leo. At about 820,000 light-years distant, it is a member of the Local Group of galaxies and is thought to be one of the most distant satellites of the Milky Way galaxy. It was discovered ...

rect 280 180 315 205

rect 320 180 335 220

Leo II (dwarf galaxy) Leo II (or Leo B) is a dwarf spheroidal galaxy about 690,000 light-years away in the constellation Leo (constellation), Leo. It is one of 24 known satellite galaxy, satellite galaxies of the Milky Way. Leo II is thought to have a core radius of ...

circle 349 213 5 Canes Dwarf rect 350 195 400 223 Canes Dwarf circle 310 350 15

rect 275 325 335 375

circle 265 415 15

rect 215 395 275 445

circle 392 445 5 Wolf-Lundmark-Melotte rect 360 435 392 455 Wolf-Lundmark-Melotte circle 370 410 5 Cetus Dwarf rect 345 395 390 430 Cetus Dwarf circle 395 373 10 IC 1613 rect 390 373 420 400 IC 1613 rect 381 335 393 357 SagDIG rect 430 450 480 475 SagDIG rect 393 335 406 356

rect 450 425 510 450

circle 412 332 12 LGS 3 rect 465 405 510 425 LGS 3 circle 460 361 21 Pegasus Dwarf rect 485 370 540 405 Pegasus Dwarf poly 440 282 455 260 511 259 493 285

rect 484 280 550 305

circle 417 304 2

rect 400 321 480 340

circle 417 254 15

rect 440 185 480 211

rect 432 237 447 260

rect 480 195 508 225

poly 450 264 434 265 431 280 442 280 M110 rect 508 205 540 225 M110 circle 461 229 17

rect 493 241 525 256

rect 524 305 550 323 M32 circle 394 272 14

Andromeda II Andromeda II (And II) is a dwarf spheroidal galaxy about 2.22 Mly away in the constellation Pisces. While part of the Local Group, it is not quite clear if it is a satellite of the Andromeda Galaxy or the Triangulum Galaxy. It was discovere ...

rect 500 345 515 360

rect 427 279 438 294

Andromeda III Andromeda III is a dwarf spheroidal galaxy about 2.44 million light-years away in the constellation Andromeda. It is part of the Local Group and is a satellite galaxy of the Andromeda Galaxy (M31). The galaxy was discovered by Sidney van den Be ...

rect 518 345 530 360

rect 438 282 450 294 Andromeda I rect 557 345 570 360 Andromeda I

List

Structure

Streams

* Magellanic Stream, a stream of gas being stripped off the Magellanic Clouds due to their interaction with the Milky Way * Monoceros Ring, a ring of stars around the Milky Way that is proposed to consist of a

stellar stream In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as ...

torn from the Canis Major Dwarf Galaxy * Virgo Strem, a steam formed from a dwarf galaxy. * Helni Stream

Future

The galaxies of the Local Group are likely to merge together under their own mutual gravitational attractions over a timescale of tens of billions of years into a single elliptical galaxy, with the coalescence of Andromeda and the Milky Way being the predominant event in this process.

Location

References

External links

* * * {{Portal bar, Stars, Outer space, Science Galaxy clusters Virgo Supercluster