The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellite galaxies form one lobe, and the Andromeda Galaxy and its satellite galaxies constitute the other. The two collections are separated by about and are moving toward one another with a velocity of . The center of the group is located at about away from the Milky Way, placing it slightly closer to the Andromeda Galaxy by roughly , in which the latter may be more massive than the former in terms of mass.
The Local Group has a total mass of the order of ,The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy. Estimates for the mass of each galaxy are compatible with , and Peñarrubia et al. (2014) estimate for the Local Group, but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of . and also a total diameter of based on density matching and the potential surface of its parent structure, Local Sheet. It is itself a part of the Local Volume and the larger Virgo Supercluster, which is a part of the even greater Laniakea Supercluster along with the Pisces–Cetus Supercluster Complex. The exact number of galaxies in the Local Group is unknown, as the Milky Way obscures some; however, a current total of 134 members is known within 1 megaparsec from the center, most of which are dwarf galaxies. The Local Group was thought to have been more spread in the early universe with by 700 million years after the Big Bang.
The two largest members, the Andromeda and the Milky Way galaxies, are both spiral galaxies with masses of about solar masses each. Each has its own system of satellite galaxies: The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And V, And VI (also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph), And VII (a.k.a. the Cassiopeia Dwarf Galaxy), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies. The Milky Way's satellite system comprises the Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy (disputed, considered by some not a galaxy), Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy), Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.
The Triangulum Galaxy (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 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 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, and Sagittarius Dwarf Irregular Galaxy.
The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy as well as Sextans B, Leo P, Antlia B and possibly Leo A, 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—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 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, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible part of the Local Group.
Component galaxies
Clickable map
List + Properties of galactic bodies in and around the Local Group Name Type Constellation Notes Spiral galaxies Andromeda Galaxy (M31, NGC 224) SAb AndromedaLargest galaxy in the groupDiameter (D25 isophote): 152,000 light-yearsMass: Number of stars: ca. . Milky Way Galaxy SBbc Sagittarius (centre)Second largest galaxy in the group, which may or may not be the most massive galaxy of the group.Diameter (D25 isophote): 87,400 light-yearsMass: Number of stars: . Triangulum Galaxy (M33, NGC 598) SAcd Triangulum Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy.Diameter (D25 isophote): 61,100 light-yearsMass: Number of stars: . Magellanic spiral galaxies Large Magellanic Cloud (LMC, Dorado Dwarf) Irr/SB(s)m Dorado Fourth largest known member of the group, satellite of Milky Way and only confirmed Magellanic Spiral Galaxy in the local groupMass: Diameter (D25 isophote): 32,200 light-years Elliptical galaxies M32 (NGC 221, Andromeda Dwarf) cE2 Andromeda Satellite of the Andromeda Galaxy, shows signs of a supermassive black hole Irregular galaxies Wolf–Lundmark–Melotte (WLM, DDO 221) Ir+ Cetus Possible size between Small Magellanic Cloud and Large Magellanic Cloud IC 10 (PGC 1305) KBm or Ir+ Cassiopeia Only known starbust galaxy in the Local Group Small Magellanic Cloud (SMC, NGC 292, Tucana galaxy) SB(s)m pec Tucana Satellite of Milky Way, 5th largest known galaxy in the local groupMass: Diameter (D25 isophote): 18,900 light-years Pisces Dwarf (LGS3,Pisces I dwarf irregular) Irr Pisces Possible satellite of the Triangulum Galaxy IC 1613 (UGC 668, UGCA 47954) IAB(s)m V Cetus Phoenix Dwarf (Phoenix I dwarf) Irr Phoenix Leo A (Leo III, PCA QA JO74974) IBm V Leo Aquarius Dwarf (DDO 210, Aquarius I, ESO 497-4054.4591 G) IB(s)m Aquarius Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999. SagDIG (Sagittarius Dwarf Irregular Galaxy, Sagittarius II) IB(s)m V Sagittarius Most remote from barycenter member thought to be in the Local Group. NGC 6822 (Barnard's Galaxy, AO 4797) IB(s)m IV-V Sagittarius Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216) Irr PegasusUGC 4879 (VV124)IAmUrsa MajorOne of the most isolated galaxies in Local Group. Situated at the edge of the Local Group.Sextans A (UGCA 205, AO 4977)Ir+VSextansMember of Antlia-Sextans GroupSextans B (UGC 5373, QRT 947748)Ir+IV-VSextansMember of Antlia-Sextans GroupLeo P (AGC 19470059)IrrLeoMember of Antlia-Sextans Group, extraordinarily low metallicity (Z = 0.03ZMW)AGC 198606 (Leo VI)Irr?LeoGas-rich ultra-faint dwarf galaxyAGC 215417(Leo VII)Irr?LeoGas-rich ultra-faint dwarf galaxyAGC 219656Irr?LeoGas-rich ultra-faint dwarf galaxyAGC 249525Irr?BoötesGas-rich ultra-faint dwarf galaxy, Situated at the edge of the Local GroupAGC 268069Irr?SerpensGas-rich ultra-faint dwarf galaxy Dwarf elliptical galaxies M110 (NGC 205) dE6p Andromeda Satellite of the Andromeda Galaxy and the 6th largest galaxy with the mass of 9.3 billion solar masses. NGC 147 (DDO 3) dE5 pec Cassiopeia Satellite of the Andromeda Galaxy Dwarf spheroidal galaxies Boötes I (DDO 9774998.074÷×47) dSph Boötessatellite of the Milky Way Cetus Dwarf dSph/E4 Cetus3.4 million light-years away size:999 light-years Canes Venatici I Dwarf and Canes Venatici II Dwarf dSph Canes Venatici Satellites of the Milky Way Andromeda III dE2 Andromeda Satellite of the Andromeda Galaxy NGC 185 dE3 pec Cassiopeia Satellite of the Andromeda Galaxy Andromeda I dE3 pec Andromeda Satellite of the Andromeda Galaxy Sculptor Dwarf (E351-G30) dE3 Sculptor Satellite of Milky Way Andromeda V (UKS 4977-94) dSph Andromeda Satellite of the Andromeda Galaxy Andromeda II (NCA 14) dE0 Andromeda Satellite of the Andromeda Galaxy Fornax Dwarf (E356-G04) dSph/E2 Fornax Satellite of Milky Way Carina Dwarf (E206-G220) dE3 Carina Satellite of Milky Way Leo I (DDO 74) dE3 Leo Satellite of Milky Way Sextans Dwarf (Sextans 1) dE3 Sextans Satellite of Milky Way Leo II (Leo B) dE0 pec Leo Satellite of Milky Way Ursa Minor Dwarf (NCA 47-49) dE4 Ursa Minor Satellite of Milky Way Draco Dwarf (DDO 208) dE0 pec Draco Satellite of Milky Way SagDSG (Sagittarius Dwarf Spheroidal Galaxy) dSph/E7 Sagittarius Satellite of Milky Way Tucana Dwarf dE5 Tucana3.2 million ly away Cassiopeia Dwarf (Andromeda VII, NCA 4) dSph Cassiopeia Satellite of the Andromeda Galaxy Pegasus Dwarf Spheroidal Galaxy (Andromeda VI) dSph Pegasus Satellite of the Andromeda Galaxy Ursa Major I Dwarf dSph Ursa Major Satellite of the Milky Way Ursa Major II Dwarf dSph Ursa Major Satellite of the Milky Way Ursa Major III dSph Ursa Major Satellite of the Milky Way Leo IV dSph Leo Satellite of the Milky Way Leo V dSph Leo Satellite of the Milky Way Leo T dSph/Irr Leo Satellite of the Milky Way Boötes II dSph Boötes Satellite of the Milky Way Boötes III (NCA 4.9) dSph Boötes Satellite of the Milky Way, Bootes lll likely formed both the Bootes Ill Stream & the Monosoros Stellar stream Boötes IV dSph Boötes Satellite of the Milky Way Coma Berenices dSph Coma Berenices Satellite of the Milky Way Segue 2 (Aries Dwarf, NCA 429) dSph Aries Satellite of the Milky Way size:220 light-years