thumb|Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way
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 .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  .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form one lobe, and the Andromeda Galaxy 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, which may be a part of the Laniakea Supercluster.
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.
The two largest members, the Andromeda Galaxy and the Milky Way, 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 (also known as 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 galaxies 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 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 membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group.
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.
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
Map
Image:Local_Group.svg|frame|center|Local Group (clickable map)
rect 392 53 405 72 ly
circle 318 239 20    Milky Way rect 345 256 375 284 Milky Way
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     Antlia Dwarf rect 109 146 145 175 Antlia Dwarf
circle 314 119 5    Leo A rect 280 97 340 125 Leo A circle 315 210 5     Leo I (dwarf galaxy) rect 280 180 315 205 Leo I (dwarf galaxy) rect 320 180 335 220 Leo II (dwarf galaxy) circle 349 213 5     Canes Dwarf rect 350 195 400 223 Canes Dwarf
circle 310 350 15    Phoenix Dwarf rect 275 325 335 375 Phoenix Dwarf circle 265 415 15    Tucana Dwarf rect 215 395 275 445 Tucana Dwarf
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 Aquarius Dwarf rect 450 425 510 450 Aquarius Dwarf 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 Andromeda Galaxy rect 484 280 550 305                 Andromeda Galaxy
circle 417 304 2     Triangulum Galaxy rect 400 321 480 340 Triangulum Galaxy
circle 417 254 15    NGC 185 rect 440 185 480 211 NGC 185 rect 432 237 447 260 NGC 147 rect 480 195 508 225 NGC 147 poly 450 264 434 265 431 280 442 280 M110 rect 508 205 540 225                 M110 circle 461 229 17    IC 10 rect 493 241 525 256 IC 10
rect 524 305 550 323 M32
circle 394 272 14    Andromeda II rect 500 345 515 360 Andromeda II rect 427 279 438 294 Andromeda III rect 518 345 530 360 Andromeda III rect 438 282 450 294 Andromeda I rect 557 345 570 360 Andromeda I
desc bottom-left List of galactic bodies
Other objects
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 torn from the Canis Major Dwarf Galaxy
See also
Galaxy cluster
List of nearest galaxies
List of galaxy clusters
IC 342/Maffei Group, the group of galaxies nearest to the Local Group
Local Supercluster
List of Andromeda's satellite galaxies
List of Milky Way's satellite galaxies
Virgocentric flow
References
External links
