| Local Group | |
| Image Scale: | 1.5 |
| Epoch: | J2000 |
| Parent: | Virgo Supercluster |
The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly 3Mpc,[1] and a total mass of the order of 2000000000000lk=inNaNlk=in.[2] 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 800kpc and are moving toward one another with a velocity of .[3] 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 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 Triangulum Galaxy (M33) is the third-largest member of the Local Group, with a mass of approximately 50000000000sigfig=1NaNsigfig=1, and is the third spiral galaxy.[6] It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart,[7] 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.[8]
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.[9]
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.[10] 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.
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.[11] 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.
rect 392 53 405 72 ly
circle 318 239 20 Milky Wayrect 345 256 375 284 Milky Way
circle 383 310 5 NGC 6822rect 348 292 380 322 NGC 6822
circle 177 27 10 Sextans Brect 155 17 215 35 Sextans Bcircle 130 37 10 Sextans Arect 95 25 155 45 Sextans A
circle 100 144 5 NGC 3109rect 65 130 100 160 NGC 3109circle 109 149 5 Antlia Dwarfrect 109 146 145 175 Antlia Dwarf
circle 314 119 5 Leo Arect 280 97 340 125 Leo Acircle 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 Dwarfrect 350 195 400 223 Canes Dwarf
circle 310 350 15 Phoenix Dwarfrect 275 325 335 375 Phoenix Dwarfcircle 265 415 15 Tucana Dwarfrect 215 395 275 445 Tucana Dwarf
circle 392 445 5 Wolf-Lundmark-Melotterect 360 435 392 455 Wolf-Lundmark-Melottecircle 370 410 5 Cetus Dwarfrect 345 395 390 430 Cetus Dwarfcircle 395 373 10 IC 1613rect 390 373 420 400 IC 1613rect 381 335 393 357 SagDIGrect 430 450 480 475 SagDIGrect 393 335 406 356 Aquarius Dwarfrect 450 425 510 450 Aquarius Dwarfcircle 412 332 12 LGS 3rect 465 405 510 425 LGS 3
circle 460 361 21 Pegasus Dwarfrect 485 370 540 405 Pegasus Dwarf
poly 440 282 455 260 511 259 493 285 Andromeda Galaxyrect 484 280 550 305 Andromeda Galaxy
circle 417 304 2 Triangulum Galaxyrect 400 321 480 340 Triangulum Galaxy
circle 417 254 15 NGC 185rect 440 185 480 211 NGC 185rect 432 237 447 260 NGC 147rect 480 195 508 225 NGC 147poly 450 264 434 265 431 280 442 280 M110rect 508 205 540 225 M110circle 461 229 17 IC 10rect 493 241 525 256 IC 10
rect 524 305 550 323 M32
circle 394 272 14 Andromeda IIrect 500 345 515 360 Andromeda IIrect 427 279 438 294 Andromeda IIIrect 518 345 530 360 Andromeda IIIrect 438 282 450 294 Andromeda Irect 557 345 570 360 Andromeda I
desc bottom-left
| + Properties of galactic bodies in and around the Local Group | ||||
| Name | Type | Constellation | Notes | |
|---|---|---|---|---|
| Spiral galaxies | ||||
| Andromeda Galaxy (M31, NGC 224) | SA(s)b | Andromeda | Largest galaxy in the group[12] Diameter (D25 isophote): 152,000 light-years Mass: 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.[13] Diameter (D25 isophote): 87,400 light-years Mass: Number of stars: . | |
| Triangulum Galaxy (M33, NGC 598) | SA(s)cd | Triangulum | Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy. Diameter (D25 isophote): 60,000 light-years Mass: Number of stars: . | |
| Magellanic spiral galaxies | ||||
| Large Magellanic Cloud (LMC) | Irr/SB(s)m | Dorado | Fourth largest member of the group, satellite of Milky Way and only Magellanic Spiral Galaxy in the local group Mass: Diameter (D25 isophote): 32,200 light-years | |
| NGC 3109 | SB(s)m | Hydra | Member of Antlia-Sextans Group | |
| Elliptical galaxies | ||||
| M32 (NGC 221) | E2 | 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 | KBm or Ir+ | Cassiopeia | ||
| Small Magellanic Cloud (SMC, NGC 292) | SB(s)m pec | Tucana | Satellite of Milky Way, 6th largest galaxy in the local groupmass: 7 × 109 | |
| Pisces Dwarf (LGS3) | Irr | Pisces | Possible satellite of the Triangulum Galaxy | |
| IC 1613 (UGC 668) | IAB(s)m V | Cetus | ||
| Phoenix Dwarf | Irr | Phoenix | ||
| Leo A (Leo III) | IBm V | Leo | ||
| Aquarius Dwarf (DDO 210) | IB(s)m | Aquarius | Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999.[14] | |
| SagDIG (Sagittarius Dwarf Irregular Galaxy) | IB(s)m V | Sagittarius | Most remote from barycenter member thought to be in the Local Group. | |
| NGC 6822 (Barnard's Galaxy) | IB(s)m IV-V | Sagittarius | ||
| Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216) | Irr | Pegasus | ||
| UGC 4879 (VV124) | IAm | Ursa Major | One of the most isolated galaxies in Local Group. Situated at the edge of the Local Group. | |
| Sextans A (UGCA 205) | Ir+V | Sextans | Member of Antlia-Sextans Group | |
| Sextans B (UGC 5373) | Ir+IV-V | Sextans | Member of Antlia-Sextans Group | |
| Leo P | Irr | Leo | Member of Antlia-Sextans Group, extraordinarily low metallicity (Z = 0.03ZMW) | |
| AGC 198606 | Irr? | Leo | Gas-rich ultra-faint dwarf galaxy | |
| AGC 215417 | Irr? | Leo | Gas-rich ultra-faint dwarf galaxy | |
| AGC 219656 | Irr? | Leo | Gas-rich ultra-faint dwarf galaxy | |
| AGC 249525 | Irr? | Boötes | Gas-rich ultra-faint dwarf galaxy, Situated at the edge of the Local Group | |
| AGC 268069 | Irr? | Serpens | Gas-rich ultra-faint dwarf galaxy | |
| Dwarf elliptical galaxies | ||||
| M110 (NGC 205) | dE6p | Andromeda | Satellite of the Andromeda Galaxy and 5th 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 | dSph | Boötes | ||
| Cetus Dwarf | dSph/E4 | Cetus | ||
| Canes Venatici I Dwarf and Canes Venatici II Dwarf | dSph | Canes Venatici | Satellites of the Milky Way[15] [16] | |
| 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 | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda II | 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 | dE3 | Sextans | Satellite of Milky Way | |
| Leo II (Leo B) | dE0 pec | Leo | Satellite of Milky Way | |
| Ursa Minor Dwarf | 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 | Tucana | ||
| Cassiopeia Dwarf (Andromeda VII) | dSph | Cassiopeia | Satellite of the Andromeda Galaxy | |
| Pegasus Dwarf Spheroidal Galaxy (Andromeda VI) | dSph | Pegasus | Satellite of the Andromeda Galaxy | |
| Ursa Major I Dwarf and Ursa Major II Dwarf | dSph | Ursa Major | Satellite of Milky Way | |
| Leo IV | dSph | Leo | Satellite of the Milky Way | |
| 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 | |
| dSph | Boötes | Satellite of the Milky Way | ||
| Boötes IV | dSph | Boötes | Satellite of the Milky Way | |
| dSph | Coma Berenices | Satellite of the Milky Way | ||
| dSph | Aries | Satellite of the Milky Way | ||
| dSph | Hercules | Satellite of the Milky Way | ||
| dSph | Pisces | Satellite of the Milky Way | ||
| dSph | Reticulum | Satellite of the Milky Way | ||
| Reticulum III | dSph | Reticulum | Satellite of the Milky Way | |
| dSph | Eridanus | Probable satellite of the Milky Way | ||
| Grus I | dSph | Grus | Satellite of the Milky Way | |
| Grus II | dSph | Grus | Satellite of the Milky Way | |
| Tucana II | dSph | Tucana | Satellite of the Milky Way | |
| Hydrus I | dSph | Hydrus | Satellite of the Milky Way | |
| Draco II | dSph | Draco | Satellite of the Milky Way | |
| Carina III | dSph | Carina | Satellite of the Milky Way | |
| Triangulum II (Laevens 2) | dSph | Triangulum | Satellite of the Milky Way | |
| Carina II | dSph | Carina | Satellite of the Milky Way | |
| Pictor II | dSph | Pictor | Satellite of the Milky Way | |
| Horologium II | dSph | Horologium | Satellite of the Milky Way | |
| dSph | Virgo | Satellite of the Milky Way | ||
| Aquarius II | dSph | Aquarius | Satellite of the Milky Way | |
| dSph | Crater | Satellite of the Milky Way | ||
| Hydra II | dSph | Hydra | Satellite of the Milky Way | |
| dSph | Antlia | Satellite of the Milky Way | ||
| Pegasus III | dSph | Pegasus | Satellite of the Milky Way | |
| Pegasus IV | dSph | Pegasus | Satellite of the Milky Way[17] | |
| Pegasus W | dSph | Pegasus | Recent star formation, could still be starforming | |
| Cetus III | dSph | Cetus | Satellite of the Milky Way | |
| Tucana B | dSph | Tucana | [18] | |
| Perseus I Dwarf Galaxy (Andromeda XXXIII) | dE | Perseus | Satellite of Andromeda | |
| Antlia Dwarf | dE3/dSph/Irr? | Antlia | Member of Antlia-Sextans Group | |
| Andromeda IX | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda X | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XI | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XII | dSph | Andromeda | Possible satellite of the Andromeda Galaxy | |
| Andromeda XIII (Pisces III) | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XIV (Pisces IV) | dSph | Pisces | Possible satellite of the Andromeda Galaxy | |
| Andromeda XV | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XVII | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XIX | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XX | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXI | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXII | dSph | Pisces | Possible satellite of the Triangulum Galaxy | |
| Andromeda XXIII | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXIV | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXV | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXVI | dSph | Andromeda | Satellite of the Andromeda Galaxy | |
| Andromeda XXVII | dSph | Andromeda | Satellite of the Andromeda Galaxy, tidally disrupted[19] | |
| Andromeda XXIX | dSph | Pegasus | Satellite of the Andromeda Galaxy | |
| Andromeda XXX (Cassiopeia II) | dSph? | Cassiopeia | Satellite of the Andromeda Galaxy | |
| Andromeda XXXI (Lacerta I) | dSph? | Lacerta | Satellite of the Andromeda Galaxy | |
| Andromeda XXXII (Cassiopeia III) | dSph? | Cassiopeia | Satellite of the Andromeda Galaxy | |
| Andromeda XXXIV (Pegasus V) | dSph | Pegasus | Satellite of the Andromeda Galaxy[20] | |
| Andromeda XVI (Pisces V) | dSph | Pisces | Possible satellite of the Andromeda Galaxy | |
| Andromeda XXVIII | dSph? | Pegasus | Possible satellite of the Andromeda Galaxy | |
| Andromeda XXXIII (Perseus I) | dSph? | Perseus | Possible satellite of the Andromeda Galaxy | |
| Andromeda XVIII | dSph | Andromeda | ||
| Centaurus I | dSph | Centaurus | Satellite of the Milky Way[21] | |
| Pisces VII (Triangulum III) | dSph? | Pisces | Candidate, possible satellite of the Triangulum Galaxy[22] | |
| Identification unclear | ||||
| Virgo Stellar Stream | dSph (remnant)? | Virgo | In the process of merging with the Milky Way | |
| Canis Major Dwarf | Irr? | Canis Major | Possibly a dwarf galaxy in the process of merging with the Milky Way | |
| Hydra 1 | Hydra | Possibly a dwarf galaxy in the process of merging with the Milky Way | ||
| Tucana III | dSph or cluster? | Tucana | Satellite of the Milky Way, tidally disrupting | |
| Tucana IV | dSph or cluster? | Tucana | Satellite of the Milky Way | |
| Tucana V | dSph or cluster? | Tucana | Possibly non-existent | |
| Columba I | dSph or cluster? | Columba | Satellite of the Milky Way | |
| dSph or Globular Cluster | Leo | Satellite of the Milky Way | ||
| Cetus II | Cetus | Likely part of Sagittarius tidal stream | ||
| Willman 1 | dSph or Globular Cluster | Ursa Major | 147,000 light-years away | |
| Horologium I | dSph or Globular Cluster | Horologium | Satellite of the Milky Way. Not to be confused with the Horologium Supercluster. | |
| Pictoris | dSph or Globular Cluster | Pictor | Satellite of the Milky Way | |
| Phoenix II | dSph or Globular Cluster | Phoenix | Satellite of the Milky Way | |
| Indus I (Kim 2) | dSph or Globular Cluster | Indus | Satellite of the Milky Way | |
| Eridanus III | dSph or Globular Cluster | Eridanus | Satellite of the Milky Way or SMC[23] | |
| Sagittarius II | dSph or Globular Cluster | Sagittarius | Satellite of the Milky Way | |
| dSph? | Andromeda | Satellite of the Andromeda Galaxy, tidally disrupting | ||
| Antlia B | Antlia | Member of Antlia-Sextans Group | ||
| Probable non-members | ||||
| Andromeda IV | Irr | Andromeda | Once considered to be associated with M31. Its distance is now known to be 22 to 24 million light years (not close to the Andromeda Galaxy at all).[24] | |
| GR 8 (DDO 155) | Im V | Virgo | Distance 7.9 million light years[25] | |
| IAB(s)m IV | Indus | Distance 5.8 million light years, possibly an outlying member of the local group[26] | ||
| SA(s)d | Sculptor | Distance 6.07 million light years | ||
| SB(s)m | Sculptor | Distance 6.5 million light years | ||
| NGC 404 | E0 or SA(s)0− | Andromeda | Distance 10 million light years[27] | |
| NGC 1569 | Irp+ III-IV | Camelopardalis | In IC 342 group of galaxies. Distance 11 million light years[28] | |
| NGC 1560 (IC 2062) | Sd | Camelopardalis | Distance 8-12 million light years | |
| Camelopardalis A | Irr | Camelopardalis | Distance 12 million light years[29] | |
| Argo Dwarf | Irr | Carina | ||
| ESO 347-8 (2318–42) | Irr | Grus | ||
| UKS 2323-326 (ESO 407-18) | Irr | Sculptor | Distance 7.2 million light-years | |
| UGC 9128 (DDO 187) | Irp+ | Boötes | ||
| KKs 3 | dSph | Hydrus | ||
| Objects in the Local Group no longer recognised as galaxies | ||||
| Palomar 12 (Capricornus Dwarf) | Capricornus | Globular cluster formerly classified as a dwarf spheroidal galaxy | ||
| Palomar 4 (Ursa Major Dwarf) | Ursa Major | Globular cluster formerly classified as a dwarf spheroidal galaxy | ||
| Palomar 5 (Serpens Dwarf) | Serpens | Globular cluster formerly classified as a dwarf spheroidal galaxy | ||
| Palomar 3 (Sextans C) | Sculptor | Globular cluster formerly classified as a dwarf spheroidal galaxy[30] | ||
| Segue 3 | Pegasus | Globular cluster formerly classified as a dwarf spheroidal galaxy[31] | ||
| Laevens 1 (Crater Dwarf) | Crater | Globular cluster formerly classified as a dwarf spheroidal galaxy[32] | ||
| DES J2038-4609 (Indus II) | Indus | Likely a chance alignment of stars[33] | ||
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 Messier 31 and the Milky Way being the predominant event in this process.[34]
See main article: Laniakea Supercluster.