Local Group

The Local Group is the galaxy group that includes the Milky Way, where Earth 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" 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 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 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 (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

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

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 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

See also

* Galaxy cluster
* IC 342/Maffei Group, possibly the group of galaxies nearest to the Local Group
* Antlia-Sextans Group, another candidate for the group of galaxies nearest to the Local Group
* List of galaxy groups and clusters
* List of nearest galaxies
* Virgocentric flow

References

External links

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{{Portal bar, Stars, Outer space, Science

Galaxy clusters
Virgo Supercluster