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 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 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 , 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 PiscesâÂÂCetus 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 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 2âÂÂ4 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, WolfâÂÂLundmarkâÂÂMelotte, 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 surfaceâÂÂwhich 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.
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. 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.
<imagemap> Image:Local_Group.svg|frame|center|Map of the local universe within 5 million light-years from Earth, including the Local Group and other nearby galaxies (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 </imagemap>
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. There is debate over whether ellipticity might be the immediate structure of the combined galaxy right after the collision or whether ellipticity might only emerge after a theoretical intermediate period of retaining a spiraling structure directly following the collision. Some even theorize a permanent superspiral or a transition toward a more lenticular galaxy, rather than a more elliptical or spiraled distribution, as the future of the Local Group's galactic merger.