Definitions

Local_group

Local Group

The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 35 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The galaxies of the Local Group cover a 10 million light-year diameter (see 1 E+22 m for distance comparisons) and have a binary (dumbbell) shape. The group is estimated to have a total mass of (1.29 ± 0.14)M. The group itself is one of many within the Virgo Supercluster (i.e. the Local Supercluster).

The two most massive members of the group are the Milky Way and the Andromeda Galaxy. These two barred spirals each have a system of satellite galaxies.

The other members of the group are gravitationally secluded from these large subgroups: IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.

History

Edwin Hubble was the first to identify the Local Group in Chapter VI of his book The Realm of Nebulae (Hubble 1936, pp. 124–151). There he describes it as "a typical small group of nebulae which is isolated in the general field." In that book, he delineated by decreasing luminosity its members to be M31, the Milky Way, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible Local Group member. In the ~70 years since his work, the number of known Local Group members has increased from his initial twelve to thirty-six as of 2003, by way of the discovery of almost two dozen low-luminosity galaxies.

Component galaxies

Clickable maps

Image:Local_Group.JPG|frame|center|Local group (clickable map)

circle 167 27 20 Sextans B circle 120 36 23 Sextans A circle 318 239 20 Milky Way circle 289 197 16 Leo I (dwarf galaxy) circle 334 201 15 Canes Dwarf rect 303 185 318 215 Leo II (dwarf galaxy) circle 357 289 28 NGC 6822 circle 288 323 24 Phoenix Dwarf circle 248 391 35 Tucana Dwarf circle 363 416 20 Wolf-Lundmark-Melotte circle 363 383 17 Cetus Dwarf circle 369 346 11 IC 1613 rect 381 335 393 357 SagDIG rect 393 335 406 356 Aquarius Dwarf circle 417 304 17 Triangulum Galaxy circle 417 254 15 NGC 185 rect 432 237 447 260 NGC 147 circle 461 229 17 IC 10 poly 440 282 455 260 511 259 493 285 Andromeda Galaxy poly 450 264 434 265 431 280 442 280 Messier 110 circle 295 110 20 Leo A circle 84 128 20 NGC 3109 circle 109 149 14 Antlia Dwarf circle 412 332 12 LGS 3 circle 460 361 21 Pegasus Dwarf circle 394 272 14 Andromeda II rect 427 279 438 294 Andromeda III rect 438 282 450 294 Andromeda I

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Image:Satellite_Galaxies.JPG|frame|center|Milky Way's satellite galaxies (clickable map)

rect 289 219 352 251 Milky Way rect 319 252 380 281 Sagittarius Dwarf Elliptical Galaxy rect 187 81 229 113 Sextans Dwarf rect 168 273 249 318 Large Magellanic Cloud rect 229 326 288 368 Small Magellanic Cloud rect 297 376 352 407 Sculptor Dwarf rect 183 446 234 476 Fornax Dwarf rect 107 297 150 332 Carina Dwarf rect 296 107 339 144 Bootes Dwarf rect 336 180 408 196 Ursa Major II rect 357 40 423 59 Ursa Major I rect 370 110 428 142 Ursa Minor Dwarf rect 430 119 470 154 Drwaco Dwarf

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Galactic bodies

Spiral galaxies
name type constellation notes
Milky Way SBbc n/a Second largest, though possibly most massive galaxy in the group.
Andromeda Galaxy (M31, NGC 224) SA(s)b Andromeda Largest member of the group, recently (2006) discovered to also be a barred spiral. May be less massive than the Milky Way.
Triangulum Galaxy (M33, NGC 598) SAc Triangulum only ordinary spiral galaxy and possible satellite of the Andromeda Galaxy?
Elliptical galaxies
name type constellation notes
M110 (NGC 205) E6p Andromeda satellite of the Andromeda Galaxy
M32 (NGC 221) E2 Andromeda satellite of the Andromeda Galaxy
Irregular galaxies
name type constellation notes
Wolf-Lundmark-Melotte (WLM, DDO 221) Ir+ Cetus
IC 10 KBm or Ir+ Cassiopeia
Small Magellanic Cloud (SMC, NGC 292) SB(s)m pec Tucana satellite of Milky Way
Canis Major Dwarf Irr Canis Major satellite of Milky Way
Pisces Dwarf (LGS3) Irr Pisces satellite of the Triangulum Galaxy?
IC 1613 (UGC 668) IAB(s)m V Cetus
Phoenix Dwarf Irr Phoenix
Large Magellanic Cloud (LMC) Irr/SB(s)m Dorado satellite of Milky Way
Leo A (Leo III) IBm V Leo
Sextans B (UGC 5373) Ir+IV-V Sextans
NGC 3109 Ir+IV-V Hydra
Sextans A (UGCA 205) Ir+V Sextans
Dwarf elliptical galaxies
name type constellation notes
NGC 147 (DDO 3) dE5 pec Cassiopeia satellite of the Andromeda Galaxy
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
Dwarf spheroidal galaxies
name type constellation notes
Boötes Dwarf dSph Boötes
Cetus Dwarf dSph/E4 Cetus
Canes Venatici Dwarf dSph Canes Venatici
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
Antlia Dwarf dE3 Antlia
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
SagDEG (Sagittarius Dwarf Elliptical 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 (And VI) dSph Pegasus satellite of the Andromeda Galaxy
Ursa Major Dwarf dSph Ursa Major satellite of Milky Way
Identification Unclear
name type constellation notes
Virgo Stellar Stream dSph (remnant)? Virgo In the process of merging with the Milky Way
Willman 1 dwarf Spherical galaxy or Globular cluster? Ursa Major 147,000 light-years away
Andromeda IV Irr? Andromeda probably not a galaxy
UGC-A 86 (0355+66) Irr, dE or S0 Camelopardalis
UGC-A 92 (EGB0427+63) Irr or S0 Camelopardalis
Probable non-members
name type constellation notes
GR 8 (DDO 155) Im V Virgo
IC 5152 IAB(s)m IV Indus
NGC 55 SB(s)m Sculptor
Aquarius Dwarf (DDO 210) Im V Aquarius
NGC 404 E0 or SA(s)0- Andromeda
NGC 1569 Irp+ III-IV Camelopardalis
NGC 1560 (IC 2062) Sd Camelopardalis
Camelopardalis A Irr Camelopardalis
Argo Dwarf Irr Carina
2318-42 Irr Grus
UKS 2323-326 Irr Sculptor
UGC 9128 (DDO 187) Irp+ Boötes
Palomar 12 (Capricornus Dwarf) Capricornus a globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 4 (originally designated Ursa Major Dwarf) Ursa Major a globular cluster formerly classified as a dwarf spheroidal galaxy
Sextans C

Other notable objects

See also

References

External links

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