Local Group

From Wikipedia, the free encyclopedia
Jump to: navigation, search
A member of the Local Group of galaxies, irregular galaxy Sextans A is 4.3 million light-years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with young blue star clusters clearly visible.
Distribution of the iron content (in logarithmic scale) in four dwarf neighbouring galaxies of the Milky Way.

The Local Group is the group of galaxies that includes the Milky Way among others. It comprises more than 54 galaxies, counting dwarf galaxies. Its gravitational center is located somewhere between Milky Way and Andromeda Galaxy. The Local Group covers a diameter of 10 megalight-years (3.1 megaparsecs) (see 1 E+23 m for distance comparisons) and has a binary (dumbbell)[1] distribution. The group is estimated to have a total mass of 1.29±0.14 ×1012 M[1] and has a velocity dispersion of 61±8 km/s.[2] The group itself is part of the Virgo Supercluster (i.e. the Local Supercluster).[3]

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


The term, "The Local Group" was introduced by Edwin Hubble in Chapter VI of his book The Realm of the Nebulae (Hubble 1936, pp. 124–151). There, he described it as "a typical small group of nebulae which is isolated in the general field." He 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 Local Group member. By 2003, the number of known Local Group members has increased from his initial twelve to thirty-six by way of the discovery of almost two dozen low-luminosity galaxies.[5]

Component galaxies[edit]


Sextans B Sextans A Milky Way Leo I (dwarf galaxy) Canes Dwarf Leo II (dwarf galaxy) NGC 6822 Phoenix Dwarf Tucana Dwarf Wolf-Lundmark-Melotte Cetus Dwarf IC 1613 SagDIG Aquarius Dwarf Triangulum Galaxy NGC 185 NGC 147 IC 10 Andromeda Galaxy M110 Leo A NGC 3109 Antlia Dwarf LGS 3 Pegasus Dwarf Andromeda II Andromeda III Andromeda I
About this image
Local Group (clickable map)

Galactic bodies[edit]

Spiral galaxies
name type constellation notes
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.
Milky Way SBbc Sagittarius (centre) Second largest, though possibly most massive galaxy in the group.[6]
Triangulum Galaxy (M33, NGC 598) SA(s)cd Triangulum Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy.
Elliptical galaxies
name type constellation notes
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 Fourth largest member of the group, 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
M110 (NGC 205) dE6p Andromeda satellite of the Andromeda Galaxy
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.[7]
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 I Dwarf and Canes Venatici II 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 (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
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
UGCA 86 (0355+66) Irr, dE or S0 Camelopardalis
UGCA 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
ESO 347-8 (2318–42) Irr Grus
UKS 2323-326 Irr Sculptor
UGC 9128 (DDO 187) Irp+ Boötes
Sextans C
objects in local group no longer recognised as galaxies
name type constellation notes
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

Other notable objects[edit]


A diagram of our location in the observable Universe. (Click here for smaller image.)

See also[edit]


  1. ^ a b Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. 
  2. ^ van den Bergh, Sidney (1999). "The local group of galaxies". The Astronomy and Astrophysics Review (Springer) 9 (3-4): 273–318 (1999). Bibcode:1999A&ARv...9..273V. doi:10.1007/s001590050019. Retrieved March 11, 2012. 
  3. ^ R. B. Tully (1982). "The Local Supercluster". Astrophysical Journal 257: 389–422. Bibcode:1982ApJ...257..389T. doi:10.1086/159999. 
  4. ^ Miller, Bryan W. et al. (December 2001), "The Star Formation History of LGS 3", The Astrophysical Journal 562 (2): 713–726, arXiv:astro-ph/0108408, Bibcode:2001ApJ...562..713M, doi:10.1086/323853 
  5. ^ van den Bergh, Sidney (May 2003). "History of the Local Group". To be published in: "The Local Group as an Astrophysical Laboratory" (Cambridge University Press): 5042. arXiv:astro-ph/0305042. Bibcode:2003astro.ph..5042V. 
  6. ^ "Milky Way 'bigger than thought'". BBC News. 2009-01-06. 
  7. ^ van den Bergh, Sidney (April 2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. 
  8. ^ Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, J. C.; Richter, P.; Beers, T. C.; Ivezić, Ž.; Howk, J. C. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal 672 (1): 298–319. arXiv:0709.1926. Bibcode:2008ApJ...672..298W. doi:10.1086/523845. 
  9. ^ "Massive Gas Cloud Speeding Toward Collision With Milky Way". Retrieved 2008-06-06. 

External links[edit]

Media related to Local Group at Wikimedia Commons