Local Group

The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 54 galaxies (including dwarf 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)^[1] shape. The group is estimated to have a total mass of (1.29 ± 0.14)×10¹²M_s.^[1] The group itself is part of the Virgo Supercluster (i.e. the Local Supercluster).^[2]

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

• The Milky Way's satellite system consists of Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major I Dwarf and Ursa Major II Dwarf.
• Andromeda's satellite system comprises M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph (aka And VI), Cassiopeia Dwarf (aka And VII), And VIII, And IX, And X, And XI, And XII, And XIII, And XIV, And XV, And XVI, And XVII, And XVIII, And XIX, and And XX.
• The Triangulum Galaxy, the third largest and only unbarred spiral galaxy in the Local Group, may or may not be a companion to the Andromeda galaxy but probably has Pisces Dwarf as a satellite.
• The membership of NGC 3109, and its companions Sextans A and the Antlia Dwarf, is uncertain due to their extreme distance from the center of the Local Group.

The other members of the group are gravitationally secluded from these large subgroups: IC10, IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.

History

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 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. 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.^[3]

Component galaxies

Map

Galactic bodies

Other notable objects

• Smith's Cloud - High-velocity cloud, between 32,000 and 49,000 light years from Earth,^[6] and 8000 light years from the disk of the Milky Way Galaxy.^[7]
• HVC 127-41-330 - High-velocity cloud, 2.3 million light-years from Earth
• Monoceros Ring - ring of stars around the Milky Way which is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy

Diagram

See also

• Groups and clusters of galaxies
• List of nearest galaxies
• List of galaxy clusters
• Maffei Group--the group of galaxies nearest to the Local Group
• Local Supercluster
• Andromeda's satellite galaxies
• Milky Way's satellite galaxies

References

External links

• The Local Group of Galaxies, SEDS Messier pages
• A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory
• van den Bergh, Sidney (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.