The Local Group
The Local Group, compromising more than 30 galaxies (including dwarf galaxies and the Milky Way), is a group of galaxies with a gravitational center located somewhere between the Milky Way and the Andromeda Galaxy.
With a binary (dumbbell) shape and a total mass of (1.29 Â± 0.14) Ã— 1012M the Local group covers a 10 million light-year diameter. The local group is part of the Virgo Supercluster. The two largest galaxies in the group are the Milky Way and the Andromeda both Spiral galaxies and both containing 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 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 And X.
The Triangulum Galaxy, which probably contains a Pisces Dwarf as a satellite, is the third largest and only other spiral galaxy within 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.
Edwin Hubble coined the term “The Local Group” in his book The Realm of the Nebulae (Hubble 1936, pp. 124″“151). He explains the the group in his book as “a typical small group of nebulae which is isolated in the general field.” In the same book he also identified IC 10 as a possible Local Group member. Through the discovery of almost two dozen low-luminosity galaxies the number of known local group members has gone from twelve to thirty-six as of 2003.