Return to: The Galaxies of the Local Group.
The Local Group - Its Discovery, Size and Distribution in Space
The information presented here is for the most part extracted from:
Hartmut Frommert and Christine Kronberg, SEDS, The Local Group.
Membership List
Galaxy Clusters
Throughout the universe, galaxies occur in clusters. Some contain thousands
of members. Our Local Group, with about 40 gravitationally bound members, is a
more typical size. It contains three of the four main types of galaxies:
spiral, dwarf elliptical and dwarf irregular, but lacks an elliptical galaxy,
a type usually found in the inner regions of larger clusters.
Discovery
The Local Group was first recognized by Hubble in the 1920s when he made the
first distance determinations and redshift measurements. Its members include
the Milky Way Galaxy and the Large and the Small Magellanic Clouds which have
been known before the invention of the telescope, as well as smaller galaxies
discovered more recently. The membership of some galaxies to the group is
still being debated, and there are possible other candidate members.
Discoveries are still being made - the nearest member to the Milky Way, the
Sagittarius Dwarf Elliptical Galaxy, was not discovered until 1994.
The group is spread across a volume of nearly 10 million light years in
diameter, centered somewhere between the Milky Way and Andromeda (M 31).
It is flattened in shape, unlike the giant clusters which are spherical.
Sub-Groupings
The Milky Way and Andromeda (M 31) are by for the most massive, and therefore
dominant members. Each of these giant spirals has accumulated a system of
satellite galaxies:
- The Milky Way System
which we see from the inside
contains many dwarf galaxies, spread all over the sky: Sag DEG,
LMC, SMC, and the dwarf galaxies in Ursa Minor, Draco, Carina,
Sextans (dwarf), Sculptor, Fornax, Leo I and Leo II,
- The Andromeda System
which we see from outside, is
grouped around its main galaxy M31 in Andromeda> It also contains the bright and
nearby M32 and M110 galaxies, the fainter and more far-out NGC 147 and NGC 185, and
the very faint systems And I, And II, And III and possibly And IV, And V,
And VI (Pegasus dwarf), and And VII (Cassiopeia dwarf).
Outlying Members
The third-largest galaxy, the Triangulum spiral M 33, may or may not be
an outlying, gravitationally bound companion of M 31. LGS3 is probably
a satellite of M 33.
The other members cannot be assigned to one of the main subgroups, and float
quite alone in the gravitational field of the giant group members.
The substructures of the group are probably not stable. The galaxies around
the large elliptical Maffei 1 were probably once part of our galaxy group.
Neighbouring Groups
The Local Group is not isolated, but rather is interacting gravitionally
and exchanging members with the nearest surrounding groups, notably:
- the Maffei 1 Group
which besides the giant elliptical galaxy Maffei 1 also contains smaller
Maffei 2, and is associated with nearby IC 342. It is highly obscurred by
dark dust near the Milky Way's equatorial plane.
- the Sculptor or South Polar Group
whose members are scattered around the South Galactic pole and dominated
by the giant spiral NGC 253.
- the M 81 Group
- the M 83 Group
Information about each of these groups can be found from links given in the
source document:
Hartmut Frommert and Christine Kronberg, SEDS, The Local Group.