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M31 is classified as a type Sb spiral galaxy, and it lies 770
kpc from the Milky Way (Sparke 2000). It is tilted 750
to the plane of the sky [figure
1A]. One of the first rectified pictures of the galaxy was
constructed at the Tuatenburg Observatory in Germany by Richter
and Weibrecht [figure 1B] (Sky &
Telescope 1964). The spiral structure of M31 has been a matter
of some contention due to its inclination to our line of sight.
Arp in 1964 described M31 as a two armed spiral with the arms
trailing. Hodge regards this as the best fit for the galaxy’s
characteristics (Hodge 1993; Arp 1964). According to Arp (1964),
“…the pitch [of M31] steepens somewhat to the inside and becomes
slightly shallower to the outside.” Braun (1991) used the
neutral hydrogen content of M31 to trace “…two continuous,
trailing spiral arms…” This currently seems to be the best model
for M31’s structure.
Examination of wide field infrared images of M31 taken by the
1.3-meter Two Micron All Sky Survey (2Mass) showed M31's central
bulge is not simply a flattened sphere. It has a boxy
shape and theoretical modeling of this data showed M31 has a bar
25,000 light years long similar to the bar of the Milky Way [figure
1C] (Beaton 2007).
The Local Group
M31 and the Milky Way are the dominant galaxies in a small
galaxy cluster known as the Local Group, a term first used by
Hubble in 1936 (Mateo 2000). The Local Group contains
approximately 40 known members, although more, small faint dwarf
galaxies probably remain to be discovered in the group. Another
prominent member of the Local Group is M33, which is a beautiful
Sc spiral galaxy, though it is considerably smaller and less
luminous than the Milky Way and M31. Most of the galaxies in the
Local Group are irregular galaxies, dwarf irregular galaxies,
dwarf elliptical galaxies, and dwarf spheroidal galaxies (Sparke
2000).
Most of the galaxies of the Local Group have low intrinsic
brightness and low surface brightness, making them difficult to
detect and study. A large number of low surface brightness
galaxies probably remain to be discovered throughout the
Universe and in the Local Group. The Milky Way hides some
members from discovery, and they will likely be discovered on
infrared and radio surveys (Mateo 2000). Moreover, the boundary
of the Local Group is uncertain, and it is sometimes difficult
to tell for a particular galaxy whether it is bound to the Local
Group.
The best way to associate galaxies with the Local Group is to
see if they are physically bound to the M31-Milky Way system,
because the mutual gravitational attraction of these two
galaxies is strong enough to overcome the expansion of the
Universe (Sparke 2000). In fact, M31 and the Milky Way are
approaching each other at >100 km/sec. They probably make up a
binary system and orbit around a common center of gravity (Mateo
2000). It is simple in theory to see if other galaxies are
physically bound to this binary system but difficult in practice
to determine this for small faint galaxies (Mateo 2000). It is
likely that some galaxies now listed as being members of the
Local Group will later be found to be unassociated with it and
passing through on their way somewhere else, while other
galaxies not now considered to be part of the Local Group will
be found to be members of it. |
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