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The Andromeda Galaxy
(also known as Messier Object 31, M31, or NGC 224; older texts often call
it the Andromeda Nebula) is a giant spiral galaxy in the Local Group,
together with the Milky Way galaxy. It is at a distance of approximately
2.9 million light years or 900 kpc, in the direction of the constellation
Andromeda.
With a mass of about 1.5 times more than the Milky Way, it is the dominant
galaxy of the Local Group, which consists of about 30 small galaxies plus
three large spirals: Andromeda, Milky Way and M33. (With improving
measurements and data, some scientists now believe that the Milky Way
contains more 'dark matter' and may be more massive than M31.)
General Information
Projections indicate that the Andromeda Galaxy is on a collision course
with the Milky Way, approaching at a speed of about 140 kilometres per
second. Impact is predicted in about 3 billion years; the two galaxies
will probably merge to form a giant elliptical. Due to the fact that it is
moving towards us, the Andromeda Galaxy is one of the few galaxies to
exhibit a blue shift.
This galaxy plays an important role in galactic studies, since it is the
nearest giant spiral. In 1943, Walter Baade was the first person to
resolve stars in the central region of the Andromeda Galaxy. Edwin Hubble
identified extragalactic cepheid variable stars for the first time on
astronomical photos of this galaxy, enabling its distance to be
determined.
Robin Barnard of the Open University has detected 10 X-ray sources in the
Andromeda Galaxy, published April 5th 2004, using observations from the
European Space Agency XMM-Newton orbiting observatory. He hypothesizes
that these are candidate black holes or neutron stars, which are heating
incoming gas to millions of kelvins and emitting X-rays. The spectrum of
the neutron stars is the same as the hypothesized black holes, but can be
distinguished by their masses.
In 1991 the Planetary Camera then onboard the Hubble Space Telescope
imaged Andromeda's core. To everyone's surprise its nucleus showed a
double structure, with two nuclear hot-spots located within a few light
years of each other. Subsequent ground-based observations have led to
speculation that indeed two nuclei exist, are moving with respect to each
other, that one nucleus is slowly tidally disrupting the other, and that
one nucleus may be the remnant of a smaller galaxy "eaten" by M31. The
nuclei of many galaxies, including M31, are known to be quite violent
places, and the existence of supermassive black holes are frequently
postulated to explain them.
Scott Chapman, from the California Institute of Technology, and Rodrigo
Ibata, from the Observatoire Astronomique de Strasbourg in France,
announced in 2005 their observations with the Keck telescopes which show
that the tenuous sprinkle of stars extending outward from the galaxy are
actually part of the main disk itself. This means that the spiral disk of
stars in Andromeda is three times larger in diameter than previously
estimated. This constitutes evidence that there is a vast, extended
stellar disk that makes the galaxy more than 220,000 light-years in
diameter. Previously, estimates of Andromeda's size ranged from 70,000 to
120,000 light-years across.
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Observation
The Andromeda Galaxy was observed in 964 by the Persian astronomer 'Abd
Al-Rahman Al Sufi, who described it as a "small cloud". The first
description of the object based on telescopic observation was given by
Simon Marius (1612), who is often wrongly credited as the discoverer of
the Andromeda Galaxy.
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Andromeda Galaxy
The Andromeda Galaxy is a spiral
galaxy similar to our own, though somewhat larger. It is the
farthest object that is visible with the naked eye. It can be seen
in the northern-sky constellation Andromeda. The Milky Way and
Andromeda galaxies are the dominant members of the Local Group of
galaxies, which in turn is an outlying part of the Virgo Cluster,
which comprises thousands of galaxies. |
In 1885, a supernova (known as "S Andromedae") was seen in the Andromeda
Galaxy, the first and so far only one observed in that galaxy.
The Andromeda Galaxy is easily visible to the naked eye in a truly dark
sky; however, such a truly dark sky is available only in relatively few,
isolated areas very far from population centers and sources of light
pollution. It appears quite small to the eye because only the central part
is bright enough to be visible, but the full angular diameter of the
galaxy is seven times that of the full moon.
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