The Andromeda Galaxy (also known as Messier Object 31, M31, or NGC 224) 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.

This galaxy plays an important role in galactic studies, since it is the nearest giant spiral. In 1943, Walter Bade 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 remains of 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.

Observation

The Andromeda Galaxy was observed in 964 by the Arab 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.

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.

Other resources

See also

• G1 (astronomy) - the largest globular cluster in the Local Group and part of M31

Related topics

• List of messier objects
• New General Catalogue
• Wikipedia Project: Astronomical Objects

External links

• Messier 31, SEDS Messier pages
• Astronomy Picture of the Day July 18, 2004
• Astronomy Picture of the Day October 17, 1998