Andromeda Galaxy

**Andromeda Galaxy**
Observation data ( Epoch J2000.0)
Galaxy	List of galaxies
The Andromeda Galaxy, with M32 (left) and M110 (right)
Type	SA(s)ab
Right ascension	00^h 42.8^m
Declination	+41° 16′
Red shift	?
Distance	2.52 million ly
Apparent magnitude (V)	+3.4
Apparent dimensions (V)	3.2 × 1.0 °
Constellation	Andromeda
Physical characteristics
Radius	110,000 ly
Absolute magnitude (V)	-21.4
Notable features	None
Other designations	M31, NGC 224

The Andromeda Galaxy (also known as 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 2.5 million light years or 770 kpc, in the direction of the constellation Andromeda.

Andromeda was believed to be the dominant galaxy of the Local Group, which consists of about 30 small galaxies plus three large spirals: Andromeda, Milky Way and M33. Due to recent findings based on improved measurements and data, scientists believe that the Milky Way contains more dark matter and may be the most massive in the grouping.

General information

This image is a Galaxy Evolution Explorer observation of the large galaxy in Andromeda, Messier 31.

The Andromeda Galaxy is approaching the Milky Way at about 300 kilometres per second, so it is one of the few blue shifted galaxies. Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometres per second. However, this does not mean it will collide with the Milky Way, since the galaxy's tangential velocity is unknown. If it is on a collision course, the impact is predicted to occur in about 3 billion years. In that case the two galaxies will merge to form a giant elliptical galaxy. Such events are frequent among the galaxies in galaxy groups.

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.

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is in fact another, dimmer type of Cepheid. In 1990s, Hipparcos satellite measurements were used to calibrate the Cepheid distances. The corrected value give the Andromeda Galaxy a distance of 2.9 million light years. Unfortunately, all Cepheids lie further than Hipparcos could measure accurately, and it became clear that Hipparcos-calibrated values for Cepheids were not reliable.

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 is 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.

Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/ JPL- Caltech/K. Gordon ( University of Arizona)

In 2005, a group of astronomers consisting of Ignasi Ribas ( CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236, has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 million light years and the whole Andromeda Galaxy at about 2.5 million light years. This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

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.

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. At the time, it was thought to be much less luminous and unrelated event called nova, and was named accordingly the Nova 1885.

The Andromeda Galaxy is easily visible to the naked eye in a moderately dark sky; however, such a sky is available only in smaller towns and isolated areas reasonably 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.

Satellites

Plane of Andromeda's satellites.

Like our Milky Way, Andromeda has satellite galaxies. The best known ones are M32 & M110.

**Andromeda's satellites**
Name	Type	Distance from Sun (million ly)	Magnitude	Year discovered
M32	dE2	2.9	+9.2	1749
M110	dE6	2.9	+9.4	1773

Other resources

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

External articles