Andromeda Galaxy

Andromeda is a spiral galaxy approximately 2.5 million light years away from Earth. The furthest galaxy, by comparison, is about 13,000,000,000 (thirteen billion) light years away.

M31
Observation data
Constellation	Andromeda
Right ascension	00h42.7m
Declination	+41o16'
Distance	2.5-2.9 million light years
Type	Spiral Galaxy
Apparent dimensions	178.63 arc minutes
Diameter in ly	250,000
Apparent magnitude	3.4
Other designations	NGC 224, M 31, Chained Maiden

The largest of our nearest galactic neighbors, its disk is about 260,000 light-years in diameter. The disc of the Milky Way galaxy, our own, is considerably smaller, approximately 100,000 light years across.

Andromeda Galaxy: Composite of Galaxy Evolution Explorer ultraviolet detectors and Spitzer's super-sensitive infrared imaging photometer disclose embryonic stars. Young, hot, high-mass stars (blue); older stars (green); Bright yellow spot at the galaxy's center - a particularly dense population of old stars; Cooler, dusty regions where stars are forming (red swaths); Populations of hot, high-mass stars and cooler, dust-enshrouded stars co-exist (pinkish purple areas).

The galaxy is in the Andromeda Constellation. Andromeda is named for a princess in Greek mythology, the name translates "to think," "to attend to" or "be mindful of." The myth of Andromeda entails a story of a princess chained to a rock as a sacrifice to a monster. Ultimately she was saved, but the story also endows the galaxy with the appellation, "The Chained Maiden."

History

Known to Al-Sufi in Isfahan about AD 905, Charles Messier was unaware of earlier Persian records and attributed its discovery to Simon Marius, who gave a telescopic description of Andromeda in 1612.

In 1654, Giovanni Batista Hodierna discovered Andromeda without knowledge of what others had recorded long before he saw the galaxy. In 1716, Edmond Halley credited the discovery of what he then labled a "nebula" to the French astronomer Bullialdus (Ismail Bouillaud), who made his observations of Andromeda in 1661. For his part Bullialdus attributed its discovery to an anonymous astronomer 150 years prior (sometime in the early1500s).

Characteristics

Andromeda has a radial or heliocentric velocity of about -300 +/- 4 km/s and the Milky Way and Andromeda are approaching each other at about 100 km/sec and may collide in about 3 billion years.

Andromeda Galaxy with two of its orbiting globular clusters.

Size

Visible to the unaided eye, Andromeda appears several degrees in diameter and contains approximately 1,000,000,000,000 stars (one trillion). The Milky Way by comparison contains about 200,000,000 (two hundred billion). Conversely, new measurements employing infrared technolgy indicate that Andromeda's energy output is about that of 4,000,000,000 (four billion) suns. The most recent measurements concur with earlier estimates of the mass of the galaxy however.

Mass of Andromeda is now calculated to be less than that of the Milky Way, at 1.23 trillion solar masses for M31, significantly less than the 1.9 trillion for the Milky Way.

Composition and structure

Galactic center

Double nucleus

Double Nucleus of the Andromeda Galaxy- Hubble Space Telescope.

Andromeda apparently has a double nucleus of two star clusters about 5 light years in distance from each other. There are two theories for this phenomenon: The entire nucleus is either partially obscured by dust clouds making it appear to be in two separate sections, or the galaxy has previously engulfed another smaller galaxy about one billion years ago and its core remains as a relic of the collision separate from the original Andromeda core. The first indication of a double core was made in 1986 by Jean-Luc Nieto (Pic du Midi Observatory in France). Nieto noticed that there was a bright nucleus separated by a few light years from the exact center of the galaxy's central bulge. The Hubble Space Telescope later revealed that the dimmer of the two light peaks is the true nucleus and that earlier ground-based observations had in fact identified the wrong peak as the nucleus.

Black hole

In 1988, data taken by John Ormandy (University of Hawaii), Alan Dressler (Observatories of Carnegie Institution, Washington) and Douglas O.Richstone (University of Michigan) showed a sudden increase in orbital velocities of stars at the center of M31. Current theories postulate a black hole of about 10 million times the mass of our sun. If in fact there is a black hole in Andromeda, this would seem to indicate that the cannibalised galaxy's core would have been destroyed within a few hundred thousand years. A possible soution would be if the smaller galaxy had its own black hole that would have kept it from being dissipated.

Halo

At present, there are about 300,000 stars that have been distinguished in the halo, a spherical cloud of stars around Andromeda. Using a photo taken with the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope capturing both faint dwarf stars and bright giant stars, estimates of the age of the halo population are made by analyzing its distribution of color and brightness.

In 2003, the surprise was that approximately one-third of the stars in Andromeda's halo formed within the last 6 to 8 billion years. The entire star population in the halo span a range of 6 to 13 billion years old, much wider than that of the stars in the Milky Way halo, where stars range from about 11- to 13-billion-year-old. One theory is that the presence of younger stars in the Andromeda halo is the result of mergers with smaller galaxies.

The younger stars in Andromeda's halo recently discovered are richer in heavier elements than the stars in our Milky Way's halo and in most of the small dwarf galaxies that surround the Milky Way. The level of chemical content in these younger stars is characteristic of relatively massive galaxies, containing at least a billion stars. There are three possible explanations for the age and chemical content of these halo stars: (1) Multiple collisions destroyed the M31 disc formed more recently and scattered many of Andromeda's stars into the halo; (2) One single collision with another massive galaxy dispersed its stars and some of Andromeda's disk stars into the halo; and/or (3) Many new and therefore younger stars formed during the collision itself.

M31 globular cluster system

Andromeda has possibly as many as 500 accompanying extragalactic clusters of stars. The two bright points in the photo above, Andromeda Galaxy with two of its orbiting globular clusters, are but two of the brightest. The M32 (NGC 221) is at the lower center of the photo, and NGC 205 (sometimes designated M110) is to the upper left.

G1 (Mayall II)

Composed of about 300,000 old stars, G1 is a dwarf elliptical in orbit around the Andromeda galaxy at about 170,000 light-years from Andromeda's nucleus. The brightest globular cluster in the Local Group of galaxies, G1 is about 2.9 million light years from Earth. With a visual magnitude of 13.7. G1 can be seen with larger amateur telescopes.