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	Galaxy
Apparent dimensions	178.63 arc minutes
Diameter in ly	250,000
Apparent magnitude	3.4

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.

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.

Andromeda Galaxy.

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.