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Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy, and one of the 88 modern constellations. Located in the northern celestial hemisphere, it is named for Andromeda, daughter of Cassiopeia, in the Greek myth, who was chained to a rock to be eaten by the sea monster Cetus. Andromeda is most prominent during autumn evenings in the Northern Hemisphere, along with several other constellations named for characters in the Perseus myth. Because of its northern declination, Andromeda is visible only north of 40° south latitude; for observers farther south, it always lies below the horizon. It is one of the largest constellations, with an area of 722 square degrees. This is over 1,400 times the size of the full moon, 55% of the size of the largest constellation, Hydra, and over 10 times the size of the smallest constellation, Crux. Its brightest star, Alpheratz (Alpha Andromedae), is a binary star that has also been counted as a part of Pegasus, while Gamma Andromedae (Almach) is a colorful binary and a popular target for amateur astronomers. With a variable brightness similar to Alpheratz, Mirach (Beta Andromedae) is a red giant, its color visible to the naked eye. The constellation's most obvious deep-sky object is the naked-eye Andromeda Galaxy (M31, also called the Great Galaxy of Andromeda), the closest spiral galaxy to the Milky Way and one of the brightest Messier objects. Several fainter galaxies, including M31's companions M110 and M32, as well as the more distant NGC 891, lie within Andromeda. The Blue Snowball Nebula, a planetary nebula, is visible in a telescope as a blue circular object. In Chinese astronomy, the stars that make up Andromeda were members of four different constellations that had astrological and mythological significance; a constellation related to Andromeda also exists in Hindu mythology. Andromeda is the location of the radiant for the Andromedids, a weak meteor shower that occurs in November.

History and mythology

The uranography of Andromeda has its roots most firmly in the Greek tradition, though a female figure in Andromeda's location had appeared earlier in Babylonian astronomy. The stars that make up Pisces and the middle portion of modern Andromeda formed a constellation representing a fertility goddess, sometimes named as Anunitum or the Lady of the Heavens. Andromeda is known as "the Chained Lady" or "the Chained Woman" in English. It was known as Mulier Catenata ("chained woman") in Latin and al-Mar'at al Musalsalah in Arabic. It has also been called Persea ("Perseus's wife") or Cepheis ("Cepheus's daughter"), all names that refer to Andromeda's role in the Greco-Roman myth of Perseus, in which Cassiopeia, the queen of Aethiopia, bragged that her daughter was more beautiful than the Nereids, sea nymphs blessed with incredible beauty. Offended at her remark, the nymphs petitioned Poseidon to punish Cassiopeia for her insolence, which he did by commanding the sea monster Cetus to attack Aethiopia. Andromeda's panicked father, Cepheus, was told by the Oracle of Ammon that the only way to save his kingdom was to sacrifice his daughter to Cetus. She was chained to a rock by the sea but was saved by the hero Perseus, who in one version of the story used the head of Medusa to turn the monster into stone; in another version, by the Roman poet Ovid in his Metamorphoses, Perseus slew the monster with his diamond sword. Perseus and Andromeda then married; the myth recounts that the couple had nine children together – seven sons and two daughters – and founded Mycenae and its Persideae dynasty. After Andromeda's death Athena placed her in the sky as a constellation, to honor her. Three of the neighboring constellations (Perseus, Cassiopeia and Cepheus) represent characters in the Perseus myth, while Cetus retreats to beyond Pisces. It is connected with the constellation Pegasus. Andromeda was one of the original 48 constellations formulated by Ptolemy in his 2nd-century Almagest, in which it was defined as a specific pattern of stars. She is typically depicted with α Andromedae as her head, ο and λ Andromedae as her chains, and δ, π, μ, β, and γ her body and legs. However, there is no universal depiction of Andromeda and the stars used to represent her body, head, and chains. Arab astronomers were aware of Ptolemy's constellations, but they included a second constellation representing a fish overlapping Andromeda's body; the nose of this fish was marked by a hazy patch that ‍we ‍now ‍know ‍as ‍the ‍Andromeda Galaxy, ‍M31. Several stars from Andromeda and most of the stars in Lacerta were combined in 1787 by German astronomer Johann Bode to form Honores Friderici (also called Friedrichs Ehre). It was designed to honour King Frederick II of Prussia, but quickly fell into disuse. Since the time of Ptolemy, Andromeda has remained a constellation and is officially recognized by the International Astronomical Union. Like all those that date back to a pattern known to Ptolemy, it is attributed to a wider zone and thus many surrounding stars. In 1922, the IAU defined its recommended three-letter abbreviation, "And". The official boundaries of Andromeda were defined in 1930 by Belgian astronomer Eugène Delporte as a polygon of 36 segments. Its right ascension is between 22h 57.5m and 2h 39.3m and its declination is between 53.19° and 21.68° in the equatorial coordinate system.