Best visible at 21:00 (9 p.m.) during the month of November.
Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy and remains one of the 88 modern constellations. Located north of the celestial equator, 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 lies below the horizon.
It is one of the largest constellations, with an area of 722 square degrees. This is 55% of the size of the largest constellation, Hydra, and over 10 times the size of the smallest constellation, Crux.
Its brightest star, Alpha Andromedae, is a binary star that has also been counted as a part of Pegasus, while Gamma Andromedae is a colorful binary and a popular target for amateur astronomers. Only marginally dimmer than Alpha, 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. Andromeda is the location of the radiant for the Andromedids, a weak meteor shower that occurs in November.
In Chinese astronomy, the stars that make up Andromeda were members of four different constellations that had astrological and mythological significance.
History and mythology
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 Andromedae representing 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 at Andromeda’s feet. The Arab constellation called “al-Hut” (the fish) was composed of several stars in Andromeda, M31, and several stars in Pisces. ν And, μ And, β And, η And, ζ And, ε And, δ And, π And, and 32 And were all included from Andromeda; ν Psc, φ Psc, χ Psc, and ψ Psc were included from Pisces.
In traditional Chinese astronomy, nine stars from Andromeda (including Beta Andromedae, Mu Andromedae, and Nu Andromedae), along with seven stars from Pisces, formed an elliptical constellation called “Legs” (奎宿). This constellation either represented the foot of a walking person or a wild boar. Gamma Andromedae and its neighbors were called “Teen Ta Tseang Keun” (天大将军, heaven’s great general), representing honor in astrology and a great general in mythology. Alpha Andromedae and Gamma Pegasi together made “Wall” (壁宿), representing the eastern wall of the imperial palace and/or the emperor’s personal library. For the Chinese, the northern swath of Andromeda formed a stable for changing horses (tianjiu, 天厩, stable on sky) and the far western part, along with most of Lacerta, became Tengshe, a flying snake.
Hindu legends surrounding Andromeda are similar to the Greek myths. Ancient Sanskrit texts depict Antarmada chained to a rock. Scholars believe that the Hindu and Greek astrological myths were closely linked.
In the Marshall Islands, Andromeda, Cassiopeia, Triangulum, and Aries are incorporated into a constellation representing a porpoise. Andromeda’s bright stars are mostly in the body of the porpoise; Cassiopeia represents its tail and Aries its head.
M31 is often referred to as a twin sister to the Milky Way, but it has only half the mass of the Milky Way despite being twice its diameter. The futures of the Andromeda and Milky Way galaxies may be interlinked: in about five billion years, the two could potentially begin an Andromeda–Milky Way collision that would spark extensive new star formation.
There is one prominent planetary nebula in Andromeda: NGC 7662 (Caldwell 22). Lying approximately three degrees southwest of Iota Andromedae at a distance of about 4,000 light-years from Earth, the “Blue Snowball Nebula” is a popular target for amateur astronomers. It earned its popular name because it appears as a faint, round, blue-green object in a telescope, with an overall magnitude of 9.2.