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 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, 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.

In Chinese astronomy, the stars that makeup 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 makeup 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 Ethiopia, 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 Ethiopia.

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. Several of the neighboring constellations (Perseus, Cassiopeia, Cetus, and Cepheus) also represent characters in the Perseus myth. 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 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.

Several stars from Andromeda and most of the stars in Lacerta were combined in 1787 by German astronomer Johann Bode to form Frederici Honores (also called Friedrichs Ehre). It was designed to honor 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, although like all modern constellations, it is now defined as a specific region of the sky that includes both Ptolemy’s pattern and the surrounding stars.

In 1922, the IAU defined its recommended three-letter abbreviation, “And“. The official boundaries of Andromeda were defined in 1930 by 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.

In non-Western astronomy

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“.

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” (heaven’s great general), representing honor in astrology and a great general in mythology.

Alpha Andromedae and Gamma Pegasi together made “Wall“, 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 and the far western part, along with most of Lacerta, became Tengshe, a flying snake.

An 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.

Hindu legends surrounding Andromeda are similar to the Greek myths. Ancient Sanskrit texts depict Antarmada chained to a rock, as in the Greek myth. Scholars believe that the Hindu and Greek astrological myths were closely linked; one piece of evidence cited is the similarity between the names “Antarmada” and “Andromeda“.

Andromeda is also associated with the Mesopotamian creation story of Tiamat, the goddess of Chaos. She bore many demons for her husband, Apsu, but eventually decided to destroy them in a war that ended when Marduk killed her. He used her body to create the constellations as markers of time for humans.

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. In the Tuamotu islands, Alpha Andromedae was called Takurua-e-te-tuki-hanga-ruki, meaning “Star of the wearisome toil“, and Beta Andromedae was called Piringa-o-Tautu.

Deep-sky objects

The constellation of Andromeda lies well away from the galactic plane, so it does not contain any of the open clusters or bright nebulae of the Milky Way.

Because of its distance in the sky from the band of obscuring dust, gas, and abundant stars of our home galaxy, Andromeda’s borders contain many visible distant galaxies.

The most famous deep-sky object in Andromeda is the spiral galaxy cataloged as Messier 31 (M31) or NGC 224 but known colloquially as the Andromeda Galaxy for the constellation.

M31 is one of the most distant objects visible to the naked eye, 2.2 million light-years from Earth (estimates range up to 2.5 million light-years); it is seen under a dark, transparent sky as a hazy patch in the north of the constellation.

M31 is the largest neighboring galaxy to the Milky Way and the largest member of the Local Group of galaxies. In absolute terms, M31 is approximately 200,000 light-years in diameter, twice the size of the Milky Way.

It is enormous – 192.4 by 62.2 arcminutes in apparent size – barred spiral galaxy similar in form to the Milky Way and at an approximate magnitude of 3.5, is one of the brightest deep-sky objects in the northern sky.

Despite being visible to the naked eye, the “little cloud” near Andromeda’s figure was not recorded until AD 964, when the Arab astronomer al-Sufi wrote his Book of Fixed Stars. M31 was first observed telescopically shortly after its invention, by Simon Marius in 1612.

The future 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.

Meteor showers

Each November, the Andromedids meteor shower appears to radiate from Andromeda. The shower peaks in mid-to-late November every year, but has a low peak rate of fewer than two meteors per hour.

Astronomers have often associated the Andromedids with Biela’s Comet, which was destroyed in the 19th century, but that connection is disputed.

Andromedid meteors are known for being very slow and the shower itself is considered to be diffuse, as meteors can be seen coming from nearby constellations as well as from Andromeda itself. Andromedid meteors sometimes appear as red fireballs.

The Andromedids were associated with the most spectacular meteor showers of the 19th century; the storms of 1872 and 1885 were estimated to have a peak rate of two meteors per second (a zenithal hourly rate of 10,000), prompting one Chinese astronomer to compare the meteors to falling rain.

The Andromedids had another outburst on December 3–5, 2011, the most active shower since 1885, with a maximum zenithal hourly rate of 50 meteors per hour.

The 2011 outburst was linked to ejecta from Comet Biela, which passed close to the Sun in 1649. None of the meteoroids observed were associated with material from the comet’s 1846 disintegration. The observers of the 2011 outburst predicted outbursts in 2018, 2023, and 2036.


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