Andromeda

Andromeda is a northern constellation, one of the 48 that the Greco-Roman astronomer Ptolemy cataloged in the 2nd century and one of the 88 modern ones recognized today. It takes its name from the figure in Greek mythology: Andromeda, the daughter of Cassiopeia, who was fastened to a rock as a sacrifice to the sea monster Cetus. The constellation is easiest to see during autumn evenings in the Northern Hemisphere. It ranks among the largest constellations, covering 722 square degrees—more than 1,400 times the area of a full moon, about 55 percent the size of the largest constellation (Hydra), and over ten times the size of the smallest (Crux). Because of its northern declination, it is only visible north of 40° south latitude; south of that line, it never rises above the horizon.

Its brightest star, Alpheratz (Alpha Andromedae), is a binary star that is also sometimes counted as part of Pegasus. Gamma Andromedae (Almach) is a colorful binary popular with amateur astronomers. Mirach (Beta Andromedae) is a red giant whose color can be seen with the naked eye, and its brightness varies similarly to Alpheratz. The most obvious deep-sky object in the constellation is the 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. It is visible without a telescope. Several fainter galaxies lie within Andromeda, including M31’s companions M110 and M32, as well as the more distant NGC 891. The Blue Snowball Nebula, a planetary nebula, appears as a blue circular object through a telescope.

In Chinese astronomy, the stars of Andromeda belonged to four different constellations with astrological and mythological meanings. A constellation related to Andromeda also appears in Hindu mythology. The Andromedids, a weak meteor shower in November, have their radiant in this constellation.

The constellation’s roots are strongest in Greek tradition, but a female figure in the same location appeared earlier in Babylonian astronomy. There, the stars of Pisces and the middle part of modern Andromeda formed a constellation representing a fertility goddess, sometimes called Anunitum or the Lady of the Heavens. In English, Andromeda is known as “the Chained Lady” or “the Chained Woman”; in Latin it was *Mulier Catenata*, and in Arabic *al-Mar’at al Musalsalah*. It has also been called *Persea* (“Perseus’s wife”) or *Cepheis* (“Cepheus’s daughter”), all referring to the Greco-Roman myth. In that story, Queen Cassiopeia of Aethiopia boasted that her daughter Andromeda was more beautiful than the Nereids, the sea nymphs. Offended, the nymphs asked Poseidon to punish Cassiopeia. He sent the sea monster Cetus to attack Aethiopia. Andromeda’s father, King Cepheus, consulted the Oracle of Ammon, who told him the only way to save the kingdom was to sacrifice his daughter to Cetus. Andromeda was chained to a rock by the sea, but the hero Perseus saved her. In one version, he used Medusa’s head to turn the monster to stone; in another, from the Roman poet Ovid’s *Metamorphoses*, he killed it with his diamond sword. Perseus and Andromeda married and had nine children—seven sons and two daughters—and founded Mycenae and its Persideae dynasty. After her death, Athena placed Andromeda in the sky as a constellation to honor her. Three neighboring constellations (Perseus, Cassiopeia, and Cepheus) represent characters from the myth, while Cetus retreats beyond Pisces. Andromeda is also connected to the constellation Pegasus.

Ptolemy included Andromeda as one of the original 48 constellations in his 2nd-century *Almagest*, defining it as a specific star pattern. She is usually depicted with Alpha Andromedae as her head, Omicron and Lambda Andromedae as her chains, and Delta, Pi, Mu, Beta, and Gamma as her body and legs, though no universal depiction exists. Arab astronomers knew Ptolemy’s constellations but added a second constellation—a fish—that overlapped Andromeda’s body; the fish’s nose was marked by a hazy patch now known as the Andromeda Galaxy (M31). In 1787, German astronomer Johann Bode combined several stars from Andromeda and most of the stars in Lacerta to form *Honores Friderici* (also called *Friedrichs Ehre*), meant to honor King Frederick II of Prussia, but it quickly fell out of use. Since Ptolemy’s time, Andromeda has remained a constellation and is officially recognized by the International Astronomical Union. Like all constellations dating back to Ptolemy, it is attributed to a wider zone and includes many surrounding stars. In 1922, the IAU gave it the three-letter abbreviation “And.” The official boundaries were defined in 1930 by Belgian astronomer Eugène Delporte as a polygon of 36 segments. Its right ascension ranges from 22h 57.5m to 2h 39.3m, and its declination from 53.19° to 21.68° in the equatorial coordinate system.

In traditional Chinese astronomy, nine stars from Andromeda (including Beta, Mu, and Nu Andromedae) and seven from Pisces formed an elliptical constellation called *Kui* (Legs), representing either a walking person’s foot or a wild boar. Gamma Andromedae and its neighbors were called *Tian Dajiangjun* (Heaven’s Great General), signifying honor in astrology and a great general in mythology. Alpha Andromedae and Gamma Pegasi together made *Bi* (Wall).