The Andromeda Galaxy also known as Messier 31, M31, or NGC 224, is a spiral
galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and
the nearest major galaxy to the Milky Way. Its name stems from the area of the
Earth’s sky in which it appears, the constellation of Andromeda. The 2006
observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy
contains approximately one trillion stars, more than twice the number of the
Milky Way’s estimated 200 to 400 billion stars. The Andromeda Galaxy’s mass is
estimated to be around 1.76 times that of the Milky Way Galaxy though a 2018 study
found that the Andromeda Galaxy’s mass is roughly the same as that of the Milky
Way.
The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.
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 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.
Andromeda is a princess from Greek mythology who, as divine punishment for her
mother’s bragging, (the Boast of Cassiopeia) was chained to a rock as a
sacrifice to a sea monster. She was saved from death by Perseus, her future
husband. Her name is the Latinized form of the Greek. The subject has been
popular in art since classical times. In the Christian period the subject was
converted into the legend of St George and the Dragon, but from the Renaissance
interest revived in the original story, typically as derived from Ovid’s
account.
In Greek mythology, Andromeda was the daughter of Cepheus and Cassiopeia, king and queen of the kingdom Ethiopia.
Her mother Cassiopeia boasted that she was more beautiful than the Nereids, the nymph-daughters of the sea god Nereus and often seen accompanying Poseidon. To punish the Queen for her arrogance, Poseidon, brother to Zeus and god of the sea, sent a sea monster (Cetus) to ravage the coast of Ethiopia including the kingdom of the vain Queen. The desperate King consulted the Oracle of Apollo, who announced that no respite would be found until the king sacrificed his virgin daughter Andromeda to the monster. She was chained naked to a rock on the coast.
Perseus was returning from having slain the Gorgon Medusa, he found Andromeda and slew Cetus by approaching invisible with Hades’s helm and slaying him. He set her free, and married her in spite of Andromeda having been previously promised to her uncle Phineus. At the wedding a quarrel took place between the rivals, and Phineus was turned to stone by the sight of the Gorgon’s head (Ovid, Metamorphoses v. 1).
Andromeda followed her husband to Tiryns in Argos, and together they became the ancestors of the family of the Perseidae through the line of their son Perses. Perseus and Andromeda had seven sons: Perseides, Perses, Alcaeus, Heleus, Mestor, Sthenelus, and Electryon, and one daughter, Gorgophone. Their descendants ruled Mycenae from Electryon down to Eurystheus, after whom Atreus attained the kingdom, and would also include the great hero Heracles. According to this mythology, Perseus is the ancestor of the Persians.
After her death, Andromeda was placed by Athene amongst the constellations in the northern sky, near Perseus and Cassiopeia. The constellation had been named after her.
