Perseus (constellation)

Perseus is a constellation in the northern sky, named after the Greek mythological hero Perseus. It is one of the 48 ancient constellations listed by the 2nd-century astronomer Ptolemy, and among the 88 modern constellations defined by the International Astronomical Union (IAU). It is located near several other constellations named after ancient Greek legends surrounding Perseus, including Andromeda to the west and Cassiopeia to the north. Perseus is also bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, and Triangulum to the west. Some star atlases during the early 19th century also depicted Perseus holding the disembodied head of Medusa, whose asterism was named together as Perseus et Caput Medusae; however, this never came into popular usage.

The galactic plane of the Milky Way passes through Perseus, whose brightest star is the yellow-white supergiant Alpha Persei (also called Mirfak), which shines at magnitude 1.79. It and many of the surrounding stars are members of an open cluster known as the Alpha Persei Cluster. The best-known star, however, is Algol (Beta Persei), linked with ominous legends because of its variability, which is noticeable to the naked eye. Rather than being an intrinsically variable star, it is an eclipsing binary. Other notable star systems in Perseus include X Persei, a binary system containing a neutron star, and GK Persei, a nova that peaked at magnitude 0.2 in 1901. The Double Cluster, comprising two open clusters quite near each other in the sky, was known to the ancient Chinese. The constellation gives its name to the Perseus Cluster (Abell 426), a massive galaxy cluster located 250 million light-years from Earth. It hosts the radiant of the annual Perseids meteor shower—one of the most prominent meteor showers in the sky.

In Greek mythology, Perseus was the son of Danaë, who was sent by King Polydectes to bring the head of Medusa the Gorgon—whose visage caused all who gazed upon her to turn to stone. Perseus slew Medusa in her sleep, and Pegasus and Chrysaor appeared from her body. Perseus continued to the realm of Cepheus whose daughter Andromeda was to be sacrificed to Cetus the sea monster.

Perseus rescued Andromeda from the monster by killing it with his sword. He turned Polydectes and his followers to stone with Medusa's head and appointed Dictys the fisherman king. Perseus and Andromeda married and had six children. In the sky, Perseus lies near the constellations Andromeda, Cepheus, Cassiopeia (Andromeda's mother), Cetus, and Pegasus.

In Neo-Assyrian Babylonia (911–605 BC), the constellation of Perseus was known as the Old Man constellation (SU.GI), then associated with East in the MUL.APIN, an astronomical text from the 7th century.

Four Chinese constellations are contained in the area of the sky identified with Perseus in the West. Tiānchuán (天船), the Celestial Boat, was the third paranatellon (A star or constellation which rises at the same time as another star or object) of the third house of the White Tiger of the West, representing the boats that Chinese people were reminded to build in case of a catastrophic flood season. Incorporating stars from the northern part of the constellation, it contained Mu, Delta, Psi, Alpha, Gamma and Eta Persei. Jīshuǐ (積水), the Swollen Waters, was the fourth paranatellon of the aforementioned house, representing the potential of unusually high floods during the end of August and beginning of September at the beginning of the flood season. Lambda and possibly Mu Persei lay within it. Dàlíng (大陵), the Great Trench, was the fifth paranatellon of that house, representing the trenches where criminals executed en masse in August were interred. It was formed by Kappa, Omega, Rho, 24, 17 and 15 Persei. The pile of corpses prior to their interment was represented by Jīshī (積屍, Pi Persei), the sixth paranatellon of the house. The Double Cluster, h and Chi Persei, had special significance in Chinese astronomy.

In Polynesia, Perseus was not commonly recognized as a separate constellation; the only people that named it were those of the Society Islands, who called it Faa-iti, meaning "Little Valley". Algol may have been named Matohi by the Māori people, but the evidence for this identification is disputed. Matohi ("Split") occasionally came into conflict with Tangaroa-whakapau over which of them should appear in the sky, the outcome affecting the tides. It matches the Maori description of a blue-white star near Aldebaran but does not disappear as the myth would indicate.

Perseus is bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis and Cassiopeia to the north, and Andromeda and Triangulum to the west. Covering 615 square degrees, it ranks twenty-fourth of the 88 constellations in size. It appears prominently in the northern sky during the Northern Hemisphere's spring. Its main asterism consists of 19 stars. The constellation's boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a 26-sided polygon. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01^h 29.1^m and 04^h 51.2^m , while the declination coordinates are between 30.92° and 59.11°. The International Astronomical Union (IAU) adopted the three-letter abbreviation "Per" for the constellation in 1922.