Perseus

Perseus is a northern constellation named for the Greek hero, one of the 48 ancient constellations cataloged by Ptolemy in the 2nd century and now one of the 88 modern ones recognized by the IAU. It sits near other constellations tied to the same myths: Andromeda lies to its west, Cassiopeia to its north. Its other neighbors include Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, and Triangulum to the west. In the early 1800s, some star atlases showed Perseus holding Medusa’s severed head as a separate asterism called Perseus et Caput Medusae, but that name never caught on.

The Milky Way’s galactic plane runs through Perseus. Its brightest star is Mirfak (Alpha Persei), a yellow-white supergiant with a magnitude of 1.79, which belongs to the Alpha Persei Cluster along with many nearby stars. The most famous star here is Algol (Beta Persei), linked to dark legends because its brightness changes noticeably to the naked eye. It’s not truly variable—it’s an eclipsing binary. Other notable systems include X Persei, a binary with a neutron star, and GK Persei, a nova that reached magnitude 0.2 in 1901. The Double Cluster, two open clusters close together in the sky, was known to ancient Chinese astronomers. The constellation also gives its name to the Perseus Cluster (Abell 426), a massive galaxy cluster 250 million light-years away, and hosts the radiant of the Perseids, one of the year’s most prominent meteor showers.

In Greek myth, Perseus was the son of Danaë. King Polydectes sent him to fetch the head of Medusa the Gorgon, whose face turned anyone who looked at her to stone. Perseus killed her while she slept, and from her body sprang Pegasus and Chrysaor. He then went to the realm of King Cepheus, whose daughter Andromeda was about to be sacrificed to the sea monster Cetus. Perseus killed the monster with his sword, rescued Andromeda, and later turned Polydectes and his followers to stone with Medusa’s head, making Dictys the fisherman king. Perseus and Andromeda married and had six children. In the sky, Perseus lies near Andromeda, Cepheus, Cassiopeia (Andromeda’s mother), Cetus, and Pegasus. In Neo-Assyrian Babylonia (911–605 BC), this constellation was called the Old Man (SU.GI) and was linked to the east in the 7th-century text MUL.APIN.

In Chinese astronomy, four constellations fall within Perseus’s area. Tiānchuán (天船), the Celestial Boat, was the third paranatellon of the third house of the White Tiger of the West, representing boats people should build for flood season. It included Mu, Delta, Psi, Alpha, Gamma, and Eta Persei. Jīshuǐ (積水), the Swollen Waters, was the fourth paranatellon of that house, symbolizing high floods in late August and early September, and contained Lambda and possibly Mu Persei. Dàlíng (大陵), the Great Trench, was the fifth paranatellon, representing trenches for criminals executed in August, and included Kappa, Omega, Rho, 24, 17, and 15 Persei. Jīshī (積屍, Pi Persei), the sixth paranatellon, stood for the pile of corpses before burial. The Double Cluster (h and Chi Persei) had special significance in Chinese astronomy. In Polynesia, Perseus was rarely a separate constellation; only the Society Islands called it Faa-iti, meaning “Little Valley.” Algol may have been called Matohi by the Māori, but the evidence is disputed. Matohi (“Split”) supposedly fought Tangaroa-whakapau over which would appear in the sky, affecting the tides. It matches a description of a blue-white star near Aldebaran but does not vanish as the myth says.

Perseus covers 615 square degrees, ranking 24th among the 88 constellations. It is most visible in the northern sky during spring in the Northern Hemisphere. Its main asterism has 19 stars. Belgian astronomer Eugène Delporte set its boundaries in 1930 as a 26-sided polygon, with right ascension from 01h 29.1m to 04h 51.2m and declination from 30.92° to 59.11°. The IAU adopted the three-letter abbreviation “Per” in 1922.

Algol (Arabic for “The Demon’s Head”), also called Beta Persei, is the best-known star in the constellation. In Greek myth it represents Medusa’s head; Egyptians called it Horus, and Hebrews called it Rosh ha Satan (“Satan’s Head”). It lies 92.8 light-years away and varies in apparent magnitude from 3.5 to 2.3 every 2.867 days. It is the prototype for a class of eclipsing binary stars called Algol variables.