Pegasus

Pegasus is a northern constellation named for the winged horse of Greek myth. It was among the 48 constellations cataloged by the 2nd-century astronomer Ptolemy and remains one of the 88 officially recognized today, lying just north of the celestial equator.

Its brightest star, Epsilon Pegasi (called Enif), is an orange supergiant that varies in apparent magnitude between 2.37 and 2.45, marking the horse's muzzle. The large asterism known as the Square of Pegasus is formed by Alpha (Markab), Beta (Scheat), and Gamma (Algenib) in Pegasus, together with Alpha Andromedae (Alpheratz). Twelve star systems within the constellation are known to host exoplanets. Among them, 51 Pegasi was the first Sun-like star found to have an exoplanet companion; it lies just over 50 light-years from Earth.

In Greek mythology, Pegasus was a winged horse with magical powers. One story says his hooves dug out the spring Hippocrene, whose water granted the ability to write poetry. Pegasus was born from the blood of Medusa after Perseus cut off her head—Medusa had been impregnated by Poseidon—and emerged alongside Chrysaor. The horse later served Bellerophon, who with Athena's help used Pegasus to kill the Chimera. After his children died, Bellerophon asked Pegasus to carry him to Mount Olympus. Pegasus agreed, but Zeus either threw a thunderbolt or sent a gadfly to make Pegasus buck him off, sending Bellerophon plummeting back to Earth.

The Babylonians had a constellation called IKU (field), with four stars, three of which later became part of the Greek Hippos (Pegasus). Persia also recognized the constellation. In the 10th century, the astronomer al-Sufi depicted Pegasus as a complete horse facing east, unlike most other uranographers who showed only the front half rising from the ocean. In al-Sufi's version, the head uses stars from Lacerta; the right foreleg is β Peg, the left foreleg η Peg, μ Peg, and λ Peg; the hind legs are marked by 9 Peg; the back by π Peg and μ Cyg; and the belly by ι Peg and κ Peg. In Chinese astronomy, Pegasus lies within the Black Tortoise of the north (北方玄武), where its stars form several separate asterisms. Epsilon and Theta Pegasi join Alpha Aquarii to form Wei 危 ("rooftop"), with Theta as the roof apex. In Hindu astronomy, Pegasus falls within the 25th nakshatra (lunar mansion) Purva Bhadrapada, representing a bedstead that served as a resting place for the Moon. For the Warrau and Arawak peoples of Guyana, the stars of the Great Square (parts of Pegasus and Andromeda) represent a barbecue taken into the sky by the seven hunters of the Siritjo myth.

Covering 1121 square degrees, Pegasus is the seventh-largest of the 88 constellations. It is bordered by Andromeda to the north and east, Lacerta to the north, Cygnus to the northwest, Vulpecula, Delphinus, and Equuleus to the west, Aquarius to the south, and Pisces to the south and east. Its three-letter IAU abbreviation (adopted in 1922) is "Peg." The official boundaries, set by Belgian astronomer Eugène Delporte in 1930, form a 35-segment polygon. In equatorial coordinates, the right ascension borders lie between 21h 12.6m and 00h 14.6m, and the declination between 2.33° and 36.61°. The whole constellation is visible to observers north of 53°S.

The constellation is dominated by the roughly square asterism called the Square of Pegasus. One of its stars, formerly Delta Pegasi (Sirrah), is now officially Alpha Andromedae (Alpheratz) in Andromeda. Traditionally, the horse's body is a quadrilateral formed by α Peg, β Peg, γ Peg, and α And. The front legs are two crooked lines of stars: one from η Peg to κ Peg, the other from μ Peg to 1 Pegasi. Another crooked line from α Peg through θ Peg to ε Peg forms the neck and head, with ε as the snout.

Bayer cataloged 23 stars in Pegasus, giving them designations Alpha through Psi. He saw Pi Pegasi as a single star of uncertain brightness (between magnitude 4 and 5). Flamsteed labeled it 29 Pegasi, but Bode concluded that Bayer had seen two stars (27 and 29 Pegasi) as one, making them Pi¹ and Pi². Flamsteed added lowercase letters e through y (omitting A–D, used on Bayer's chart for neighboring constellations and the equator) and numbered 89 stars (now Flamsteed designations), though 6 and 11 turned out to be in Aquarius. Within the constellation's borders are 177 stars of apparent magnitude 6.5 or brighter.

Epsilon Pegasi (Enif), the brightest star, is an orange supergiant of spectral type K21b, about 12 times the Sun's mass and roughly 690 light-years away. It is an irregular variable, with magnitude between 2.37 and 2.45. Near Enif lies AG Pegasi, an unusual star that brightened to magnitude 6.0 around 1885 before dimming to magnitude 9. It consists of a red giant and a white dwarf (about 2.5 and 0.6 solar masses, respectively), and its outburst—taking over 150 years—has been called the slowest nova ever recorded. Three Bayer-designated stars within the Great Square are variable: Phi and Psi Pegasi are pulsating red giants, and Tau Pegasi (named Salm) is also variable.