Pegasus is a constellation in the northern hemisphere. It is the seventh largest constellation in the sky. It was named after the mythological winged horse and introduced by the Greek astronomer Ptolemy in the 2nd century.
In Greek mythology, Pegasus leapt from the body of the Gorgon Medusa, the monster whose gaze turned people into stone, when Perseus decapitated her. It was tamed by the hero Bellerophon, son of the Corinthian king Glaucus and grandson of Sisyphus. Bellerophon was helped by the goddess Athena. In Greek mythology, he is best known for slaying the Chimaera, a monster that breathed fire and had the head of a lion, body of a goat and tail of a serpent. When Bellerophon tried to ascend to Olympus on Pegasus to join the gods, he fell. Pegasus, however, made it and stayed with Zeus, carrying the god's thunder and lightning. It was Zeus who placed the horse among the stars.
The constellation Pegasus occupies an area of 1121 square degrees and contains eight stars with known planets. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of October.
The brightest star in the constellation is 
epsilon Pegasi, also known as Enif ("nose"). It is an orange supergiant 672 light-years distant. It marks the nose of Pegasus.
The constellation Pegasus is dominated by a large, recognizable asterism known as the Square of Pegasus. The asterism represents Pegasus’ body and is formed by 
beta and 
gamma Pegasi together with 
alpha Andromedae, Alpheratz.
alpha Pegasi, the third brightest star in the constellation, is a large main sequence star lying approximately 140 light-years from Earth. It is also known as Markab or Marchab, which is Arabic for "the saddle of the horse."
beta Pegasi is also known as Scheat ("wish"), a name it shares with 
delta Aquarii. It is a red giant classified as an irregular variable star. It is the second brightest star in Pegasus.
gamma Pegasi, or Algenib ("the side") is a Beta Cephei type variable approximately 335 light-years distant. It is a hot blue star, 4,000 times more luminous than the Sun, with an apparent magnitude of 2.83. It shares the name Algenib with 
eta Pegasi, or Matar ("lucky star of rain") is a binary star, possibly a multiple star system, 215 light-years distant. The brighter component is a class G giant star, while the companion star is a class A dwarf.
zeta Pegasi, or Homam ("the lucky star of high minded"), is a class B dwarf, 209 light-years distant.
51 Pegasi is another notable star. It was the first star similar to the Sun discovered to have a planet in orbit. It is a yellow dwarf, approximately 50 light-years away. It has an apparent magnitude of 5.49 and can be seen in binoculars. The extrasolar planet orbiting it is the prototype for a class of planets known as Hot Jupiters; planets with mass equal or greater than that of the planet Jupiter, that orbit the parent star within 1.05 AU. (Jupiter, for reference, orbits the Sun at 5 AU.) The planet was named Bellerophon, or 51 Pegasi b.
IK Pegasi is a binary star, lying about 150 light-years away, visible to the naked eye. The primary component is a class A main sequence star classified as a Delta Scuti type variable, while the companion star is a massive white dwarf, designated IK Pegasi B and notable for being the nearest supernova progenitor candidate. When the primary star starts to evolve into a red giant and the dwarf reaches the limit of 1.44 solar masses, it will likely explode as a supernova.
Pegasus is home to a couple of interesting deep sky objects. Messier 15 (NGC 7078) is a globular star cluster that can be spotted near 
epsilon Pegasi. The cluster contains a relatively high number of variable stars, at least eight pulsars, and a rare planetary nebula discovered within a globular cluster, named Pease 1. The age of the cluster is estimated to be around 13.2 billion years, making it one of the oldest globular clusters ever discovered. The cluster is approximately 33,600 light-years distant.
NGC 7742 is an unbarred spiral galaxy that can be seen face-on. It is classified as a Seyfert 2 active galaxy, one likely powered by a central black hole, and notable for having a very bright central region and a ring structure but no bar, which is usually needed to produce the ring. The ring is a very active region of star formation.
Pegasus belongs to the Perseus family of constellations, along with Cassiopeia
Constellations directly bordering Pegasus are Andromeda