The Great Square of Pegasus is a well-known asterism in Pegasus constellation formed by three stars in Pegasus – Markab (Alpha Pegasi), Scheat (Beta Pegasi), and Algenib (Gamma Pegasi) – and Alpheratz (Alpha Andromedae), the brightest star in Andromeda constellation.
Markab is a class B giant star with a radius almost five times that of the Sun, about 133 light years distant from the solar system.
Scheat is the second brightest star in Pegasus. It is a red giant or subgiant star approximately 196 light years distant from Earth.
Algenib is a B class subgiant about nine times more massive than the Sun. It is approximately 390 light years distant from Earth.
Alpheratz (sometimes also known as Sirrah) is a double star, with two components closely orbiting each other. The star used to be known as Delta Pegasi, but is now treated as a star in Andromeda constellation. It is about 97 light years distant.
The Great Square of Pegasus can be seen rising above the horizon in the eastern sky in late September.