Pegasus [key], in astronomy, northern constellation lying SW of Andromeda and SE of Cygnus. It is named for the mythological winged horse Pegasus. The constellation is easily recognized by the Great Square formed by the bright stars Markab (Alpha Pegasi) at the southwest corner, Scheat (Beta Pegasi) at the northwest corner, Algenib (Gamma Pegasi) at the southeast corner, and Alpheratz in Andromeda at the northeast corner. The constellation reaches its highest point in the evening sky in October. In 1995 a planet at least half the size of Jupiter was discovered orbiting the star 51 Pegasus, which is about 40 light-years from the earth; this marked the first time a planet was detected orbiting a sunlike star outside the solar system.
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