The Pegasus constellation is a highly recognizable figure in the night sky, known for the distinct square shape that forms its body. Asking how many stars are in this constellation seems like a straightforward question, yet the answer depends entirely on how one defines a “star” and the boundaries used for counting. The difference between the few dozen stars visible to the unaided eye and the countless celestial bodies cataloged by modern instruments is immense. Understanding this complexity requires examining the official borders and the various methods astronomers use to count the stellar population within those limits.
Defining the Celestial Boundaries
The International Astronomical Union (IAU) formally established the precise, fixed boundaries of all 88 modern constellations, including Pegasus, in 1930. This process transformed the ancient, fluid star patterns into rigid, three-dimensional sections of the night sky. Pegasus is a large constellation, ranking as the seventh largest overall, encompassing a celestial area of 1,121 square degrees.
The constellation lies in the Northern Celestial Hemisphere and is most easily visible to observers at mid-northern latitudes during the autumn months. Its visibility spans a wide range of the globe, observable from latitudes between +90 degrees and approximately -60 degrees south. These official borders determine which stars, regardless of their distance or brightness, are considered to be within the constellation.
The Variable Answer to “How Many Stars?”
The exact number of stars within the Pegasus constellation changes depending on the specific criteria used for counting. For a casual observer looking up on a clear night, the number of stars is relatively small. Only about six to ten stars are bright enough to clearly outline the recognizable winged horse figure with the naked eye. This small group forms the famous “Great Square of Pegasus” asterism.
Moving beyond what the human eye can perceive, the count increases significantly when referring to officially cataloged stars. The IAU borders contain 177 stars that possess an apparent magnitude of 6.5 or brighter. Stars at this magnitude are generally considered the limit of visibility for the unaided eye under perfectly dark conditions. Earlier star maps, such as the one compiled by the astronomer John Flamsteed, designated 88 stars within the constellation’s area, demonstrating the historical variation in catalogs.
When modern professional instruments are utilized, the number becomes astronomical. Surveys like the European Space Agency’s Gaia mission, which maps billions of stars, reveal that the total number of stellar bodies within the three-dimensional volume of the Pegasus boundaries is in the millions, or even billions. This immense number includes stars too dim to ever be seen by ground-based telescopes, such as faint red dwarfs and brown dwarfs.
Identifying the Brightest Stars
The shape of the winged horse is dominated by the four stars that form the Great Square, though only three of them are actually within the official Pegasus boundaries. These three are Markab (Alpha Pegasi), Scheat (Beta Pegasi), and Algenib (Gamma Pegasi). The fourth corner, Alpheratz, was historically considered part of Pegasus but is now formally designated as Alpha Andromedae.
The brightest star in the entire constellation is Epsilon Pegasi, commonly known as Enif, which marks the horse’s muzzle. Enif is an orange supergiant star with an apparent magnitude of 2.38, lying approximately 690 light-years from Earth. This star is an irregular variable, meaning its brightness fluctuates slightly over time.
Scheat, the second brightest star, is a cool red giant located about 196 light-years away, and it contributes to the upper-right corner of the Great Square. Markab, which is a blue-white star, defines the opposite corner of the square and is situated 140 light-years distant. The third star of the square, Algenib, is a blue-white subgiant about 331 light-years away that exhibits slight variability in its brightness.