Great Square of Pegasus Asterism Has Four Second Magnitude Stars

Summary: The Great Square of Pegasus asterism dominates the large constellation of Pegasus the Winged Horse constellation with four second magnitude stars.

Pegasus the Winged Horse constellation (bottom left) with Andromeda (upper left); Pegasus the Winged Horse illustration, Alexander Jamieson’s A Celestial Atlas, Plate XII (1822): Public Domain, via U.S. Naval Observatory

The Great Square of Pegasus asterism has four second magnitude stars, with three corners contributed by Pegasus the Winged Horse constellation and one corner borrowed from Pegasus’ northeastern neighbor, Andromeda the Chained Princess constellation.

As a pattern of stars recognizable in Earth’s nighttime skies, an asterism may comprise stars from one constellation or may bring together stars from several constellations. For ages, stargazers have constructed the Great Square of Pegasus asterism with four second magnitude stars, all participating in Pegasus the Winged Horse constellation.

In the early 20th century, one of the asterism’s stars, known traditionally as Alpheratz, was removed from Pegasus the Winged Horse, the seventh largest constellation, and assigned officially to Andromeda the Chained Princess, the 19th largest constellation. The founding of the International Astronomical Union (IAU) in 1919 as an international association led to the designation of 88 official constellations and the establishment of clearly defined boundaries for constellations.

Relocating Alpheratz has jostled the list of brightest stars in both constellations. Nudging Beta Andromedae (β Andromedae; Beta And, β And) into second place, Alpheratz shines as the brightest star in the Chained Princess’s constellation.

Alpheratz’s reassignment moves Epsilon Pegasi (ε Pegasi; Epsilon Peg; ε Peg), which does not participate in the Great Square of Pegasus asterism, from second place to first place as the Winged Horse’s brightest star. The variable orange supergiant has the traditional name of Enif, derived from the Arabic word for “nose.” Located southwest of the Great Square of Pegasus asterism, Enif, indeed, marks the Winged Horse’s nostril.

Celestial cartographer Johann Bayer (1572-March 7, 1625) acknowledged claims from two constellations by assigning two designations for Alpheratz in his system of stellar nomenclature. Alpha Andromedae (α Andromedae; Alpha And; α And) is now the official designation while Delta Pegasi (δ Peg), obviously, has fallen into disuse.

Alpha Andromedae’s traditional name, Alpheratz, comes from Arab astronomy. Alpheratz derives from al-faras, the Arabic word for “horse.”

Alpha Andromedae is actually a close binary star system. The hot blue supergiant has a close orbit of 96.7 days with its dimmer companion.

Beta Pegasi (β Pegasi; Beta Peg; β Peg) lies west of Alpha Andromedae. The variable red giant star shines as the second brightest star, both in the asterism and in the constellation. Prior to Alpheratz’s reassignment to Andromeda, Beta Pegasi ranked as third brightest in the Winged Horse constellation.

Beta Pegasi’s traditional name is Scheat, derived from Al Sā'id, Arabic for “upper arm.” Beta Pegasi marks the Winged Horse’s upper foreleg.

Alpha Pegasi (α Pegasi; Alpha Peg; α Peg) lies south of Beta Pegasi. Alpha Pegasi is the third brightest star, both in the asterism and in the constellation. Prior to Alpheratz’s removal to Andromeda, Alpha Pegasi ranked as fourth brightest in the Winged Horse constellation. The white star’s traditional name is Markab, which may be a mistranscription of mankib, the Arabic word for “shoulder.” Alpha Pegasi marks the Winged Horse’s shoulder.

Gamma Pegasi (γ Pegasi; Gamma Peg; γ Peg) resides to the east of Alpha Pegasi. Gamma is the fourth brightest star, both in the asterism and in the constellation. Prior to Alpheratz’s reallocation to Andromeda, Gamma Pegasi ranked as fifth brightest in the Winged Horse constellation.

The variable blue-white subgiant star has the traditional name of Algenib. American astronomer James B. “Jim” Kaler notes that the name derives from the Arabic word for “side.” A Celestial Atlas, published by Scottish rhetorician Alexander Jamieson (1782-1850) in 1822, places Algenib near the tip of the Winged Horse’s wing, which is folded along its side.

The takeaway for the Great Square of Pegasus asterism is that its four second magnitude stars form a nightly reunion of three stars in Pegasus the Winged Horse constellation with one star now sited in Andromeda the Chained Princess constellation.

Great Square of Pegasus asterism and other stars in Pegasus the Winged Horse constellation: International Astronomical Union and Sky & Telescope, CC BY 4.0 International, via International Astronomical Union (IAU)

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Pegasus the Winged Horse constellation (bottom left) with Andromeda (upper left); Pegasus the Winged Horse illustration, Alexander Jamieson’s A Celestial Atlas, Plate XII (1822): Public Domain, via U.S. Naval Observatory @ http://aa.usno.navy.mil/library/

Great Square of Pegasus asterism and other stars in Pegasus the Winged Horse constellation: International Astronomical Union and Sky & Telescope, CC BY 4.0 International, via International Astronomical Union (IAU) @ https://creativecommons.org/licenses/by/4.0/ (image URL @ https://www.iau.org/public/images/detail/peg/)

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