With anapparent magnitude varying between 2.37 and 2.45, the brightest star in Pegasus is the orange supergiantEpsilon Pegasi, also known as Enif, which marks the horse's muzzle.Alpha (Markab),Beta (Scheat), andGamma (Algenib), together withAlpha Andromedae (Alpheratz) form the largeasterism known as theSquare of Pegasus. Twelve star systems have been found to have exoplanets.51 Pegasi was the first Sun-like star discovered to have an exoplanet companion.
The Babylonian constellation IKU (field) had four stars of which three were later part of the Greek constellationHippos (Pegasus).[2] Pegasus, in Greek mythology, was a winged horse with magical powers. One myth regarding his powers says that his hooves dug out a spring,Hippocrene, which blessed those who drank its water with the ability to write poetry. Pegasus was born whenPerseus cut off the head ofMedusa, who was impregnated by the godPoseidon. He was born withChrysaor from Medusa's blood.[3] Eventually, it became the horse forBellerophon, who was asked to kill theChimera and succeeded with the help ofAthena and Pegasus. Despite this success, after the death of his children, Bellerophon asked Pegasus to take him toMount Olympus. Though Pegasus agreed, he plummeted back to Earth after Zeus either threw a thunderbolt at him or sent a gadfly to make Pegasus buck him off.[4][5]
In mediaeval Persia, Pegasus was depicted byal-Sufi as a complete horse facing east, unlike most other uranographers, who had depicted Pegasus as half of a horse, rising out of the ocean. In al-Sufi's depiction, Pegasus's head is made up of the stars ofLacerta the lizard. Its right foreleg is represented by β Peg and its left foreleg is represented by η Peg, μ Peg, and λ Peg; its hind legs are marked by 9 Peg. The back is represented by π Peg and μ Cyg, and the belly is represented by ι Peg and κ Peg.[4]
In Hindu astronomy, Pegasus is contained within the 25th nakshatralunar mansion Purva Bhadrapada. More specifically, it represented a bedstead that was a resting place for the Moon.[4]
For theWarrau andArawak peoples inGuyana the stars in the Great Square, corresponding to parts of Pegasus and of Andromeda, represented a barbecue, taken up to the sky by the seven hunters of the myth of Siritjo.[4][8]
Covering 1121 square degrees, Pegasus is the seventh-largest of the 88 constellations. Pegasus is bordered by Andromeda to the north and east,Lacerta to the north,Cygnus to the northwest,Vulpecula,Delphinus andEquuleus to the west,Aquarius to the south andPisces to the south and east. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is "Peg".[9] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930, are defined as a polygon of 35 segments. In theequatorial coordinate system theright ascension coordinates of these borders lie between21h 12.6m and00h 14.6m, while thedeclination coordinates are between 2.33° and 36.61°.[1] Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 53°S.[10][a]
Pegasus with the foalEquuleus next to it, as depicted inUrania's Mirror, a set of constellation cards published in London c.1825. The horses appear upside-down in relation to the constellations around them.
Pegasus is dominated by a roughly square asterism (the Square of Pegasus) although one of the stars, formerly known as Delta Pegasi or Sirrah, is now officially assigned toAndromeda and is known asAlpha Andromedae, or Alpheratz.[11] Traditionally, the body of the horse consists of a quadrilateral formed by the starsα Peg,β Peg,γ Peg, andα And. The front legs of the winged horse are formed by two crooked lines of stars, one leading fromη Peg toκ Peg and the other fromμ Peg to1 Pegasi. Another crooked line of stars fromα Peg viaθ Peg toε Peg forms the neck and head; ε is the snout.
Bayer catalogued what he counted as 23 stars in the constellation, giving them theBayer designations Alpha to Psi. He sawPi Pegasi as one star, and was uncertain of its brightness, wavering between magnitude 4 and 5. Flamsteed labelled this star 29 Pegasi, but Bode concluded that the stars 27 and 29 Pegasi should bePi1 andPi2 Pegasi and that Bayer had seen them as a single star.[12] Flamsteed added lower case letters e through to y, omitting A to D as they had been used on Bayer's chart to designate neighbouring constellations and the equator.[13] He numbered 89 stars (now with Flamsteed designations), though 6 and 11 turned out to be stars in Aquarius.[14] Within the constellation's borders there are 177 stars ofapparent magnitude 6.5 or greater.[b][10]
Epsilon Pegasi, also known as Enif, marks the horse's muzzle. The brightest star in Pegasus, is an orangesupergiant ofspectral type K21b that is around 12 timesas massive as the Sun and is around 690light-years distant from Earth.[16] It is an irregular variable, itsapparent magnitude varying between 2.37 and 2.45.[17] Lying near Enif isAG Pegasi, an unusual star that brightened to magnitude 6.0 around 1885 before dimming to magnitude 9. It is composed of ared giant andwhite dwarf, estimated to be around 2.5 and 0.6 times the mass of the Sun respectively. With its outburst taking over 150 years, it has been described as the slowestnova ever recorded.[18]
Three stars with Bayer designations that lie within the Great Square are variable stars.Phi andPsi Pegasi are pulsating red giants, whileTau Pegasi (the proper name is Salm[19]), is aDelta Scuti variable—a class of short period (six hours at most) pulsating stars that have been used asstandard candles and as subjects to studyastroseismology.[20] Rotating rapidly with aprojected rotational velocity of 150 km s−1, Kerb is almost 30 times as luminous as the Sun and has a pulsation period of 56.5 minutes. With an outer atmosphere at aneffective temperature of 7,762 K, it is a white star with a spectral type of A5IV.[21]
Zeta,Xi,Rho andSigma Pegasi mark the horse's neck.[22] The brightest of these with a magnitude of 3.4 is Zeta, also traditionally known as Homam. Lying seven degrees southwest of Markab, it is a blue-white main sequence star of spectral type B8V located around 209 light-years distant.[23] It is aslowly pulsating B star that varies slightly in luminosity with a period of 22.952 ± 0.804 hours, completing 1.04566 cycles per day.[24] Xi lies 2 degrees northeast, and is ayellow-white main sequence star of spectral type F6V that is 86% larger and 17% more massive that the Sun, and radiate 4.5 times thesolar luminosity.[25] It has a red dwarf companion that is 192.3 au distant.[26] If (as is likely) the smaller star is in orbit around the larger star, then it would take around 2000 years to complete a revolution.[27]Theta Pegasi marks the horse's eye.[22] Also known as Biham, it is a 3.43-magnitude white main sequence star of spectral type A2V, around 1.8 times as massive, 24 times as luminous, and 2.3 times as wide as the Sun.[28]
Alpha (Markab),Beta (Scheat), andGamma (Algenib), together withAlpha Andromedae (Alpheratz or Sirrah) form the largeasterism known as theSquare of Pegasus. The brightest of these, Alpheratz was also known as both Delta Pegasi and Alpha Andromedae before being placed in Andromeda in 1922 with the setting of constellation boundaries. The second brightest star is Scheat, ared giant of spectral type M2.5II-IIIe located around 196 light-years away from Earth.[29] It has expanded until it is some 95 times as large, and has a totalluminosity 1,500 times that of the Sun.[30] Beta Pegasi is asemi-regular variable that varies frommagnitude 2.31 to 2.74 over a period of 43.3 days.[31] Markab and Algenib are blue-white stars of spectral types B9III and B2IV located 133 and 391 light-years distant respectively.[32][33] Appearing to have moved off the main sequence as their core hydrogen supply is being or has been exhausted, they are enlarging and cooling to eventually become red giant stars.[34][35] Markab has an apparent magnitude of 2.48,[32] while Algenib is aBeta Cephei variable that varies between magnitudes 2.82 and 2.86 every 3 hours 38 minutes, and also exhibits some slow pulsations every 1.47 days.[36]
Eta andOmicron Pegasi mark the left knee and Pi Pegasi the left hoof, whileIota andKappa Pegasi mark the right knee and hoof.[22] Also known as Matar, Eta Pegasi is the fifth-brightest star in the constellation. Shining with an apparent magnitude of 2.94, it is a multiple star system composed of a yellow giant of spectral type G2 and a yellow-white main sequence star of spectral type A5V that are 3.2 and 2.0 times as massive as the Sun. The two revolve around each other every 2.24 years. Farther afield is a binary system of two G-type main sequence stars, that would take 170,000 years to orbit the main pair if they are in fact related.[37] Omicron Pegasi has a magnitude of 4.79. Located 300 ± 20 light-years distant from Earth,[38] it is a white subgiant that has begun to cool, expand and brighten as it exhausts its core hydrogen fuel and moves off the main sequence.[39] Pi1 and Pi2 Pegasi appear as an optical double to the unaided eye as they are separated by 10 arcminutes, and are not a true binary system.[40] Located 289 ± 8 light-years distant,[38] Pi1 is an ageing yellow giant of spectral type G6III, 1.92 times as massive and around 200 times as luminous as the Sun.[41] Pi2 is a yellow-white subgiant that is 2.5 times as massive as the Sun and has expanded to 8 times the Sun's radius and brightened to 92 times the Sun's luminosity. It is surrounded by a circumstellar disk spinning at 145 km a second,[40] and is 263 ± 4 light-years distant from Earth.[38]
Twelve star systems have been found to haveexoplanets.51 Pegasi was the first Sun-like star discovered to have an exoplanet companion;[45]51 Pegasi b (unofficially named Bellerophon,[46] officially named Dimidium[47]) is ahot Jupiter close to its star, completing an orbit every four days. Spectroscopic analysis ofHD 209458 b, anextrasolar planet in this constellation, has provided the first evidence of atmosphericwater vapor beyond theSolar System,[48][49][50] while extrasolar planets orbiting the starHR 8799 also in Pegasus are the first to be directly imaged.[51][52][53]V391 Pegasi is a hot subdwarf star that has been found to have a planetary companion.[54]
M15 (NGC 7078) is aglobular cluster of magnitude 6.4, 34,000 light-years from Earth. It is a Shapley class IV cluster,[55] which means that it is fairly rich and concentrated towards its center. M15 was discovered in 1746 byJean-Dominique Maraldi.[56] Pease 1 is aplanetary nebula located within theglobular cluster and was the first planetary nebula known to exist within a globular cluster.[57] It has an apparent magnitude of 15.5.[58]
NGC 7331 is aspiral galaxy located in Pegasus, 38 million light-years distant with a redshift of 0.0027. It was discovered by musician-astronomerWilliam Herschel in 1784 and was later one of the first nebulous objects to be described as "spiral" byWilliam Parsons. Another of Pegasus's galaxies isNGC 7742, a Type 2Seyfert galaxy. Located at a distance of 77 million light-years with a redshift of 0.00555, it is anactive galaxy with asupermassive black hole at its core. Its characteristic emission lines are produced by gas moving at high speeds around the central black hole.[59]
Pegasus is also noted for its more unusual galaxies and exotic objects.Einstein's Cross is aquasar that has beenlensed by a foreground galaxy. Theelliptical galaxy is 400 million light-years away with aredshift of 0.0394, but the quasar is 8 billion light-years away. The lensed quasar resembles a cross because the gravitational force of the foreground galaxy on its light creates four images of the quasar.[59]Stephan's Quintet is another unique object located in Pegasus. It is a cluster of five galaxies at a distance of 300 million light-years and a redshift of 0.0215. First discovered byÉdouard Stephan, a Frenchman, in 1877, the Quintet is unique for its interacting galaxies. Two of the galaxies in the middle of the group have clearly moms tocollide, sparking massive bursts of star formation and drawing off long "tails" of stars. Astronomers have predicted that all five galaxies may eventually merge into one large elliptical galaxy.[59]
^While parts of the constellation technically rise above the horizon to observers between the 53°S and 87°S, stars within a few degrees of the horizon are to all intents and purposes unobservable.[10]
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[15]
^Conner, Nancy. The Everything Classical Mythology Book: from the Heights of Mount Olympus to the Depths of the Underworld - All You Need to Know about the Classical Myths. 2nd ed., Adams Media, 2010.
^Templeton, Matthew (16 July 2010)."Delta Scuti and the Delta Scuti Variables".Variable Star of the Season. AAVSO (American Association of Variable Star Observers). Retrieved16 March 2016.
^Ghezzi, L.; Cunha, K.; Smith, V. V.; de Araújo, F. X.; Schuler, S. C.; de la Reza, R. (2010). "Stellar Parameters and Metallicities of Stars Hosting Jovian and Neptunian Mass Planets: A Possible Dependence of Planetary Mass on Metallicity".The Astrophysical Journal.720 (2):1290–1302.arXiv:1007.2681.Bibcode:2010ApJ...720.1290G.doi:10.1088/0004-637X/720/2/1290.S2CID118565025.
^Moro-Martín, A.; Marshall, J. P.; Kennedy, G.; Sibthorpe, B.; Matthews, B. C.; Eiroa, C.; Wyatt, M. C.; Lestrade, J.-F.; Maldonado, J.; Rodriguez, D.; Greaves, J. S.; Montesinos, B.; Mora, A.; Booth, M.; Duchêne, G.; Wilner, D.; Horner, J. (2015). "Does the Presence of Planets Affect the Frequency and Properties of Extrasolar Kuiper Belts? Results from the Herschel Debris and Dunes Surveys".The Astrophysical Journal.801 (2): 28.arXiv:1501.03813.Bibcode:2015ApJ...801..143M.doi:10.1088/0004-637X/801/2/143.S2CID55170390. 143.
^Kaler, James B. (30 November 2007)."Xi Pegasi".Stars. University of Illinois. Retrieved15 March 2016.
^Boyajian, Tabetha S.; von Braun, Kaspar; van Belle, Gerard; Farrington, Chris; Schaefer, Gail; Jones, Jeremy; White, Russel; McAlister, Harold A.; ten Brummelaar, Theo A.; Ridgway, Stephen; Gies, Douglas; Sturmann, Laszlo; Sturmann, Judit; Turner, Nils H.; Goldfinger, P. J.; Vargas, Norm (2013). "Stellar Diameters and Temperatures. III. Main-sequence A, F, G, and K Stars: Additional High-precision Measurements and Empirical Relations".The Astrophysical Journal.771 (1): 31.arXiv:1306.2974.Bibcode:2013ApJ...771...40B.doi:10.1088/0004-637X/771/1/40.S2CID14911430. 40. See Table 3.
^Skiff, B. A. (October 2014), "Catalogue of Stellar Spectral Classifications",Lowell Observatory, VizieR On-line Data Catalog: B/mk,Bibcode:2014yCat....1.2023S.
^Barstow, M. A.; Holberg, J. B.; Koester, D. (1994), "Extreme Ultraviolet Spectrophotometry of HD16538 and HR:8210 Ik-Pegasi",Monthly Notices of the Royal Astronomical Society,270 (3): 516,Bibcode:1994MNRAS.270..516B,doi:10.1093/mnras/270.3.516
^Wonnacott, D.; Kellett, B. J.; Stickland, D. J. (1993), "IK Peg - A nearby, short-period, Sirius-like system",Monthly Notices of the Royal Astronomical Society,262 (2):277–284,Bibcode:1993MNRAS.262..277W,doi:10.1093/mnras/262.2.277
^Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters".Harvard College Observatory Bulletin.849 (849):11–14.Bibcode:1927BHarO.849...11S.
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