Constellations: Pegasus 'the Winged Horse' Content from the guide to life, the universe and everything

Constellations: Pegasus 'the Winged Horse'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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The constellation Pegasus next to Andromeda

Short form:Peg
Area:1,121 square degrees
Co-ordinates1:Right Ascension 22h, Declination +20°

The constellation of Pegasus2 the winged horse is easily definable. Imagine a line from Ursa Major to the Pole Star then onto Cassiopeia; the great square of Pegasus follows on. The head of the horse is bent down towards the equator, extending to Cygnus and Delphinus.

The convention when referring to one celestial object in relation to another is to refer to it being north, south, east or west of it. To orientate yourself, imagine you are standing with your back to north, looking south. East is to your left and west is to your right. Now look at the portion of sky just above the horizon. South is at the bottom of this piece of sky so north must be at the top (right up and over your head behind you). East is still to the left and west to the right. Therefore in this case, Pegasus being apparently to the right of Andromeda can be said to be west of it. For observers in the southern hemisphere the positions are reversed. North is now down and south is up, while east is now to the right and west to the left.


There are two stories concerning the birth of Pegasus. In one version, Pegasus sprang to life from the blood of Medusa, the Gorgon, who had just been killed by Perseus. The other story has Medusa's blood spilling into the seafoam, Poseidon's domain, creating a demigod offspring.

The vain queen Cassiopeia offended the god of the oceans, Poseidon, when she boasted that her own and her daughter's beauty was greater than that of the sea nymphs, the Nereids, who were his handmaidens. Angry Poseidon sent storms to ravage the coast of their kingdom as punishment for the queen's vanity. To bring an end to the suffering, King Cepheus went to the Oracle of Ammon for advice and was told to sacrifice his daughter Andromeda to the sea monster  Cetus, to placate Poseidon. Andromeda was duly chained to the rocks on the coast for the sea monster to devour. She was saved at the last moment by the hero Perseus, riding in on his winged horse Pegasus.

Later on, the goddess Athena (the favourite daughter of Zeus) gave Bellerophon a golden bridle which he used to capture Pegasus. With the flying horse at his command, Bellerophon was invincible; he even defeated the feared Chimera3. Bellerophon went too far, though, when he attempted to get Pegasus to take him to Mount Olympus, the realm of the gods, which was forbidden. Pegasus refused to obey and threw Bellerophon off his back, causing him severe injuries. Because Pegasus was mortal, at the end of his life he was placed in the heavens as a constellation by Zeus.

Another interesting story, before the International Astronomical Union (IAU) reclassified the constellations, concerns the four bright stars of the original Square of Pegasus. They were supposed to be King Diomedes' flesh-eating mares, the capture of which was the eighth of Hercules' 12 labours decreed by the Oracle of Delphi. Alexander the Great's famous mount Bucephalus, who really existed, was supposed to be descended from these mares.

Principal Stars

The scientific star names are simple to understand (if you know your Greek alphabet). 'Alpha Pegasi' means that it is the brightest star in the constellation Pegasus. The next brightest is 'beta', and so on. Combined with the genitive name, this is known as the 'Bayer designation'. Some stars have proper names as well; for example alpha Pegasi is known as Markab. Other stars are known by their catalogue number.

The Great Square of Pegasus is an easily recognisable feature, with four bright corners and very few visible stars inside the square. Three of the four corners are alpha Pegasi, Markab, in the south-west, which is the brightest star in Pegasus; gamma Pegasi, Algenib, in the south-eastern corner; and beta Pegasi, Scheat, in the north-western. Scheat, a red giant, has a strong solar wind causing the star to be surrounded by a thin outer layer of gases.

The fourth corner, however, presents one of those anomalies that occurs occasionally in the nomenclature of astronomy. In 1930 the IAU decided to clear up the ambiguous borders to the constellations and set clearly defined limits for all of the 88 constellations. In doing so they allocated the north-eastern star of the square, Alpheratz, sometimes known as Sirrah, to the neighbouring constellation of Andromeda. Up to that time Alpheratz had been classified delta Pegasi, but with this change was re-classified as alpha Andromedae.

Another clearly identifying feature to the square is that Scheat is one corner of a triangle of stars; the other two, eta Pegasi Matar and mu Pegasi, lie outside the square to its western side.

Star Table

StarDesignationName or
catalogue number
(light years4)
α Pegalpha PegasiMarkab
'the Saddle'
+2.49140Blue-white giant
β Pegbeta PegasiScheat
'the foreleg'
+2.42200Red giant
γ Peggamma PegasiAlgenib
'the flank'
ε Pegepsilon PegasiEnif
'the muzzle'
+2.39670Orange supergiant
ζ Pegzeta PegasiHomam
'high spirit'
+3.4142Blue-white giant
η Pegeta PegasiMatar
'the shoulder'
+2.9215Double star system
θ Pegtheta PegasiBaham
+3.567White type A2
ι Pegiota Pegasi24 Pegasi+3.738Multiple star system
κ Pegkappa PegasiJih+4.14115Yellow-white dwarf
51 PegSAO 90896Helvetios+5.4950Yellow dwarf: an extrasolar
planet discovered Oct 1995
HD 209458SAO 107623V376 Pegasi+4.29150Yellow dwarf: an extrasolar
planet discovered Nov 1999
HD 210702SAO 107729unnamed+5.94182Orange subgiant: an extrasolar
planet discovered in April, 2007

New General Catalogue Objects

The NGC was compiled by John Louis Emil Dreyer, the director of the Armagh Observatory from 1882 to 1916. The objects in the catalogue appear in order of their position in the sky; the object with the lowest Right Ascension is first. NGC 1 and NGC 2 both appear in the constellation of Pegasus.

(light years)
NGC 1UGC 57SB Spiral5  galaxy+13.7150 millionFirst object listed in the
New General Catalogue
NGC 2UGC 59SB Spiral galaxy+14.2300 millionAppears as a neighbour of NGC 1
but is twice as far away from Earth
NGC 7078M15Globular cluster+6.232,600Contains planetary nebula Pease 1
NGC 7217UGC 11914SA spiral galaxy+11.042 millionRing of dust surrounds the nucleus
NGC 7318
Stephan's Quintet5 galaxies+14300 millionColliding galaxies
NGC 7331Caldwell 306Edge-on
spiral galaxy
+9.542 millionSupernova SN 1959D was
discovered in this galaxy
NGC 7742UGC 12760Type 2 Seyfert galaxy+12.472 millionFace-on SA spiral
galaxy with ring

Stephan's Quintet is a set of five colliding galaxies, labelled NGC 7317, NGC 7318A, NGC 7318B, NGC 7319 and NGC7320. The whole cosmic dance provides a stunning view in small telescopes. Stephan's Quintet was featured on the BBC's monthly astronomy TV programme The Sky At Night in November, 2007.

Meteor Showers

Meteor showers occur when the Earth passes through the debris path left by the tail of a comet. There are two annual meteor showers every July/August called the Pegasids, so named because they appear to originate from the constellation of Pegasus. The July Pegasids are typically active from 7 July for around a week. The Upsilon Pegasids commence around the end of July and last until the end of August.

Extrasolar Planets in Pegasus

The planet orbiting Helvetios (51 Pegasi) was the first one to be found by the Doppler spectroscopy technique: a plan­et's gravita­t­ional pull causes a 'wob­ble' in the par­ent star as it orbits it; this in turn causes a change in the frequency of the star's light, due to the Doppler Effect. Designated 51 Pegasi b, and later named Dimidium by the IAU, this was the first extrasolar planet found to be orbiting a stable sun. Three others had been found previously but their parent star is a pulsar7. The discovery of 51 Pegasi b was announced on 6 October, 1995, causing immense excitement throughout the field of astronomy. Bellerophon is the unofficial name this planet has been dubbed, after the mythical captor of Pegasus. 51 Pegasi b is half Jupiter's mass8 but bigger in actual size. However, it is extremely close to its star, a mere 7.5 million km (4.6m miles) distant, so its year only lasts 4.25 days. This orbit compares to around 13% of the orbital period of our innermost planet, Mercury.

A planet designated HD 209458 b orbiting yellow dwarf star HD 209458 (aka V376 Pegasi) was detected in November 1999. Astronomers discovered that the planet's atmosphere contains sodium. This planet is in an even closer orbit around its sun than the previous one mentioned, and its atmosphere is literally boiling away into space, so it leaves a comet-like trail of blue-glowing energised hydrogen gas in its wake as it orbits the star. HD 209458 b has been given the official name 'Osiris'.

Yet another extrasolar planet was discovered in Pegasus in April, 2007, orbiting HD 210702, an orange subgiant. HD 210702 b is twice the mass of Jupiter but its orbital period (year) is 341 days. HAT-P-8 b is a superjovian world positively whizzing around its star in just over three Earth days. V391 Peg b is a superjovian planet orbiting a dying star at 1.7AU. HD 219828 b is a so-called 'hot Neptune' planet due to its mass being similar to that of Neptune, and its close proximity to the parent star.

In November 2008 it was announced that a solar system, discovered earlier in 2008, had been imaged around HR 8799, a 6th mag gamma Doradus-variable star almost 125 light years distant. This star is located midway between alpha and beta Pegasi which form two points of the easily-recognisable 'Square of Pegasus', so it should be easy to locate with binoculars. Three gas giant planets were noted at distances of 24AU (HR 8799 d), 38AU (HR 8799 c) and 68AU (HR 8799 b), with a fourth, HR 8799 e detected in 2010.

HR 8799's dust disc stands out as one of the most massive in orbit around any star within 300 light years of Earth.
– Ben Zuckerman of UCLA

A further discovery in 2009 revealed a gas giant planet BD14 4559 b orbiting the orange dwarf star BD14 4559. The robotic observatory WASP (Wide Angle Search for Planets) searches for extrasolar planets using the transiting technique. WASP-10 b is a superjovian world which orbits its star in just over three Earth days. Another hot gas giant planet, WASP-21 b, was discovered in 2010. HD 220773 b orbits a sun-like star at a distance of approximately 5 AU. WASP-52 b, WASP-59 b and WASP-60 b were discovered in 2011. HD 220773 b is a Jovian-sized world discovered in 2012. Planet WASP-102 b is a Saturn-sized world orbiting its star in just 2.7 days. WASP-114 b, almost twice the mass of Jupiter, is so close to its star that its year is just one and a half days long!

Planet HD 214823 b, discovered in 2016, is over 19 × the mass of Jupiter, and likely a brown dwarf companion to its sun-like parent star. Also discovered in 2016 was planet HIP 109600 b, a gas giant orbiting its sun-like star in 232 days.

Pegasus in Modern Culture

  • There is a Pegasus Theatre in Oxford, UK.
  • Pegasus is the name of the official car of the British Prime Minister. It's bullet-proof and bomb-proof9.
  • The Pegasus was the spaceship in the BBC TV series Space Odyssey: Voyage To The Planets.
  • The Rolls-Royce Pegasus is a jet engine powering all versions of the Harrier military aircraft.
  • The Pegasus Award is an American music award, usually in the science fiction genre.
  • In the James Bond spoof Johnny English starring Rowan Atkinson, Pegasus is the head of MI7.
  • Crossing the Canal de Caen à la Mer at the village of Bénouville, in Calvados, France, is the Pegasus Bridge, an important defensive point in the Battle of Normandy during the Second World War.
  • There is a Pegasus Bay on the east coast of the South Island of New Zealand, named after the vessel Pegasus which surveyed the bay in 1809.
  • The Ferranti Pegasus computer of 1955 is arguably the zenith of the British contribution to computer development, and with a decent claim to being the world's first business computer.
1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2From the Greek word pégasos meaning 'strong'.3Homer described the Chimera as having a lion's head, a goat's body and a serpent's tail.4A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.5There are three main classes of galaxy: elliptical, spiral and lenticular. Spirals are further sub-divided into barred (SB) and unbarred (SA).6Sir Patrick Moore's catalogue.7A small, ultradense star that rotates rapidly and flashes like a lighthouse.8The mass of extrasolar planets are described in comparison to the largest planet in our own Solar System, Jupiter, mainly because most of the extrasolar planets discovered have been gas giants.9Please don't take that as a challenge.

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