The Pegasus Constellation

Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and is one of the 88 constellations recognised today. 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 and Equuleus to the west, Aquarius to the south and Pisces to the south and east. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined as a polygon of 35 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 21h 12.6m and 00h 14.6m, while the declination coordinates are between 2.33° and 36.61°. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 53°S. Pegasus with the foal Equuleus next to it, as depicted in Urania’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, although one of the stars, Delta Pegasi or Sirrah, is now officially considered to be Alpha Andromedae, part of Andromeda, and is more usually called “Alpheratz”.

Bayer catalogued what he counted as 23 stars in the constellation, giving them the Bayer designations Alpha to Psi. 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. He numbered 89 stars (now with Flamsteed designations), though 6 and 11 turned out to be stars in Aquarius. Within the constellation’s borders, there are 177 stars of apparent magnitude 6.5 or greater. Epsilon Pegasi, also known as Enif, marks the horse’s muzzle. It’s the brightest star in Pegasus, is an orange supergiant of spectral type K21b that is around 12 times as massive as the Sun and is around 690 light-years distant from Earth. It is an irregular variable, its apparent magnitude varying between 2.37 and 2.45. Alpha (Markab),  Beta (Scheat), and Gamma (Algenib), together with Alpha Andromedae (Alpheratz or Sirrah) form the large asterism known as the Square 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, a red giant of spectral type M2.5II-IIIe located around 196 light-years away from Earth. It has expanded until it is some 95 times as large, and has a total luminosity 1,500 times that of the Sun. Beta Pegasi is a semi-regular variable that varies from magnitude 2.31 to 2.74 over a period of 43.3 days. Markab and Algenib are blue-white stars of spectral types B9III and B2IV located 133 and 391 light-years distant respectively. 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. Markab has an apparent magnitude of 2.48, while Algenib is a Beta 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.

M15 (NGC 7078) is a globular cluster of magnitude 6.4, 34,000 light-years from Earth. It is a Shapley class IV cluster, which means that it is fairly rich and concentrated towards its centre. M15 was discovered in 1746 by Jean-Dominique Maraldi. NGC 7331 is a spiral galaxy located in Pegasus, 38 million light-years distant with a redshift of 0.0027. It was discovered by musician-astronomer William Herschel in 1784 and was later one of the first nebulous objects to be described as “spiral” by William Parsons. Another of Pegasus’s galaxies is NGC 7742, a Type 2 Seyfert galaxy. Located at a distance of 77 million light-years with a redshift of 0.00555, it is an active galaxy with a supermassive black hole at its core. Its characteristic emission lines are produced by gas moving at high speeds around the central black hole. Pegasus is also noted for its more unusual galaxies and exotic objects. Einstein’s Cross is a quasar that has been lensed by a foreground galaxy. The elliptical galaxy is 400 million light-years away with a redshift 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. 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 by Édouard 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 begun to collide, 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.

The Eta Pegasids radiate from the area near Eta Pegasi every year on May 30. Credit: Constellation Guide, Wikipedia.