Stars Bayer catalogued what he counted as 23 stars in the constellation, giving them the
Bayer designations Alpha to Psi. He saw
Pi 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 be
Pi1 and
Pi2 Pegasi and that Bayer had seen them as a single star. 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. It is an irregular variable, its
apparent magnitude varying between 2.37 and 2.45. Lying near Enif is
AG Pegasi, an unusual star that brightened to magnitude 6.0 around 1885 before dimming to magnitude 9. It is composed of a
red giant and
white 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 slowest
nova ever recorded. Three stars with Bayer designations that lie within the Great Square are variable stars.
Phi and
Psi Pegasi are pulsating red giants, while
Tau Pegasi (the proper name is Salm), is a
Delta Scuti variable—a class of short period (six hours at most) pulsating stars that have been used as
standard candles and as subjects to study
astroseismology. Rotating rapidly with a
projected 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 an
effective temperature of 7,762 K, it is a white star with a spectral type of A5IV.
Zeta,
Xi,
Rho and
Sigma Pegasi mark the horse's neck. 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. It is a
slowly pulsating B star that varies slightly in luminosity with a period of 22.952 ± 0.804 hours, completing 1.04566 cycles per day. Xi lies 2 degrees northeast, and is a
yellow-white main sequence star of spectral type F6V that is 86% larger and 17% more massive that the Sun, and radiate 4.5 times the
solar luminosity. It has a red dwarf companion that is 192.3 au distant. 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.
Theta Pegasi marks the horse's eye. 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.
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,
Eta and
Omicron Pegasi mark the left knee and Pi Pegasi the left hoof, while
Iota and
Kappa Pegasi mark the right knee and hoof. 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. Omicron Pegasi has a magnitude of 4.79. Located 300 ± 20 light-years distant from Earth, 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. 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. Located 289 ± 8 light-years distant, 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, and
white dwarf in very close orbit; the latter a candidate for a future
type Ia supernova as its main star runs out of core hydrogen fuel and expands into a giant and transfers material to the smaller star. Twelve star systems have been found to have
exoplanets.
51 Pegasi was the first Sun-like star discovered to have an exoplanet companion;
51 Pegasi b (unofficially named Bellerophon, officially named Dimidium) is a
hot Jupiter close to its star, completing an orbit every four days. Spectroscopic analysis of
HD 209458 b, an
extrasolar planet in this constellation, has provided the first evidence of atmospheric
water vapor beyond the
Solar System, while extrasolar planets orbiting the star
HR 8799 also in Pegasus are the first to be directly imaged.
V391 Pegasi is a hot subdwarf star that has been found to have a planetary companion.
Book of Fixed Stars, dated 1009-10
Named stars Deep-sky objects photographed by the
James Webb Space Telescope 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 center. M15 was discovered in 1746 by
Jean-Dominique Maraldi. Pease 1 is a
planetary nebula located within the
globular cluster and was the first planetary nebula known to exist within a globular cluster. It has an apparent magnitude of 15.5.
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.
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 moms 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. ==Namesakes==