Pegasus
“The Winged Horse” · Northern · best around October evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
The flying horse born of Medusa's blood, marked by the Great Square — four medium stars framing an area of sky strikingly empty to the naked eye. Counting the faint stars you can see inside the Square is a classic sky-quality test. The first exoplanet found around a normal star (51 Pegasi b, 1995) orbits a star just off the Square's edge.
How to find it
Autumn evenings: face south and find the huge, slightly tilted square of four stars.
✦ What lives inside it
- The Great Square
- M15 globular cluster
The deep dive
Researched for the Atlas from Wikipedia — Pegasus (constellation) (13,439 characters read) · updated Sep 20, 2026
01 The seventh-largest patch of sky
Pegasus commands a genuinely imposing slice of the celestial sphere, covering 1,121 square degrees and ranking as the seventh-largest of all 88 modern constellations. To put that in perspective, you could stack roughly 2,200 full Moons side by side and still not fill it. Its official borders were drawn in 1930 by Belgian astronomer Eugène Delporte, who traced a polygon of 35 straight segments to fence it in. The right ascension of those borders runs from 21h 12.6m all the way to 00h 14.6m, and the declination spans from just 2.33° north of the celestial equator up to 36.61° — meaning Pegasus sits comfortably in the northern sky but dips close enough to the equator that observers as far south as 53°S can see the whole figure on the right night. Eight constellations press against it: Andromeda and Lacerta to the north, Cygnus to the northwest, Vulpecula, Delphinus, and Equuleus to the west, and Aquarius and Pisces crowding in from the south.
02 A square that lost one of its corners
The Great Square of Pegasus is one of the most recognizable signposts in the autumn sky, yet one of its four corner stars officially belongs to a different constellation entirely. The star now called Alpheratz — Alpha Andromedae — was once labeled both Delta Pegasi and Sirrah, counted as part of Pegasus for centuries. When the International Astronomical Union formalized constellation boundaries in 1922, the star was reassigned to Andromeda, leaving Pegasus geometrically short a corner. The remaining three Square stars each have their own character: Scheat (Beta Pegasi) is a bloated red giant of spectral type M2.5II-IIIe sitting about 196 light-years away, roughly 95 times the Sun's diameter, and 1,500 times its luminosity, varying between magnitudes 2.31 and 2.74 over 43.3 days. Markab (Alpha) is a blue-white giant of spectral type B9III at 133 light-years, magnitude 2.48. Algenib (Gamma) is a hotter Beta Cephei variable of type B2IV at 391 light-years, flickering between magnitudes 2.82 and 2.86 every 3 hours and 38 minutes while also showing slower pulsations every 1.47 days.
03 Enif and the slowest nova ever
The brightest star in Pegasus is not one of the famous Square stars but Enif, the horse's muzzle. This orange supergiant of spectral type K2Ib is around 12 times as massive as the Sun and sits approximately 690 light-years away — so far that the light you see left during the medieval period. Its apparent magnitude wanders irregularly between 2.37 and 2.45, making it a mild variable as supergiants go. Lying close to Enif on the sky is AG Pegasi, a far stranger object. It is a symbiotic binary — a red giant of roughly 2.5 solar masses paired with a white dwarf of about 0.6 solar masses in a single system. Around 1885, AG Pegasi brightened dramatically to magnitude 6.0, just at the edge of naked-eye visibility, before slowly fading back toward magnitude 9. Because the outburst has taken more than 150 years to play out, astronomers have described it as the slowest nova ever recorded, a kind of stellar explosion stretched into a human lifetime rather than days or weeks.
04 Variable stars scattered across the figure Deeper
Beyond the well-known semi-regular giant Scheat, Pegasus harbors a surprisingly varied zoo of variable stars. Tau Pegasi — properly named Salm — is a Delta Scuti variable, a class of short-period pulsators that beat in six hours or less and have been exploited both as distance standard candles and as probes of stellar interiors through astroseismology. Salm rotates rapidly with a projected rotational velocity of 150 km per second, shines with nearly 30 times the Sun's luminosity, and pulses every 56.5 minutes, with an outer atmosphere at an effective temperature of 7,762 K giving it a white color and a spectral classification of A5IV. Phi and Psi Pegasi are pulsating red giants, adding lower-frequency variability to the constellation's light show. Zeta Pegasi, known as Homam and the brightest neck star at magnitude 3.4, is a slowly pulsating B star of spectral type B8V at about 209 light-years; it varies slightly with a period of 22.952 ± 0.804 hours, completing 1.04566 cycles per day. In total, 177 stars within Pegasus's borders reach apparent magnitude 6.5 or brighter — a rich hunting ground for variable star observers.
05 Stellar systems with hidden companions Deeper
Several of Pegasus's named stars reveal remarkable hidden architecture on closer inspection. Xi Pegasi is a yellow-white main-sequence star of spectral type F6V, 86 percent larger and 17 percent more massive than the Sun, radiating 4.5 times the solar luminosity. Orbiting it at a separation of 192.3 astronomical units is a red dwarf companion; if the smaller star truly circles the larger, one orbit would take roughly 2,000 years. Eta Pegasi, called Matar and the constellation's fifth-brightest star at magnitude 2.94, is itself a multiple system: a yellow giant of spectral type G2 and a yellow-white main-sequence star of type A5V, carrying 3.2 and 2.0 solar masses respectively, revolving around each other every 2.24 years. Even farther out, a binary pair of G-type main-sequence stars may be gravitationally linked to the inner pair — if so, their orbital period around Matar would be around 170,000 years. IK Pegasi presents a more urgent case: it pairs an A-type main-sequence star with a white dwarf in very close orbit, and astronomers flag the white dwarf as a candidate for a future Type Ia supernova once mass transfer from its expanding companion tips it over the limit.
06 51 Pegasi and the exoplanet revolution
One star in Pegasus quietly rewrote astronomy. In 1995, 51 Pegasi — a Sun-like star just over 50 light-years from Earth — became the first solar-type star confirmed to host a planet. Its companion, 51 Pegasi b, is a gas giant now officially named Dimidium and unofficially long called Bellerophon after the mythological hero who rode Pegasus. Dimidium is a hot Jupiter, sitting so close to its star that it completes a full orbit in just four days. That shocking orbital period overturned earlier assumptions that gas giants could only form far from their stars and forced a complete rethinking of planetary migration. Pegasus continued to deliver firsts: spectroscopic analysis of HD 209458 b, also in this constellation, provided the first detection of atmospheric water vapor on a planet outside the Solar System. And the star HR 8799 in Pegasus hosts the first extrasolar planets to be directly imaged. In total, twelve star systems within Pegasus's borders have confirmed exoplanets, making the constellation a landmark address in the history of planetary science.
07 M15 and the nebula hidden inside it
Messier 15, also catalogued as NGC 7078, is the showpiece deep-sky object of Pegasus — a globular cluster shining at magnitude 6.4 and lying 34,000 light-years from Earth. It falls in Shapley concentration class IV, meaning it is fairly rich and noticeably concentrated toward its center, giving it a bright, compressed core that even a modest telescope resolves into a mist of individual stars. Jean-Dominique Maraldi discovered M15 in 1746, making it one of the earlier entries in the globular cluster catalog. Tucked inside this dense swarm is Pease 1, a planetary nebula — the expanding shell of gas expelled by a dying star. Pease 1 carries an apparent magnitude of 15.5, well beyond casual telescopic reach, but its historical importance is enormous: it was the first planetary nebula ever identified within a globular cluster, a discovery that confirmed such nebulae could form in the ancient stellar populations that globulars contain.
08 A Babylonian field before the winged horse
The stars of Pegasus carry a history that predates the Greek myth by centuries. In Babylonian astronomy, the region was known as IKU, meaning "field," and was defined by four stars — three of which were later incorporated into the Greek figure of Hippos, the horse. When Greek astronomers took up these stars, they attached them to the myth of Pegasus, a winged horse born from a violent moment: when Perseus severed the head of Medusa, who had been made pregnant by the sea god Poseidon, Pegasus and his brother Chrysaor sprang from her blood. The horse later became the mount of the hero Bellerophon, who used Pegasus and the help of Athena to slay the fire-breathing Chimera. Bellerophon's story ends in hubris — after losing his children, he asked Pegasus to carry him to Mount Olympus, but Zeus either hurled a thunderbolt or sent a gadfly to unseat him, and Bellerophon plunged back to Earth. Pegasus alone continued to the heavens, where he became a constellation.
09 How al-Sufi drew a complete horse Deeper
Most classical and medieval sky-mappers depicted Pegasus as the forepart of a horse only — a torso and front legs rising from the ocean, with no hindquarters at all. The tenth-century Persian astronomer al-Sufi broke from that tradition deliberately, drawing Pegasus as a full horse facing east. To do so he had to borrow stars from neighboring constellations: the stars of Lacerta the lizard became the horse's head in his scheme. He mapped Beta Pegasi as the right foreleg and assigned Eta, Mu, and Lambda Pegasi to the left foreleg, while 9 Pegasi marked the hind legs. Pi Pegasi and Mu Cygni represented the back, and Iota and Kappa Pegasi outlined the belly. This expansive, anatomically complete vision stands as one of the most distinctive re-imaginings of the figure in the history of astronomy, showing how the same stars could tell very different stories depending on cultural perspective and the willingness to recruit stars from across constellation borders.
10 Pegasus across Asian and Indigenous skies
Greek mythology is only one lens through which humanity has read these stars. In Chinese astronomy the region of Pegasus falls within the vast northern sky palace known as the Black Tortoise of the North. Within it, Epsilon and Theta Pegasi join Alpha Aquarii to form the asterism Wei, meaning "rooftop," with Theta Pegasi marking the roof's apex — a domestic, architectural image far removed from a flying horse. In Hindu astronomy Pegasus sits inside the 25th nakshatra, the lunar mansion called Purva Bhadrapada, where the figure was understood not as a horse at all but as a bedstead, a resting place for the Moon as it traveled its monthly path across the sky. For the Warrau and Arawak peoples of Guyana, the stars of the Great Square — spanning parts of both Pegasus and Andromeda — represented a barbecue grill that seven hunters carried up to the sky in the myth of Siritjo. Each tradition found its own meaning written in the same scattered points of light.
11 Gravitational lenses and colliding galaxies Deeper
Beyond its stars, Pegasus is a showcase for some of the most dramatic phenomena in the deep universe. Einstein's Cross is a quasar 8 billion light-years away whose light passes through a foreground elliptical galaxy only 400 million light-years distant. The galaxy's gravity bends the quasar's light into four separate images arranged in a cross pattern — a textbook example of strong gravitational lensing that validates a core prediction of general relativity in one glance through a telescope. Stephan's Quintet, first spotted by French astronomer Édouard Stephan in 1877, is a cluster of five galaxies roughly 300 million light-years away with a redshift of 0.0215. Two of the galaxies near the group's center are clearly in the process of colliding, which triggers massive bursts of star formation and draws long tidal tails of stars out into intergalactic space. Astronomers predict that all five galaxies may eventually merge into a single large elliptical galaxy. NGC 7742, a Type 2 Seyfert galaxy 77 million light-years distant, adds an active galactic nucleus to the collection, its characteristic emission lines produced by gas racing around a central supermassive black hole.
12 NGC 7331 and its spiral pedigree
NGC 7331 is the kind of galaxy that rewards even a modest telescope on a dark night. Discovered in 1784 by the musician-turned-astronomer William Herschel, it is a spiral galaxy 38 million light-years from Earth with a redshift of 0.0027. Its historical significance reaches beyond its discovery date: NGC 7331 was later among the first nebulous objects that William Parsons — the third Earl of Rosse, who built the great Leviathan telescope in Ireland — described as "spiral," helping to establish the concept of spiral structure in what were then still called nebulae and are now understood to be entire galaxies. The galaxy's angular size on the sky and its relatively modest distance make it one of the better examples of an external spiral visible from northern latitudes in autumn, offering a view of a system broadly comparable in scale to our own Milky Way seen from tens of millions of light-years away.