Pegasus is a huge constellation that dominates a large area of the autumn sky and is especially prominent during autumn evenings in the Northern Hemisphere. Best known for the Great Square of Pegasus, it contains several notable stars and deep-sky objects and provides a useful landmark for finding other parts of the autumn sky.
Pegasus is the 7th largest constellation
Pegasus is the seventh-largest of the 88 constellations, covering about 1,121 square degrees, or nearly 3% of the entire sky. Despite its size, many of its stars are relatively faint, so its most recognizable feature is the Great Square of Pegasus. This large asterism is formed by four stars of similar brightness: Markab, Scheat and Algenib in Pegasus, and Alpheratz in neighboring Andromeda. The pattern forms much of the Winged Horse’s body, with each side spanning roughly 15° of sky, making the Square much larger than it may first appear when looking at a star chart.
Named after the mythological winged horse
Pegasus represents the famous winged horse of Greek mythology. According to the myth, Pegasus sprang from the body of Medusa after she was beheaded by Perseus. The hero Bellerophon later captured and rode Pegasus, most famously when he defeated the fire-breathing Chimera. Pegasus eventually reached Mount Olympus, where he served Zeus by carrying his thunderbolts, and Zeus later placed him among the stars. Fittingly, Pegasus appears in the same region of sky as Andromeda and Perseus, linking several characters from Greek mythology across the autumn constellations.
The brightest stars in Pegasus
Pegasus contains several prominent stars that help trace the shape of the Winged Horse, although none rank among the brightest stars in the night sky. Its principal stars include:
- Enif (Epsilon Pegasi) — the brightest star in Pegasus at around magnitude 2.4. Its name means “nose,” reflecting its position at the muzzle of the Winged Horse. Enif is an orange supergiant roughly 700 light-years away.
- Scheat (Beta Pegasi) — a red giant marking the northwestern corner of the Great Square. It is also a variable star whose brightness changes over time.
- Markab (Alpha Pegasi) — a blue-white star of around magnitude 2.5 marking the southwestern corner of the Great Square.
- Algenib (Gamma Pegasi) — a hot blue-white star marking the southeastern corner of the Great Square. It is a Beta Cephei variable, meaning its brightness varies slightly as the star pulsates.
Alpheratz completes the Great Square but actually belongs to neighboring Andromeda, rather than Pegasus. Pegasus is also home to 51 Pegasi, the Sun-like star around which 51 Pegasi b was discovered in 1995. It was the first confirmed exoplanet found orbiting a Sun-like star and became one of the landmark discoveries in exoplanet astronomy.
Find the Andromeda Galaxy using the Great Square
The Great Square of Pegasus provides one of the easiest routes to the Andromeda Galaxy (M31). Starting at Alpheratz, the corner star shared by the Great Square and the constellation Andromeda, follow the line of stars extending away from the Square toward Mirach. From there, Mu Andromedae helps point the way to M31. Earth’s closest large spiral galaxy sits a little more than 2.5 million light-years away, yet under a dark, moonless sky it can still be seen with the naked eye as a faint, fuzzy patch. Binoculars make it much easier to spot.
Pegasus contains one of the oldest globular clusters
Messier 15 (M15) is one of the oldest known globular clusters, with an estimated age of around 12 billion years. Located about 34,000 light-years away, it contains hundreds of thousands of ancient stars packed into an exceptionally dense cluster around 175 light-years across. M15 shines at around magnitude 6.2 and can be spotted with binoculars under a reasonably dark sky. It is also relatively easy to locate from Enif, lying only a few degrees from the brightest star in Pegasus.
Pegasus is home to some remarkable galaxies
Pegasus contains numerous distant galaxies, including NGC 7331, a large spiral galaxy about 50 million light-years away. Nearby in our line of sight is Stephan’s Quintet, a famous grouping of five galaxies that appears remarkably compact through a telescope.
The name is slightly deceptive: only four of the five galaxies are actually close together and interacting, at around 290 million light-years away. The fifth, NGC 7320, is a foreground galaxy only about 40 million light-years from Earth. The four interacting galaxies are being distorted by their mutual gravity, producing huge shock waves and streams of gas and stars.
Einstein’s Cross is actually one quasar seen four times
Pegasus is home to the famous Einstein’s Cross, one of the most striking examples of gravitational lensing. A quasar around 8 billion light-years away lies almost directly behind a galaxy roughly 400 million light-years from Earth. The foreground galaxy’s gravity bends the quasar’s light along different paths, causing us to see four separate images of the same quasar arranged in a cross around the galaxy.
The July Pegasids radiate from Pegasus
The July Pegasids are a minor annual meteor shower whose radiant lies in the constellation Pegasus. They are active during July, with peak activity usually occurring around the middle of the month. The shower produces relatively few meteors, but the ones that do appear tend to be fast-moving as they streak across the sky.
Next: Learn how to find the Great Square of Pegasus and use it as a landmark for exploring the autumn sky.