Pegasus

The winged horse of autumn

Pegasus, the winged horse, fills a large area of the autumn sky. As summer's bright stars sink in the west, its Great Square rises in the east and becomes the guide to the whole autumn sky.

Only the front half of the horse is pictured: the Square is its body, and lines of stars mark its neck, head and front legs. From mid-northern latitudes the figure appears upside down, with its legs in the air. The Square's north-east corner, Alpheratz, officially belongs to the neighboring constellation Andromeda.

How to find it

On October evenings, look high in the south-east to south for a large, nearly empty square of four second-magnitude stars. Each side is about 15 degrees, roughly one and a half fists at arm's length. Clockwise from the top left, as seen from the north, the corners are Alpheratz, Scheat, Markab and Algenib.

From Markab, a crooked line of stars runs west toward the horse's head and ends at Enif, the nose. From Scheat, two lines of stars extend to the north-west as the front legs.

The Square also points the way to other targets. The line from Scheat through Markab, continued south, leads to Fomalhaut, the bright lone star of autumn. A line from Alpheratz east along Andromeda leads to the Andromeda Galaxy.

The main stars and their names

Enif (Epsilon Pegasi), at magnitude about 2.4, is usually the brightest star in Pegasus. Its name comes from the Arabic for 'nose'. It is an orange supergiant about 670 light-years away, though estimates vary.

Scheat (Beta Pegasi) is a red giant about 200 light-years away whose brightness wanders between about magnitude 2.3 and 2.7, so on some nights it rivals Enif. Markab (Alpha Pegasi), magnitude about 2.5 and about 140 light-years away, has a name meaning 'saddle'. Algenib (Gamma Pegasi), at about 2.8, is a hot blue-white star at the Square's south-east corner.

51 Pegasi is a Sun-like star of magnitude about 5.5, about 50 light-years away, just west of the Square. In 1995 Michel Mayor and Didier Queloz discovered a giant planet orbiting it every 4.2 days, the first planet found around a Sun-like star. The discovery earned them a share of the 2019 Nobel Prize in Physics. The star is now officially named Helvetios and the planet Dimidium.

Deep-sky objects for binoculars and small telescopes

M15 lies about 4 degrees north-west of Enif. It is a globular cluster, a dense ball of more than 100,000 old stars, about 33,000 light-years away. In binoculars it looks like a fuzzy star; a small telescope shows a bright, tightly packed core. It is also one of the very few globular clusters known to contain a planetary nebula.

NGC 7331, about 40 to 45 million light-years away, is a spiral galaxy often compared with our Milky Way. A small telescope under a dark sky shows it as an elongated glow. Nearby, Stephan's Quintet is a tight group of galaxies, four of which are about 290 million light-years away. It is faint and needs a large telescope, but it was one of the first targets shown by the James Webb Space Telescope in July 2022.

Mythology

In Greek myth Pegasus sprang from the blood of the Gorgon Medusa after the hero Perseus cut off her head. The hero Bellerophon tamed the horse with a golden bridle given by the goddess Athena and rode it to defeat the Chimera, a fire-breathing monster.

Bellerophon later tried to fly Pegasus up to Mount Olympus. Zeus sent an insect to sting the horse, Bellerophon fell, and Pegasus flew on alone to become a constellation. Another story tells that a stamp of its hoof opened the Hippocrene spring on Mount Helicon, sacred to the Muses. In Turkish the constellation is called Kanatlı At, the Winged Horse, or simply Pegasus.

Practical observing tips

The Square is easy from a suburb but looks surprisingly empty. Try counting the stars inside it: from a city you may see none or one; from a dark rural site you can count a dozen or more. Repeating the count is a good way to compare observing sites.

For M15, find Enif, then move a binocular field to the north-west. Use a small telescope at medium power to start resolving the cluster's outer stars. 51 Pegasi is visible to the naked eye only under dark skies, but binoculars show it easily: you will not see the planet, but you will be looking at a star that changed astronomy.