How to Spot the Pegasus Constellation

Northern hemisphere autumn skies
Look for Pegasus in the northern hemisphere autumn skies looking north.

Carolyn Collins Petersen 

Stargazers looking for an easy-to-spot star pattern can't go wrong with the constellation Pegasus, the Winged Horse. Although Pegasus doesn't exactly look like a horse—more like a box with legs attached—its shape is so easily recognizable that it's hard to miss.

Finding Pegasus

Pegasus is best spotted on dark nights beginning in late September and early October. It's not far from W-shaped Cassiopeia and lies just above Aquarius. Cygnus the Swan is not too far away, either. Look for a group of stars in the shape of a box, with several lines of stars extending out from the corners. One of those lines marks the Andromeda constellation. 

the constellation Pegasus with its neighbors and some deep-sky objects.
Pegasus is one of three northern hemisphere autumn constellations that are easy to spot. It contains the globular cluster M14. Carolyn Collins Petersen

Stargazers looking for the Andromeda Galaxy can use Pegasus as a guide. Some like to think of it as a baseball diamond, with the bright star Alpheratz as the "first base" mound. A batter hits a ball, runs to first base, but instead of going to second base, runs up the first base foul line until they run into the star Mirach (in Andromeda). They turn right to run into the stands, and before long, they run right into the Andromeda Galaxy. 

The Story of Pegasus

Pegasus the Winged Horse has a long history with stargazers. The name we use today comes from ancient Greek myths about a flying steed with mystical powers. Before the Greeks were telling tales of Pegasus, ancient Babylonian mystics called the star pattern IKU, meaning "field." The ancient Chinese, meanwhile, saw the constellation as a giant black tortoise, while indigenous people of Guyana saw it as a barbecue.

The Stars of Pegasus

Twelve bright stars make up the outline of Pegasus, plus numerous others in the official IAU chart of the constellation. The brightest star in Pegasus is called Enif, or ε Pegasi. There are brighter stars than this one, such as Markab (alpha Pegasi), and of course Alpheratz.

The stars that make up the "Great Square" of Pegasus form an unofficial pattern called an asterism. The Great Square is one of several such patterns that amateur astronomers use as they find their way around the night sky.

The IAU chart of constellation Pegasus.
The official IAU chart of the constellation Pegasus shows its brighter stars plus numerous others. It also shows a few deepsky objects, such as M15 and the Andromeda Galaxy. IAU/Sky & Telescope 

Enif, which can be seen as the "muzzle" of the horse, is an orange supergiant that lies nearly 700 light-years from us. It is a variable star, which means that it varies its brightness over time, mostly in an irregular pattern. Interestingly, some of the stars in Pegasus have planetary systems (called exoplanets) orbiting them. The famous 51 Pegasi (which lies on a line in the box) is a Sun-like star that was found to have planets, including a hot Jupiter. 

Deep Sky Objects in Pegasus Constellation

Although Pegasus is one of the largest constellations, it doesn't have a lot of easily-spotted deep-sky objects. The best object to spot is the globular cluster M15. M15 is a spherically shaped collection of stars bound together by mutual gravitational attraction. It lies just off the horse's muzzle and contains stars that are at least 12 billion years old. M15 is about 33,000 light-years away from Earth and contains more than 100,000 stars. It's almost possible to see M15 with the naked eye, but only under very dark conditions.

findingm15.jpg
How to find the globular cluster M15. Carolyn Collins Petersen

The best way to view M15 is through binoculars or a good backyard telescope. It will look like a fuzzy smudge, but a good telescope or an image will reveal much more detail.

M15Hunter.jpg
An amateur view of M15 through a backyard-type telescope. Hunter Wilson/Wikimedia Commons

The stars in M15 are so tightly packed together that even the Hubble Space Telescope, with its eye for detail, cannot make out individual stars at the core of the cluster. Currently, scientists use radio telescopes to find X-ray sources in the cluster. At least one of the sources is a so-called X-ray binary: a pair of objects that are giving off X-rays. 

hs-2000-25-a-large_web.jpg
A Hubble Space Telscope view of the central region of globular cluster M15, which is so densely packed with stars that HST has trouble spying out individual ones. NASA/ESA/STScI

Far beyond the limits of backyard telescopes, astronomers are also studying galaxy clusters in the direction of the Pegasus constellation, as well as the gravitationally-lensed object called the Einstein Cross. The Einstein Cross is an illusion formed by the gravitational influence of light from a distant quasar that passes by a galaxy cluster. The effect "bends" the light and ultimately causes four images of the quasar to appear. The name "Einstein Cross" comes from the cross-like shape of the images and the famous physicist Albert Einstein. He predicted that gravity affects space-time and that gravity could bend the path of light that passes near a massive object (or collection of objects). That phenomenon is called a gravitational lens.