A horse flies through the sky on feathered wings in late summer and early fall. This celestial horse is the mythical steed Pegasus – though you will find only its front half in the sky, as its hind quarters are cut off where the neighboring figure of Andromeda begins. Here is Johann Bode's depiction of Pegasus in his 19th century atlas Uranographia.
The birth of Pegasus was odd even by the standards of Greek mythology. According to the ancient story, his mother was Medusa the evil-eyed Gorgon. In her youth, however, Medusa had been a noted beauty. She was ravished by Poseidon, the god not only of the sea but also of horses. Unfortunately for Medusa, the seduction happened in the temple of the goddess Athene. Outraged by this sacrilege, Athene turned Medusa into a snake-haired ogre. Poseidon, of course, went unpunished. When Medusa was decapitated by Perseus, the winged horse flew out of her body and took residence on Mount Helicon.
The Great Square
The best-known feature of this high-flying horse is a large square of four stars called the Square of Pegasus.
One of the stars in the square doesn't really belong to Pegasus. Sirrah actually marks the head of the neighboring figure of Andromeda. Old-time astronomers regarded this star as being shared by both constellations. But the International Astronomical Union (IAU) decreed in the 1930s that stars could belong to one constellation only. They could hardly decapitate poor Andromeda - she was in enough trouble as it was, being chained to a rock as a sacrifice to a sea monster - so they let her keep the star at the expense of the horse.
Pegasus is best seen in the fall. A star chart will help you locate it, but you shouldn't have much trouble seeing it, as the Great Square of Pegasus is so big that a row of 30 full Moons could fit from one side to the other. Even though it boxes in such a large area of sky, only a handful of stars are visible with the naked eye within it. If you find it in the sky, how many can you see? It will depend on your eyesight and how dark your skies are.
The Square marks the body of Pegasus, although horse owners might prefer to imagine it as a paddock for equine exercise. At one corner of the Square is the star called Alpha Pegasi. From this corner stretches a crooked line of stars that represents the horse's neck. Another two lines of stars extend from the star Beta Pegasi, which marks one of the northern corners of the Square. These two lines trace out the horse's forelegs.
Beta Pegasi, popularly known as Scheat, is the type of star known as a red giant; you might notice an orange tinge to it, particularly through binoculars and small telescopes. As with so many red giants, it's unstable and varies slightly in brightness, although the variability is not particularly obvious to the naked eye.
A star with a planet
Look about halfway along the side of the Square joining Alpha and Beta Pegasi. Just outside the Square lies a star called 51 Pegasi, too faint to be seen with the naked eye under town skies, but easy enough to pick out with binoculars. This star became a celebrity in 1995 when a planet was discovered in orbit around it, the first known planet of an ordinary star like our own Sun.
This planet was designated 51 Pegasi b, but it had been known informally as Bellerophon after the Greek hero who tamed Pegasus. However the planet is a giant with a mass about half that of Jupiter. So when the International Astronomical Union asked for star names for a number of exoplanets, a Swiss astronomical society proposed Dimidium, which is Latin for half. This name was accepted officially by the IAU.
Dimidium orbits its star every four days at a distance of under eight million kilometers, which is far closer to its star than Mercury is to our Sun. It was the first discovery of a star of the type now known as hot Jupiters. These are massive planets similar to Jupiter, but they orbit close to their parent stars. Current theories suggest that these planets formed farther out but moved in closer over time.
Great ball of stars
If you go back to the neck of the horse, you can see that the line of stars forming it ends with a moderately bright star called Enif. The name comes from the Arabic meaning nose, since it lies on the horse's muzzle. If you continue sweeping with binoculars in the same direction, you will come to a hazy-looking object called M15, the 15th entry in Charles Messier's catalog of comet-like objects. M15 is what is known as a globular cluster, a ball of countless thousands of stars. M15 is regarded as one of the finest examples of such a cluster in the northern sky.
This ends our tour of Pegasus, except to note that the steed is not alone as it grazes in the autumn skies. Next to it is the constellation Equuleus, representing the head of a foal, one of the smallest and faintest constellations in the sky. You can just see the foal's head in Johann Bode's drawing.