Astronomy for Everyone

If you look up in the sky on a crisp October night, you will see a large square of stars overhead. This is part of the constellation Pegasus, The Winged Horse.

According to Greek and Roman mythology, Pegasus sprang forth from the sea when drops of blood from the severed head of the horrible Medusa mixed with seafoam. He was befriended by a young man named Bellerophon and together the two performed many heroic deeds. But as so often happens with mortals, Bellerophon let his reputation get the better of him, and decided to ride Pegasus all the way to Mount Olympus, to join the gods. This would not do, so Zeus, king of the gods, sent a horsefly to sting Pegasus, who bucked and threw Bellerophon to earth. Pegasus was then allowed to join the gods in the sky, where we see him today. Bellerophon, on the other hand, was doomed to wandering the earth, lame and blinded, as punishment for his impudence.

The constellation of Pegasus today represents the head and forequarters of the famous horse. It is a large constellation, the seventh largest in the sky, in terms of area. The distinctive Great Square of Pegasus is marked by the stars Alpha Pegasi (or Markab), Beta Pegasi (or Scheat), Gamma Pegasi (or Algenib), and Alpha Andromedae (Alpheratz) and so isn’t officially completely within the constellation of Pegasus. The horse is upside-down in the sky for northern hemisphere observers, with his head facing south, and it doesn’t take much imagination to see him prancing through the night sky.

Pegasus is home to many wonderful deep-sky objects, including the magnificent globular cluster M15 (M for Messier, a French astronomer of the 18th century who began cataloguing non-stellar objects in 1758). It is a large ball of tightly packed stars that brightens noticeably toward the center. M15 emits a significant amount of x-rays, leading astronomers to suspect that there may be a black hole at the center of the cluster. M15 is located west and a little north of Epsilon Pegasi, or Enif, which marks the hoof of the horse’s front leg.

There are several galaxies located in Pegasus, but for the most part, they require a telescope to observe them, as they are too faint to be picked up with most binoculars. One of these, NGC 7331, which is located just north of Eta Pegasi, the star that marks the first bend in the rear leg of the horse, is a spiral galaxy very similar in size and form to our own Milky Way. There are several smaller, fainter galaxies to the east of NGC 7331 that may be associated with it in some way; they may be traveling through space together. About half a degree to the southwest of NGC 7331 is a very curious grouping of galaxies known as Stephan’s Quintet (NGC 7317, NGC 7318 A/B, NGC 7319, NGC 7320). These five galaxies are all bound together gravitationally and four of them appear to be interacting with each other. Four of these are fairly easy to pick out with a telescope of at least 10" in aperture on a dark night, with NGC 7318A and B appearing as one faint wispy ball of fuzz.

There are several fine double stars in Pegasus that are well worth a look. Struve 2799, located just 1 degree south/southwest of M15, is a triple star system. Kappa Pegasi is a very close double star, with a faint, distant companion, consisting of a yellow-white and an orange star; but since they are so close together, we see them, even though large telescopes, as one star. Kappa was discovered to be a binary star by S. W. Burnham in 1880, using the 36-inch refractor on Mt. Hamilton. It took all the great telescope’s power and very steady seeing before Burnham was able to split the two. Most of us amateurs do not have 36-inch refractors at our disposal, so we must content ourselves with the knowledge that the star is a double, but we aren’t likely to see each of its components. However, there are several other double stars in Pegasus that are not such demanding targets.

The Great Square of Pegasus is a most useful grouping of stars, or asterism, to locate and remember, because it serves as a "jumping-off" point for easy location of surrounding constellations, such as Andromeda, Aquarius, Pisces, and Cassiopeia. So look up often during those autumn nights and seek out the big square in the sky. From there we will travel to other wonders in the starry nights.