The Great Square of Pegasus dominates fall evenings. Learn the stars and deep-sky objects found in the Winged Horse.
In mythology, Pegasus was the winged horse that Perseus rode when he saved Andromeda from Cetus the Whale. The constellation of Pegasus is dominated by a large square shape that runs into the constellation Andromeda.
Stars
Pegasus is easy to find. On fall evenings it is above the eastern horizon, reaching to nearly overhead by late fall. The Great Square of Pegasus is a wide 20 degrees from top to bottom. The star closest to the horizon as Pegasus rises is magnitude 2.83 Algenib. It lies 333 light-years away. The star on the opposite corner of the square from Algenib is Scheat, a magnitude 2.44 star lying 199 light-years away. The star to the south in the square is Markab, a magnitude 2.49 star at a distance of 140 light-years. And the final star in the square is Alpheratz. Technically, Alpheratz lies just across the border of Pegasus and is actually a member of the constellation Andromeda. Alpheratz is the brightest of the four stars at magnitude 2.06 and lies 97 light-years away.
The Great Square marks the body of the winged horse and trails leading off the west side of the square mark the front legs and head of Pegasus. The head of Pegasus leads off from Markab. Two stars at magnitude 3.4 and 3.5 named Homam and Biham lead the way to the head star, magnitude 2.38 Enif. This star will be helpful in finding the globular cluster M15.
Find the forelegs of Pegasus off of Scheat. Five degrees west of Scheat is magnitude 2.93 Matar. As the brightest leg star in Pegasus, it is helpful in finding a couple notable galaxies.
Galaxies
Four notable galaxies (or galaxy clusters) are found in Pegasus. The brightest is magnitude 9.5 and has the curious name Deer Lick Group. Follow Scheat to Matar and then about 4 and a half degrees further and slightly north of the direction you were heading. This will bring you to the Deer Lick Group, NGC 7331. Here you'll find one large spiral galaxy and a spattering of smaller ones. Only a half degree south of the Deer Lick Group is the famous Stephan's Quintet. This tight gathering of five galaxies has a magnitude of 13.6. The largest and brightest, NGC 7320, has a small redshift compared to the other four, revealing that it is probably not a physical member of the group and just a line-of-sight coincidence.
The Pegasus I Cluster lies on the southern edge of the constellation not far from the circlet of Pisces. Eight degrees away from Markab, the Pegasus I Cluster is a magnitude 11.1 grouping. The galaxy cluster requires a large telescope to see or a long-exposure photograph, but it reveals a beautiful and striking number of galaxies.
The Pegasus II Cluster lies back within the square of Pegasus. Halfway between Alpheratz and Scheat, it lies just inside the border of a line that would be drawn connecting these two stars. A bit dimmer at magnitude 12.6, the Pegasus II Cluster (or NGC 7720) is a powerful radio source and receives much scientific study.
Globular Cluster
One other deep-sky object of note in Pegasus is the globular cluster M15. M15 can be easily found using the head and neck stars of Pegasus. Start with the star Markab and follow it along the two dimmer stars that mark the neck. From the last star of the neck (Biham) to the brighter head star Enif continue a line straight out for a little more than four degrees. Here you will find the magnitude 6.4 globular cluster M15. It lies about 33,600 light-years away and will show up nicely in a pair of binoculars.
