The Great Square of Pegasus
The flying horse Pegasus is one of the most prominent constellations in the fall. By the end of September it’s high enough in the sky to be visible as soon as it gets dark.
The constellation is easy to spot and easy to remember because it’s big and because of the four second-magnitude stars that define the Great Square. The two most southern stars of the Square are Algenib and Marcab. Algenib is the Arabic word for “the side,” or “the wing.” Marcab means “the saddle.” The star in the northwest corner is Scheat, meaning “the foreleg.”
Obviously all of these have to do with parts of a horse. The fourth star of the Square is called Alpheratz, or less commonly Sirrah; the two names together mean “the navel of the horse.” Alpheratz used to belong to the constellation Pegasus, but now astronomers count it as part of the constellation Andromeda.
There’s one other second magnitude star in Pegasus. It’s Enif, meaning “the horse’s nose.” Enif is at the end of an imaginary line running southwest through Algenib and Marcab, and is about the same distance beyond Marcab as Marcab is beyond Algenib. Enif comes up before the whole Square. With it as the nose and Algenib as the wing, Pegasus is flying west trying to overtake Cygnus or the Swan. In a way, the main stars of Andromeda look also like the left rear leg of the horse stretched out in a gallop.
Enif is also helpful in locating the very rich globular cluster M15 which is 4º NW of it. Barely visible to the naked eye in an absolutely dark sky, in binoculars M15 is a fuzzy blob. If you look through a small telescope you can begin to make out some of the individual stars. Globulars being the oldest kind of clusters, these stars are almost all the same color, one of their qualities that suggests that they are all about 15 billion years old—the age when the universe began. Measurements also suggest that M15 is about 39,000 light years (ly) distant, and that its total luminosity is that of 200,000 Suns.
Northwest of Scheat is the spiral galaxy NGC7331, the one sometimes used in astronomy textbooks to show what our own galaxy might look like from a distance. Spiral galaxies when they are viewed edge-on show a central bulge; when face-on, the concentrated center is surrounded by pin-wheel arms. Unlike the globular clusters, the spiral galaxies contain some stars that are being formed. Astronomers have calculated that NGC is 50 million light years away; if that is the case, those new stars are only that old. NGC7331 is thought to be made up of 140 billion stars. Its size is 64,000 ly from one edge of the disk to the other.
The visible distance between the edges of our own galaxy is about 100,000 ly, and our own Sun, in contrast, is middle-aged at 4.5 billion years.