by Shaphan Shank | Nov 1, 2022 | 0 comments

M15 star cluster
M15. Photo © Hubble and NASA.

Pegasus is one of the most familiar autumn constellations. The most prominent part of the constellation is a large square of stars (~15° across) known as the Great Square of Pegasus. Many people recognize only this part of Pegasus, although the traditional constellation pattern also includes two chains of stars that extend westward from the Great Square. Also, the star in the northeastern corner of the Great Square (Alpheratz) is officially within the boundaries of the adjoining constellation Andromeda. However, this technicality won’t affect you in any practical way as long as you’re not drawing maps of the official constellation boundaries.

The most impressive telescopic target in Pegasus is the globular star cluster M15. To find this cluster, follow the chain of stars that extends southwest from Markab, the star in the southwest corner of the Great Square. The last two stars in this chain, Biham and Enif, point northwest instead of southwest. Take an imaginary line connecting Biham to Enif, then continue it past Enif another 4° (half the distance between Biham and Enif) to find M15. This cluster is fairly dense as globular clusters go, with a strong concentration of stars at its core. The cluster is visible with binoculars as a fuzzy, star-like object. A mid-sized (8”–10”) telescope at 100–200× will resolve the cluster’s brighter stars.

Enif itself is a wide double star, consisting of an 8th magnitude secondary star separated from the primary star by about 144 arcseconds. (Most of the double stars featured in this column have separations of perhaps 3–40 arcseconds.) This double is far too wide to be visually striking, but it exhibits an interesting phenomenon that gives it the nickname “Pendulum Star.” To observe this phenomenon, locate the star using a magnification of 50–100×. Then, lightly jiggle the telescope tube perpendicularly to the angle of the pair of stars. The bright primary star will appear to move smoothly back and forth, but the secondary will likely appear to swing back and forth relative to the primary instead of maintaining the same orientation relative to it. This effect occurs because the secondary star is so much dimmer than the primary. Our eyes take longer to register the light of the secondary star, so it appears to lag behind and swing around the primary star when the telescope is jiggled.

Pegasus is home to a number of galaxies, most of which are quite faint. The best and brightest is an edge-on spiral galaxy cataloged as NGC 7331. This galaxy lies about 4.5° north-northwest of the star Matar, which lies about the same distance northwest of Scheat, the star marking the northwest corner of the Great Square. You won’t be able to see any spiral structure in NGC 7331, but an 8” telescope should clearly show the galaxy’s oblong shape and brighter nucleus.

Star map of Pegasus

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