Now that we are on the winter side of fall, constellations of the passing season are already high and in prime viewing position early after astronomical dark. Tonight, that time comes at 6:28 p.m. PST. In the constellation Pegasus, high in the southeast, are two wonderful galaxy groups. One, NGC 7331, is visible in modest telescopes from suburban skies, and the other, Stephan's Quintet is a challenge of varying degree.
NGC 7331 is named the Deer Lick Group. It is a prominent enough galaxy to be included in a list of highlight objects by Sir Patrick Moore, in his Caldwell list. At 40 million light years distance, it is a bright and dusty spiral galaxy that is thought to maybe be a "twin" of our home Milky Way galaxy. It is one of the brighter galaxies we can observe at magnitude 10.4, and gives plenty to look at with a dimension of 10.5'x3.7'. This jewel was discovered by renowned astronomer Sir William Herschel, in 1784. One curious feature of the galaxy is its central core or bulge, which rotates in the opposite direction than its disk! The name "Deer Lick" is thought to be attributed to an amateur astronomer, naming it after a mountain site where he first observer it.
Aside from this great galaxy itself, there are smaller satellite galaxies, four of them, very close by visually, and they are not that difficult to see if you can find some reasonably dark skies (often, less than an hour drive). Don't be put off by the magnitudes of the dimmer members, as they have a high surface brightness; all their light is contained in a very small area, making the galaxies appear brighter than their magnitudes indicate.
All in all, the group is made of NGCs 7331 (m10.5), 7337 (m15.2), 7335 (m14.4) and 7340 (m14.7). The smaller and dimmer galaxies are often referred to as The Fleas. But are these a physical group, bound in space by gravity? No! NGC 7331 is actually gravitationally allied with NGC 7320, another galaxy designated DDO 213, and an anonymous galaxy (not catalogued). The Fleas are not physically part of the NGC 7331 group!
The group is easy to find. Identify the Great Square of Pegasus, and start at the upper leading star, Sheat.
Five degrees (a fist width) west is a dim wide pair of stars. Use those and imagine a line almost 6 degrees north-northeast to magnitude 2.9 Matar, above Sheat, then continue beyond about another 4 degrees. You are in the neighborhood, and within a few houses! Use a low power eyepiece to locate the bright galaxy, then mag up (magnify) to see detail and look for The Fleas.
Stephan's Quintet is a group of interacting galaxies, included in astronomer Halton Arp's Catalog as Arp 319. Move your telescope just half a degree southwest of NGC 7331 to find this dim group appearing at first as a smudge. But with dark and steady skies, you'll be able to magnify to the point of breaking several of the components apart. Only one pair in the group is difficult to separate, NGC 7318A and NGC 7318B. NGC 7320 is not a physical member of the group, at 35 million light years distance. The other four galaxies are much further away, at approximately 300 million light years.
Here are some observing notes, so you'll know what to expect:
- NGC 7331, reported by Jere Kahanpää of Norway in a 5.2" f/4.8 newtonian, at 133X.
- "The brightest and largest galaxy is NGC 7331. Almost edge-on, size about 7'x2'. A dust lane along the W edge was suspected; the edge is sharper than it's eastern counterpart. Faintly mottled. The mottling is brightest near the core, and two especially bright brightenings are visible about 1.2' N of the core and 1.5' SE. The galaxy has a mag. 13.5 stellar core. Brightness 2.
- NGC 7337: An extremely faint glow around a mag. 14 stellar core.
- NGC 7340: Slightly fainter than -37. Very small (25"). A 14.5m core in a very faint glow.
- NGC 7335: Largest but most difficult of the companions. An extremely faint glow. Elliptical (SE-NW). A stellar core suspected."
- And here are observing notes for Stephan's Quintet by Matthias Juchert in an 8":
- NGC 7317: Very faint and very small. The bright adjoining star disturbs the perception. Only blinkingly visible with averted vision as tiny, round smudge.
- NGC 7318A/B: Most prominent object of the group. Obvious with averted vision but still faint. Elongated 2:1 E-W. Both objects can not be spit. 126x
- NGC 7319: Very faint with averted vision. Elongated with faint center. 126x
- NGC 7320: Very faint only with averted vision. Orientation at PA 330°. Largest of the group. Difficult due to low surface brightness. 126x
Next week is good for deep sky hunting as well, and we'll go after some different targets then.
Charts are from Starry Night Pro. Images are courtesy Sloan Digital Sky Survey and NASA.