|NGC 7317 to NGC 7320 "Stephan's Quintet": Galaxy Cluster (Pegasus) RA: 22h 36.0m / DEC: +33° 57'.9|
Instrument: 10-inch Starfinder
Stephan's Quintet is a group of five galaxies about one-half degree southwest of NGC 7331. The cluster includes NGCs 7317, 7318A, 7318B, 7319 and 7320. Just a few arc minutes to the east, NGC 7320C chimes in as a magnitude 15.5 object and requires larger aperture to detect. The radial velocity measurements for the Quintet members, four of which fall within a 5700-6700 km/sec range, suggest that these galaxies belong to the same cluster. The sole exception is NGC 7320. Its radial velocity clocks in at a modest 786 km/sec. Could this be a foreground object at roughly the same distance as NGC 7331?
NGC 7320 is the brightest of the five and the first to be seen. This 12.6 magnitude Sc-type galaxy shows an oval contour, 1'x0'.5, in the 10-inch at 190X. NGCs 7318A/B are next to show themselves. They're due north of 7320 and separated by just a couple of arc minutes. Initially, they appear as a unified misty oval. However after a few minutes, I start to notice an occasional dip in brightness in the middle of the oval. Eventually, I am able to hold this dimming with direct vision and the galaxies are clearly split.
NGC 7317 is next to appear. This little 13.6 magnitude stinker kept distracting me by winking in-and-out near a 12th magnitude star directly west of NGC 7320. I turned my attention to it after splitting NGCs 7318A/B and, after a few minutes, was able to hold it with direct vision. Last to appear is NGC 7319. At 13.1 magnitude, one might assume it would have been easier to detect than 7317. However, 7319's surface brightness is 21.8 magnitude per square arc second compared to NGC 7317's 21.6 magnitudes per square arc second surface brightness.
|Instrument: 18-inch Obsession|
In the 18-inch Obsession at 236X (8.8-mm UWA), the quintet members are larger and more obvious to the eye. NGC 7320 covers a 2' by 1' patch of sky. Also, notice the foreground star just southeast of NGC 7320's core. NGC 7318 appears 1' by 0'.6 in extent. Of greatest interest, however, is a sixth galaxy has emerged. Looking 3'.5 to the east of NGC 7319, the faint galaxy NGC 7320C is teased from the night with averted vision. This 15.5 magnitude spiral covers a scant 30" diameter patch of sky. No core or structure is seen, but just detecting this faint stellar metropolis is quite a catch. A 14th magnitude GSC star is seen about 1' to the northeast. A fainter pair resides about half that distance to the west.
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Revised: October 4, 2005 [WDF]