Object of the Week August 27th 2017 - NGC 7331, the "neglected" galaxy of Pegasus
NGC 7331, the "neglected" galaxy of Pegasus
Type: Galaxy
Other designations: UGC 12113, PGC 69327
RA 22 37 04
DEC +34 24 56
Mag 9.5
Size 10.5' x 3.7'
Class SA(s)b
Almost all deep sky aficionados are familiar with NGC 7331, the bright spiral galaxy tucked in the northwestern corner of Pegasus, for it is the main stepping stone for the well known ‘Stephan’s Quintet” of faint galaxies. Yet, NGC 7331 itself tends to be relatively neglected in favor of the much fainter “prize” half a degree away.
I must admit that I was guilty of the same attitude towards NGC 7331. I had seen it hundreds of times, but I had never OBSERVED it. Yet all this suddenly changed last week, when –once again – I used it as a landmark to show “Stefan’s Quintet” to a friend. It just dawned to me that I had never really observed NGC 7331, or the group of galaxies surrounding it. So, NGC 7331 and its companions became my company over the last few evenings.
NGC 7331 is a very large non-barred spiral galaxy, with a mass of more than 300 billions suns, thus it is larger than our Milky Way, and about equal to M31, at a distance of about 47 millions light years. The spiral arms are tightly wound, and we observed it about 30 degrees from edge on. Recession speed studies indicate that it is unrelated to the group of at least six smaller and fainter galaxies around it, but it is related to NGC 7320 in Stefan’s Quintet (the brighter galaxy in that group). Radio emission studies have also suggested the same idea, as NGC 7331’s radio emissions extend all the way to 7320.
dss_search7331.jpg
NGC 7331 from the Deep Sky Survey
Observationally, NGC 7331 is a great object, and is very satisfying for it can even detected with binoculars as a tiny fuzzy patch. As noted above, I have seen it many times, and my notes indicate that I have seen it in both 15X70 and 22X60 binoculars, as well as scopes ranging from 3 inches to 18 inches. But almost all I have had was that it is an oval bit of light. So I decided that this had to end and I would observe NGC 7331 in great detail.
I first examined NGC 7331 on a night of average conditions using my 18” dob. The galaxy is immediately obvious with small magnifications as an oval patch of light with pointed edges. Increasing the magnification brings out a wealth of details. The not-quite-stellar nucleus is surrounded by a spindle-shaped core, which in turn is surrounded by a fainter halo – the spiral arms. Both the northern and southern arcs of the arms could be held with direct vision, and the sharp western edge had a detectable dark shadow before continuing as a faint glow – this shadow nicely corresponds to the most prominent dust lane visible in hi-res photos! Another distinct linear shadow frames the eastern edge, further betraying the spiral pattern of the galaxy.
A faint oval galaxy is readily visible to the east of the core, this being NGC 7335. Two more galaxies can be seen to the north and east, NGC 7337 and 7340. Both are fainter and smaller than 7335, but are easy to see in 18 inches. A fourth galaxy, NGC 7336 is much harder as it is far smaller and was barely seen with my 18”, mainly showing as a very faint, impossible to focus star.
7331A18A1.jpg
NGC 7331 and companions with the 18" dob
I re-observed the NGC 7331 group three nights later, from the same spot but under superb conditions, with my 5” apo refractor. Well, I was literally shocked with all the details that could be seen. Under low magnifications the galaxy was evident as a small spindle in the more than two degrees of sky. Increasing the magnification to 139X unleashed tons of details, while further pushing the magnification to 278X confirmed many of the details. The bright oval core was again surrounded by dust lanes which seemed more distinct than when I observed the galaxy with the 18” three nights before, I suspect due to the better conditions of the night. What I could not detect with the 5-inch was the faint glow east and west of the main core, which resulted in NGC 7331 having a more elongated shape in the smaller scope, with better defined edges. I could readily detect NGC 7335 to the east of 7331 as a very faint oval patch, but despite my effort, and my knowledge of their positions, I could only detect NGC 7340 from the other companions, as a tiny barely non-stellar patch.
7331B5A1.jpg
NGC 7331 with a 5" APO refractor
Checking wide field photos of NGC 7331I could see that there are more faint galaxies in the vicinity, such as PCG 69291 and CGCG 514-66, both located to the west of 7331, but I missed them even with the 18”, they will have to wait for the next new moon period. I also plan to examine the galaxy with my 80mm refractor and see what details I can squeeze with a small instrument. But evidently, NGC 7331 is a superb object that can be very interesting for telescopes of all sizes!
N7331_marked.jpg
NGC 7331 and companions, marked (From http://martingermano.com/N7331_1024.htm)
So, during the next new moon, turn your scopes to the NGC 7331 group, and…
GIVE IT A GO AND LET US KNOW!!!!
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