[Author’s note - I originally wrote this for the November newsletter, but I did not get it to Mark in time. Believe it or not, Vulpecula still should be high enough in the sky immediately after the end of twilight to be able to check these out. Otherwise, you’ll have to wait until next summer!]
Many of you may already know that I am currently working on the so-called Herschel 400 list of deep sky objects. Two of them, cataloged as NGC 6882 and NGC 6885, are a bit puzzling. Both are located in Vulpecula (the Fox), and are literally one on top of each other. Supposedly, anyway.
These two open clusters are located very near each other, with NGC 6882 at 20h 11.7m +26 deg 33’ and NGC 6885 at 20h 12.0m +26 deg 29’, about five degrees north-northeast of M27 (the popular open house showpiece Dumbbell Nebula). In fact, they both “enclose” the 6th magnitude star 20 Vulpeculae (which is not considered to be a member of either cluster - it is rather believed simply to be a foreground star). Even in November, they should be high enough in the sky to observe, at least early in the evening after twilight ends.
NGCs 6882 and 6885 are both listed in The Deep Sky Field Guide to Uranometria 2000 as being at magnitude 8.1, with diameters of 18 and 7 arc minutes, respectively. Given their coordinates, and their listed diameters, they ought to overlap. But what we see on star charts is another story. Sky Atlas 2000 plots only NGC 6885, as a small cluster centered exactly on 20 Vul. Uranometria 2000 (first edition) plots both clusters, also with NGC 6885 centered on 20 Vul, and with the larger NGC 6882 completely enclosing it, and with its center to the northwest of 20 Vul. Millennium Sky Atlas also plots both, and also with NGC 6882 enclosing NGC 6885 (also centered on 20 Vul), but shows NGC 6882’s center to the southwest of NGC 6882. And, the totally revamped Uranometria 2000 second edition shows NGC 6882 only, and doesn’t even plot its smaller companion! Obviously, there’s confusion regarding these two objects even among the sky cartographers!
So what does one see when one looks at these in a telescope? In my 13” at the Black Forest Star Party this past September, NGC 6882 appeared as a nice, relatively “loose” cluster in a rich Milky Way field. Despite the rich background, it was still relatively easy to spot as a cluster. Yes, 20 Vul was there near the “edge” of the cluster, but I did not pay attention to which direction the star was from the cluster’s center. [I did not notice the discrepancy in the positions of NGC 6882 relative to 20 Vul until I compared star charts while writing this article. I’ll have to revisit them the next time I’m out observing.] At low power, around 55x, I saw no evidence of a smaller cluster centered on 20 Vul. At higher power, around 200x, I thought I detected a faint “circle” of stars surrounding 20 Vul. Was this NGC 6885? If so, then it’s awfully small and hardly worth cataloging in the Herschel 400 list. The Herschel 400 is advertised not as having the most challenging objects, but rather to be a collection of objects reachable with moderate apertures from less than perfectly dark skies. If this is NGC 6885, then it seems out of place in it.
So what’s up with these two clusters? Are they really two objects? Why does each sky atlas plot them differently? Why are they listed with identical brightness’s, when obviously if one “encloses” the other, then they cannot be? And is NGC 6885 really a cluster? Is it even there at all?
Perhaps the next time you are looking at M27, you might want to take a little side trip and check these out for yourself. I’d like to know - what do you see?