[AZ-Observing] Re: Sunday not too hot either....

Brent Archinal and Steve Hynes, in their recently published "Starclusters," 
offer a thorough examination of the murky identifications for NGC 6882 and NGC 
6885. (It's a must-have resource in your astronomy library, if you're 
interested in the deep-sky.) I'll not reproduce the entire discussion, here. 
But 
quoting from the last two graphs:
"In the catalog here, the large group 'A' around 20 Vul is assumed to be NGC 
6885 = Cr 417, probably equivalent to NGC 6882. The smaller group or cluster 
'B' to the northwest of 20 Vul, discovered by Trumpler, is Cr 416.  The 
position of Cr 416 has been measured from the DSS and Trumpler's size of 8' for 
it is 
confirmed (and used here), although the cluster is actually elongated NE to 
SW.  It should be noted that modern studies should probably still be done to 
confirm Trumpler's assertion that these are two independent (or even physically 
bound) groups of stars.

"The star 20 Vul is assumed to be the brightest star in the NGC 6885 group 
(also known as the '20 Vulpeculae Cluster''), so its V magnitude (5.91 from 
SIMBAD) is used as that of the brightest star. However that star was apparently 
not used by Skiff in his determination of the total magnitude of the cluster 
(probably because it is so much brighter than the other stars of the cluster 
[private communication from Skiff, 1996}), given here as 8.1." ////

Brent subscribes to this list, so I'll leave it to him to comment further on 
his interpretation of the identifications.

Question for Brian Skiff, did you not include 20 Vul in the magnitude for NGC 
6885 because it is likely not a member of the cluster?

Regards,

Bill

bobe@xxxxxxxxx writes:
NGC 6882 is probably a duplicate observation of NGC 6885.  Both clusters
were found by WH on subsequent nights (9 and 10 Sept 1784; N6882 is from 10
Sept), were refered to the same star, and have almost identical descriptions 
("A
cluster of coarsely scattered stars."  For NGC 6885, he adds, "... not
rich").

There is nothing striking near the position of N6882, but that for N6885 is
in the middle of a large scattered cluster also observed by JH.  Over the
years, there has been considerable speculation about what WH saw.  Some 
observers
have made the clusters identical, while others (notably Reinmuth) have
pointed at the wide group of three bright (m = 6) stars about 20 arcmin north 
of
N6885.  Brent Archinal believes that the clump of nine stars at 20 09 51,
+26 35.1, including HD xxxxxx (the southernmost of Reinmuth's three stars),
is N6882.  This is unlikely as the clump is only two arcmin across.  Had WH
seen this, he would most likely have put it into his 7th class; it certainly 
is
not "coarsely scattered."

Neither of these matches WH's description, so I'm more inclined to the
identity of the two NGC objects.  This would imply an error of 15 arcmin in
WH's declination; the RA's are 12 seconds different, but both are still well
within the central part of the cluster (which is over 20 arcmin across).

Adding to my conviction that N6882 = N6885 is the fact that, of the seven
objects found by WH on 10 Sept 1784, four have significant offsets in WH's
positions (the three besides N6882 are:  N6800, -1 minute off in RA; N7720,
+40 seconds off in RA; and N7741, +4 arcmin off in Dec).  WH was clearly not
up to snuff that night, and the +15 arcmin error in the declination of N6882
fits right in with the other problems.
=======================================================

Hope this helps,

/Bob
===============
Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net


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