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RA: 20h 31m 30s
Dec: +60° 39′ 42″
Ch: MSA:1075, U2:56, SA:3
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 21r
Mag: B=8.85, V=7.8
Synonyms: H VI-042
Discovered in 1798 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "a beautiful compressed cluster of small stars, extremely rich, of an iF. The preceding part of it is round, and branching out on the following side, both towards the north, and towards the south. 8' or 9' diameter."
In the 5th edition of Webb's Celestial Objects for Common Telescopes it is described as "an obscure cluster of very minute stars."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 8' and the class as 2 1 r.
Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part I. M.N.R.A.S., 35(5), 159.
The RNGC (Sulentic and Tifft 1973) notes that this is a 10.0 mag open cluster.
Photo Index by Jim Lucyk: Sky&Tel. 1/88 p115, Astronomy mag. 9/85 p90, Burnhams V1 p620, HUAT p270, Deep Sky #20 Fa87 p23.
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Harrington calls it a "bright grouping of 80 stars crammed into an 8' diameter disk. Although their total mag is equal to that of a single 8th mag star, no individual members shine brighter than 12th mag. Resolving the cluster will be difficult in small telescopes. With 6- to 8-inchers, most observers will spot only four stars set in a diamond pattern filled with the subtle glow of still fainter stars."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8M; 8' diameter; rich, faint and irregular in form; 100-plus 12M and dimmer members; umbrella'ed by three 11M stars, one in N, and the other two down toward the E and W."
In Revue des Constellations, Sagot and Texereau describe it as "Not very notable in a 3-inch 18-power refractor; a round milky spot with very faint stars in a 3.8-inch at 45x."
[amastro] A Handful of DSOs; Date: 09/10/1999 ; Loc.: Anderson Mesa, Flagstaff AZ; Weather: Clear, calm, temps in mid 40's; Scope: 10-inch, f/4.5 equatorial mount Newtonian;
I visited a few old friends during my LONEOS run, last week. First up, NGC 6939 and NGC 6946, one of my favorite deep-sky pairings. NGC 6946 is a magnitude 8.8 barred spiral, its arms loosely bound. Observing at 129x (8.8 mm UWA), the galaxy covered an area roughly 13'x 5' and was elongated nearly east-to-west. The axis of its bar ran along a PA of 40 degrees. A triangle of 10th and 11th magnitude stars resides immediately to the south. A pair of stars, 10th and 11th magnitude, sits just a few arcminutes to the north.
This galaxy shows wonderful structure in moderate aperture instruments. The core region showed as an elongated brightening along the axis of the bar. Two distinct spiral arms at the eastern extent were recorded in my sketch. The eastern half also had a more mottled appearance. Three somewhat less prominent spiral arms were detected at the western boundary.
NGC 6939 is listed in the DSFG to Uranometria as a magnitude 7.8 open cluster with approximately 80 members, the brightest members shining at 11.91 magnitude. The cluster resides some 75' to the northwest of NGC 6946. Together, they make a wonderful pair for any 8-inch or larger instrument capable of producing a 90' or larger true field of view.
My sketch records the cluster with a trio of 10th and 11th magnitude stars along the northern boundary, and with magnitude 6.9 PPM 22236 just 10' to the south. Although not highly concentrated, the densest section of the cluster covered a 6'x 4' expanse, a triangle of 12th magnitude stars marking the southern extent of this concentration. My sketch shows approximately ten 13th and 14th magnitude stars at specific locations within the cluster.
NGC 7331 is often used as a landmark for the popular galaxy cluster known as Stephan's Quintet. However, the NGC 7331 group is interesting in its own right. NGC 7331 is a magnitude 9.5 spiral galaxy, its plane slightly inclined. Observing at 190x (18 mm SWA w/ Tele Vue 3x Barlow), the galaxy occupied a 8'x 2' area and was elongated along a PA of 170 degrees. A pair of 13th magnitude stars bounded the northwest extent of the nebulosity. NGC 7331 featured a stellar core with a bright surrounding core region. The northeastern edge of the galaxy had a sharper cutoff in brightness than the opposite edge.
Megastar shows four galaxies, NGCs 7335, 7336, 7337 and 7340, to the east of NGC 7331. My sketch recorded two of the four, 13.3 magnitude NGC 7335 and 14.4 magnitude NGC 7337. Both reside about 5' to the east and appeared as faint circular patches roughly 40" in diameter. NGC 7340, a 13.7 magnitude galaxy another 5' to the east, was not captured in my drawing. I may have been so focused on the intermittent glimpses of challenging NGC 7337 that the brighter neighbor escaped my attention. I've recorded this galaxy in other sketches.
I turned my attention to Jupiter, very high in the south, following my observation of the NGC 7331 galaxy group. But that's another topic for another forum.
Bill Ferris Flagstaff, AZ
Re: NGC 6939 and 6946. I too have enjoyed looking at these two jewels since I first ran into them in the early 1980's. Both sit within roughly a degree of each other, so they make a nice pair in the low-power eyepiece. NGC 6939 I called "the Seacrest Cluster", since at first glance, some of its stars seemed to look like a row of stadium lights at Lincoln-East High School's Seacrest Field (how's that for a naming stretch!). The Galaxy NGC 6946 seemed more like an regular spiral than SB due to the looseness of its arm structure. It seemed to have a small round nuclear region in a somewhat patchy oval of haze with faint tattered arm fragments running off the ends. It almost looks like the inner arms don't want to join up with the nucleus! In a 12.5 inch, it almost resembled a smaller fainter version of M83. On NGC 7331, that is another favorite of mine. I like to show people M31 in binoculars then show them NGC 7331 in my ten inch, since the views are quite similar. It gives people an idea of just how far away NGC 7331 really is! Clear skies to you. --
Chincarini: *s N, NE, & W m~10+. * SW m11.3.
7x35mm - mod f diffuse patch w/o res, broad concen. brtr than N6946 to SE. BS, 28Jun1992, Hutch Mtn.
8cm - sim to gx N6946 @ 20x. brtr than gx and somewhat concen. unres. BS, 15Sep1982, Anderson Mesa.
15cm - found sweeping SW from eta Cep. lg patch of neb S of a triangle of *s. fntr * on S forms shield w/triangle. ten *s res @ 120x in 10' diam.
- mod br cl sim in size and total brtness to gx N6949 in Cyg SE. 8' diam, w/60 *s m11.5+, brtst one on S side. few *s S of *, which is at apex of dk wedge opening S. mod close pair SE of this *, dm 1.0. cl core 1'.5 across lying N of br *, has wk haziness underlying it @ 140x. most *s m12.0+. BS, 16Jun1991, Anderson Mesa.
- mod f mod rich cl mostly res @ 80x w/sl residual haziness. 10' diam, reaching nrly to m10 *s W, N, & ENE (W one defines edge). brtst cl * is m11.5 in SW side of 4' core. 140x shows 60 *s in core, mostly m12.5+. total close to 100 *s; well concen. BS, 8Sep1991, Anderson Mesa.
25cm - 12'-15' diam, bounded on E, W, & N by m8 *s. 100 *s and some haze @ 180x. broad concen. SE of center is a sm clump that is nebulous @ 180x.
30cm - nice sprinkling of f *s, m10.5+. on SW is m10 *. 50 *s in 12' fld, vlittle concen, and a sl haze in 3' cen area. 238x: outliers lie mostly NE to 8' radius. NE 3'.5 is nebulous knot 45" across w/four or five *s. loose.
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