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Object of the Week August 23, 2015 - NGC 6934
Object of the Week August 23, 2015 - NGC 6934
NGC 6934, Melotte 230, Caldwell 47
Type: Globular Cluster
Constellation: Delphinus
RA: 20h 34m 11.5s
Dec: +07° 24’ 15”
Size: 8.4’
Mag(v): 8.9
There are 157 cataloged globular clusters in the Milky Way, our home galaxy. This is the story of one – NGC 6934.
Discovered in 1785 and cataloged as I 103 by William Herschel with the infamous 18.7-inch f/13 speculum telescope. He logged it as: “vB, L, gmbM, easily resolvable, beautiful object”, i.e., very bright, large, and gradually much brighter in the middle, easily resolvable, beautiful object.
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NGC 6934 is brighter and larger than NGC 7006 the only other globular cluster found in the constellation Delphinus. Despite being the more prominent of the two, NGC 6934 may be considered unremarkable – no unusual science going on, no crazy properties, it doesn’t even have a nickname – it is your basic, garden variety globular cluster. Yet take a look this HST Advanced Camera for Surveys image and you’ll see that it is anything but unremarkable. Though one should never judge a deep sky object by its HST glamour portrait, this one is evocative. Perhaps it is because of the proliferation of blue stars, which by the way are a false color artifact of the filters used to capture the image, or because it lies within a rich star field. Decades before this HST image was captured Walter Scott Houston commented that “Because of its setting, I find it a particularly pretty object for a RFT.” If I were creating this globular cluster I would begin by taking the Double Cluster (NGC 869 and NGC 884), compressing it until there is a very condensed central core, leaving a nearly symmetrical halo of somewhat less density and being sure to allow a few random sprays of stars to escape from the dense core of the cluster.
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More than many other deep sky objects, this globular cluster magically transforms visually with the use of high power and in exceptional transparency. However before you go there I recommend starting with a rich field view - not only to enjoy its splendor but to also gain visual context for its setting. NGC 6934 is not bright enough to see with the naked eye, and even in ideal conditions it will be difficult to view with handheld binoculars. A much better bet, although still requiring ideal conditions, is a 80mm or larger short focal length scope in combination with a wide-field eyepiece. Under mostly darker, highly transparent skies and with the right combination of eyepiece and power, this view should knock your socks off.
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Now step up to 8” to 15” of aperture. At low power many observers report that this cluster reminds them of M15 viewed with a small aperture scope. The bright core remains mostly unresolved. As power increases more stars in the outer portion of the halo begin to resolve. The inner halo appears like a fine grain haze and even at the highest powers it doesn’t yield any significant stellar resolution. Scott Hogsten, observing in the summer of 1998 with a 12.5” f/5 dob at Perkins Observatory in Delaware Ohio, shares his description of NGC 6934: “A nice bright globular, easily spotted at 110x. At 110x the globular appeared bright and round (slightly brighter towards the center). Jumping to 150x began to resolve some of the outer stars and gave a pincushion effect to the rest of the globular. 220x resolved some additional outer stars, did not help much as the seeing was very poor.”
As previously mentioned exceptional transparency is probably the most critical factor in gaining an exceptional visual observation of this cluster. Noted observer and author Steve Coe, observing with a 13" f/5.6, opines: "This is the type of object that responds with a much better view on a clear, transparent night. On a night I rated 6/10 for seeing and transparency, down on the floor on the desert…I could only resolve 3 stars with a mottled core at 160X. At the same power on a beautiful night…at 5000 ft., this globular blazed with 40 stars resolved, 6 of them in the core area…This is the kind of observation to postpone until those rare 9/10 evenings when stars twinkle very little and the Milky Way blazes overhead."
My recent and first visual observation of NGC 6934 (11 August 2015 0710UT) with a 15” f/4.5 under less than ideal conditions (SQM 18.78 and transparency poor) closely mirrored those of Scott and Steve. I can’t wait to give it a go again under better conditions.
Meanwhile you should:
"Give it a go and let us know! Good luck and great viewing!"
