4 Attachment(s)
Object of the Week – May 29, 2016 – NGC 6210, The Turtle Planetary
NGC 6210
Aliases: PK 43+37.1 = PN G043.1+37.7 = Turtle Nebula
RA: 16h 44m 29.4s
DEC: +23° 48' 00"
Class: 2(3b)
Size: 20" x 13"
Mag: V ~ 8.8
Wilhelm Struve, the famous double star observer, discovered NGC 6210 in 1825 while searching for double stars with the 9.6" Fraunhofer refractor at the Dorpat Observatory in Estonia. It was included as #5 in a list of 9 "Nebulae dectae" in an appendix to his main catalogue of double stars published in 1827. He described a "bright planetary nebula, diameter 6", like a star of mag 7". The planetary was earlier catalogued as a star in Lalande's catalogue of 1801 (LL 30510) and is also known as HD 151121 and BD+24 3048.
Deep Sky historian Wolfgang Steinicke explains why William Herschel missed this bright planetary -- "On 15 May 1784 he swept the region (sweep 217). However he stopped for about 30 minutes for star counts, i.e. the sky passed horizontally through the eyepiece. Unfortunately the strip runs about 1° below the nebula, starting 25 tmin west and ending 6 tmin east of it. Thus the object could not enter his field of view (15'). On 10 April 1785 he inspected the field again (sweep 395), but saw only some Flamsteed stars in the morning twilight."
John Herschel observed NGC 6210 on 25 May 1830 and recorded "Struve's fifth nebula, very bright, equal to a star of 8 to 9m, 6" diameter and of uniform light, but with the edges boiling and ragged. A fine object like a star out of focus". As far as the challenging central star, Hermann Vogel observed and sketched the central star with the 27" Grubb refractor at the Vienna observatory on 4 Jun 1883. He noted a "Bright planetary nebula of elliptical shape. A delicate star is in the nebula, nearly at the centre. The nebula is surrounded by a faint elliptical shell."
On 5 May 1850, Lord Rosse (or observing assistant) logged "intense blue centre fading off to some distance all around. Small star nf to which neb nearly extends. Fancied a projection from the bright centre towards np, but I think it was only fancy."
As far as Lord Rosse's "projection towards np [northwest], James Keeler observed the planetary with the 36-inch Clark refractor. He produced this remarkable sketch (rotated so that north is approximately up) around 1890 showing the two main wings. The sketch is directly from his Lick notebook, and as far as I know was never published.
Attachment 2112
Before 1918, Heber Curtis photographed the planetary with the Crossley reflector at Lick Observatory and described the planetary as "Central star...surrounded by bright matter in the form of an oblong 8" [seconds of arc] in length. Outside of this is an oblong of somewhat fainter matter 20" X 13", from the ends of which extend two fainter ansae...From the north and south sides of the central oblong extend two longer ansae...making the nebula 43" long...". This sketch was based on the photograph.
Attachment 2115
So, there are several challenges involved with NGC 6210. The central star, clearly shown on Keeler's sketch, is listed at mag 12.7, but it's buried in the high surface brightness glow and quite a challenging object. My first view as using my 24-inch, using a "reverse O-III filter" that Jimi Lowrey brought along to the Golden State Star Party in northern California.
24" (7/8/13): Viewed at 375x with an "inverse OIII" or central star filter, which dramatically dimmed this extremely high surface brightness green planetary. Without the filter the central star was not seen at this magnification (the planetary had a strong green color), but with the filter the central star was easily visible
It was much easier (unfiltered) in Jimi's 48-inch earlier this month during the Texas Star Party week. I noted, "The central star was continuously visible and situated slightly east of center." What's the minimum aperture that will clearly show the central star?
The very high surface brightness central region is surrounded by a thin fainter outer halo. These notes were made using my 18-inch
18" (7/22/06): Appears as a beautiful light blue cosmic egg at 225x with an extremely high surface brightness. Appears slightly elongated ~E-W, ~25"x20" with a very faint thin outer envelope. At 325x and 435x the faint outer halo is obvious, increasing the size ~35"x28" with the elongation possibly WSW-ENE. At 807x, the surface brightness was irregular and the faint outer shell fairly prominent. The bright inner disc was oval but not symmetric.
And what about those remarkable wings or ansae in Keeler's sketch?
They are shown very well on this HST image from the 1990's
Attachment 2113
I took these notes with my 24-inch from remote Likely, California in the the northeast corner of the state
24" (6/12/15): At 375x and 500x; slightly elongated oval disc E-W or WSW-ENE. The very high surface brightness central region is encased in a small, but obvious lower surface brightness shell. Extremely faint "wings" appear to extend to the north and south, though the northern extension is more definite. These are roughly in the direction of the minor axis.
Again the ansae were better seen in Jimi's 48-inch (perhaps Jimi or Akarsh will share their observations),
48" (5/5/15): On close inspection the main body has an irregular outline, is slightly brighter along the north side and bulges out on the southwest end. A thin fainter "strip" or shell with a much lower surface brightness and a pinkish color encases the main body. Two or three broad "jets" or wings (broadest at the base) extend out of the halo on the NNW side and from the SSE end, with the northern wing more obvious. A third wing was sometimes visible on the west side.
Attachment 2114
GIVE IT A GO AND LET US KNOW!
