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RA: 19h 01m 38s
Dec: −36° 53′ 31″
Con: Corona Australis
Ch: MSA:1434, U2:379, SA:22
Ref: [2003A&A...399..141M], Corwin (2004)
Type: reflection nebula
Mag: B=?, V=?
Size: 2′ x 2′
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NGC 6726, NGC 6727, and NGC 6729 are all stars immersed in nebulae. Delisle Stewart found them associated with a much larger and fainter nebulosity, IC 4812 (which see), on a 5-hour Harvard plate. The positions I give apply to the stars.
This object, along with NGC 6727, is a reflection nebulae, also known as Bernes 160. It lies on the border of Sag-CrA, between the globular cluster NGC 6723 and Gamma CrA.
Discovered by Marth, observing from Malta. He recorded it in the NGC as a "Star of 6.7 mag in faint, pretty large nebulosity."
In the JBAA (1896, vol. 6, no. 6) Innes reports on observations made in 1895. "It will be noted," he wrote, "that all the observations have been made with home-made (ie amateur) instruments, or with the naked eye."
He goes on to report:
"SCHMIDT's NEBULAE IN CORONA AUSTRALIS. - I came across these three remarkable nebulae some time ago, and do not remember seeing any similar to them before. Two of the nebulae are 'nebulous stars,' viz.,-
Lac. 7955, mag 7.5, RA 18h 54m 54s, D -37deg 1.8' (1900)
CZC 18h2868, mag 9.25, RA 18h 54m 56s, D -37deg 0.9' (1900)
from the Arg. G. Catalogue.
These two stars are involved in a double nebula, which is densest around the fainter star; each star occupies a centre. The distance between the stars is about 56" and the diameter of the nebula around the fainter star is at least 70"; around the brighter it is about 60". It is curious that Sir J. Herschel and others overlooked these, as the nebulosity can be seen through the 1.5-inch finder quite easily. A very careful comparison makes the brighter star = 7.2, the lesser = 8.5, ruddy, whilst the brighter is white.
About 5' S. is a faint irregular nebula (making the third discovered in this region by Schmidt), in which is a suspected variable star. I am glad that I can confirm this variability; my observations give -
7 October 1895 = 11.0 mag
14 October 1895 = 9.0 mag
16 October 1895 = 9.0 mag
22 October 1895 = 9.5 mag
6 November 1895 = Invisible.
16 November 1895 = 10.0 mag.
The first observation only gives the magnitude of the nebula, as the star was unnoticed through faintness on that occasion. The fine cluster Dunlop 573 and gamma Coronae Aust., a binary pair, are in the immediate neighbourhood. The whole should make a fine photograph." [1896JBAA....6..257I]
Region around S Corona Austrina. Transvaal Observatory Circular, No 5, 57.
[see also NGC 6723]
The variable stars and nebulae near R Corona Australe. Union Obs. Circ. No.36, 282-284.
Lengthy discussion, illustrated, with several referneces to previous thorough discussions.
Ced 165b (NGC 6726/7)
Position (1900): RA 18 54.9, Dec - 37 2
Star: -37 13023/4 (Mp=7.1, V=7.1, SpT=B9)
Spectrum of nebula: continuous spectrum (observed)
Classification: Neb associated with mainly one star (which may be multiple) - star surrounded by a neb envelope without conspicuous structure (eg. lambda Scorpii)
Size: (not given)
Notes: "165 b = NGC 6726 + NGC 6727. NGC 6726 = GC 5935. NGC 6727 = GC 5936. Disc. Schmidt 1860. (375). FA 40. FA 63. (366, 376, 385, 386, 486, 560, 832, 856). R. -37 13023 = HD 176386 = Boss 26118. The star -37 13024 and the nebula NGC 6727 are variable."
The RNGC (Sulentic and Tifft 1973) notes that this is a diffuse nebula.
Bernes, C. (1977) A catalogue of bright nebulosities in opaque dust clouds.
Bernes 160 = NGC 6726 / NGC 6727
Bernes notes in his Catalogue of Bright Nebulosities in Opaque Dust Clouds that these objects are also known as Cederblad 165b. He gives it a diameter of 4' and classifies it as a reflection nebula. He notes that it is part of the R CrA complex.
= Bernes 160, N6726-7, GN 18.58.3, Ced 165b, Other designations: TY CrA
Class: C+E (emission and reflection neb)
Photo Index by Jim Lucyk: Sky&Tel. 8/62 p69, Sky&Tel. 9/83 p277, Astronomy mag. 7/86 p94, Burnhams V2 p696.
For Hartung's comments, see NGC 6729.
Bernie Sanden notes there are two sets of double stars, one with nebulosity glowing faintly around its southernmost component, to the southeast of the globular. Between the two doubles, slightly to the east, is a 12th magnitude star with a wisp of nebulosity extending away from the globular. The nebulosity is fan-shaped, giving this object a comet-like appearance similar to Hubble's Variable in Mon. With direct vision the fan is small and rectangular, but with averted vision it grows to twice the length. With a little imagination, the 'comet' appears to be in orbit around the globular!
Sanford notes "the figure-8 shape of the nebulae NGC 6726 and 6727 surrounds two stars, one of which is a variable, TY CrA. This variable star changes magnitude erratically from 8.8 to about 12.5 and the nebula generally follows."
William P. Clarke (1992, Webb Society Quarterly Journal, 89, 1-7, 'A Visual Survey of the Complex Field Around R CrA & TY CrA') writes about NGC 6726-6727: "this nebular complex was found as a bright, hazy envelope around 2 stars, the brightest of mag. 7.2, and the other a variable - TY CrA - about one minute of arc away at PA 60-65 degrees; at the time of the observation, TY CrA was of about 8th magnitude. Some contrast enhancement is evident when using an O III filter with x48 on the 25-cm telscope. The size of the nebulosity was about 2'."
William P. Clarke (San Diego, California, USA) writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "NGC 6726-7 appears as a conspicuous haze around a close pair of stars, the brightest of which is TY Cra. Some contrast enhancement was found using an O III filter. NGC 6729, a reflection nebula, is found in the same field as NGC 6726-7; this is faint and highly elongated in PA 170-350 degrees: rather comet-like. Faint star (R CrA) at south end. NGC 6729 disappeared when then O III filter was introduced. The general field is fairly devoid of stars. (21-inch f/20, x140 & 10-inch, x48)."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "13M; Double-lobed complex nebular region surrounds TY and R CRA; includes comet-like N6729 just 4' to SSW; 30' ESE of GLOB N6723."
Steve Coe, using a 13" f/5.6, notes: "Pretty bright, pretty large, round, with two blue-white stars involved at 135X. The UHC filter enhances the contrast of this nebula somewhat. It is difficult to tell where NGC 6726 starts and 6727 ends, however in moments of good seeing there is a hint of a dark lane through the nebula."
variable nebula lit by R CrA.
8cm - much higher sfcbr than -23, brtr overall, yet smlr. cen * consp
15cm - sm but pretty br. NE of m7 *. R CrB m~10.
25cm - 3'-4' diam w/indef border. R CrB seems bluish. best @ 90x.
The Messier objects
The Bennett objects
The Caldwell list
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