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RA: 18h 02m 58s
Dec: −27° 52′ 6″
Ch: MSA:1415, U2:339, SA:22
Ref: SIMBAD, VII/220A
Type: dark nebula
Somewhat north of the centre of the Sagittarius Cloud lies this dark nebula, one of the most prominent in the sky. It rims the west edge of the compact little open cluster NGC 6520, and appears as a distinct inky spot against the surrounding star-shimmer. Burnham comments that "this is one of the few dark nebulae that may be appreciated in amateur telescopes."
See New nebulae - Small black hole in the milky way - Duplicity of beta-1 Capricorni. Astron. Nachrichten, 108, 369-372.
"A small triangular hole in the milky-way, perfectly black, some 2' diameter, very much like a jet black nebula. A bright orange star on n.p. border. A very remarkable object."
Dark regions in the sky suggesting an obscuration of light. Astrophys.J., 38, 496-501.
Barnard writes of this object in his article "On the Dark Markings of the Sky", (Astrophysical Journal, January 1919) as follows: "This black spot, known to me in my early days of comet seeking [with a 5" refractor], is very sharply defined on its east edge, but less definite on the west. There is a 12th magnitude star near the middle." and adds "It was found in my comet-seeking in the early eighties. On account of its extreme blackness it was one of the most impressive objects in the Milky Way. I examined it with the 36-inch refractor of the Lick Observatory with a power of 350 and a field of 6', on August 18, 1895. It nearly filled the field of view. The western half was fairly well defined, while the eastern half was more diffused. Considerable nebulosity seemed to be connected with it. A photograph on July 11, 1917, with the Crossley reflector, kindly sent me by Dr. H.D. Curtis, shows this black spot to be very remarkable, having considerable nebulosity connected with it. On his photograph, its southwest side is very sharply defined,... Its east side...is more or less diffused. The star CD-27°12302 (mag 7.4) is on the northwest border, while CD -27°12310 (mag 9.0) is close east of the spot. The beautiful cluster of small bright stars, NGC 6520, is also close east."
Note that when Barnard observed it with a 5-inch telescope, he reported its diameter as 2 arc minutes, but in his catalogue he calls it: "Diameter 5'; several small stars in it."
Note (supplied by Bill Gray, Project Pluto): Diam. 5'; edge of diffused nebulosity. For the visual discovery of this object see Astronomische Nachrichten 108, 370, 1884.
"The following observations, therefore, were made with the 6-inch Cooke refractor at Brockhurst, usually with a fine achromatic Kellner eyepiece of 1-inch focus and field of 36' ... observed on two nights, commencing in 1929 August and ending in December ... on very clear nights, and in the total absence of the moonlight, when the Milky Way was conspicuous."
Barnard 86: "A feeble dingy cloud in oval ring of stars, its n edge touching the cluster NGC 6520; the latter curiously placed at the s end of a chain of stars running np and ending in a brighter one. (This is Secchi's dark nebula.)"
The following article, authored by H E Houghton, appeared in MNASSA Vol 1, p 107, Feb. 1942 titled "Sir William Herschel's 'Hole in the Sky'." "When Sir John Herschel was leaving for the Cape in 1834, his aunt, Miss Caroline Herschel, wrote to him as folows: 'Dear Nephew. As soon as your instrument is erected I wish you would see if there is not something remarkable in the lower part of the Scorpion to be found, for I remember your Father returned several nights and years to the same spot, but could not satisfy himself about the uncommon appearance of that part of the heavens. It was something more than a total absence of stars (I believe)'. Soon after his arrival at the Cape, Sir John wrote in reply: "I have not been unmindful of your hint about Scorpio. I am now rummaging the recesses of that constellation and find it full of beautiful globular clusters.' Miss Herschel was not satsified and replied to her nephew: "It is not the Clusters of Stars I want you to discover in the body of the Scorpion (or thereabouts) for that does not answer my expectation, remembering having once heard your Father, after a long awful silennce, exclaim `Hier ist wahrhaftig ein Loch im Himmel` and as I said before, stopping afterwards at the same spot but leaving it unsatisfied. . . . It is now generally thought to be a dark nebula reported by Secchi in RA 17h 54.6m and Decl. -27° 53'. . . Father Hagen holds that Sir William Herschel did not consider the appearance to be anything but a starless space, but it seems from Miss Herschel's first letter quoted above that her brother had some doubt about this object and that he saw something in the nature of a faint nebulous appearance. This provides anbother example of his visual acuteness, by the detection of such a faint nebulous appearance in an object which culminated at an altitude of only about ten degrees above his horizon."
A Preliminary Survey of Nebulosities and Associated B-Stars in Carina.
Seeking summer's dark nebulae. Sky&Telescope, Aug, 114-118.
by Jim Lucyk: Sky & Tel. 8/86 p130, Astronomy mag. 11/84 p11, Deep Sky #3 Su83 p8, Burnhams V3 p1643, Astronomy mag. 9/87 p50
Coe, observing with a 13" f/5.6, notes: "is a DARK Barnard Nebula right next to NGC 6520. It has an hourglass shape and is about 5 minutes in size. There is a lovely orange star in the field. Don't miss this very nice area."
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