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NGC 6526

NGC 6526, V 9, GC 4363

RA: 18h 02m 36s
Dec: −23° 35′ 0″

Con: Sagittarius
Ch: MSA:1392, U2:339, SA:22

Ref: SIMBAD

(reference key)

Type: reflection nebula

Mag: B=?, V=?

Size: ?
PA: ?

History and Accurate Positions for the NGC/IC Objects (Corwin 2004)

NGC 6526 = H V 9 is probably the part of M8 southeast of the dark lane. The nebula sweeps on up to the northeast to encompass NGC 6530, the bright, well-known cluster in M8.

WH found this the 22nd of May 1784, and measured the position with respect to 51 Ophiuchi. When re-reduced using the modern position for that star, WH's position for N6526 falls at 18 01 14, -24 27.6, well within the M8 complex. As Dreyer notes in the Herschel papers, the GC and NGC positions are one degree too far north due to an error by Caroline Herschel in her reduction of the position. WH describes V 9 only as "Large, extended, broad, milky figure." Thus, this could apply to any part of (or even all of) M8 (look at WH's second description of M20 = IV 41 for another almost discrepant description of the same object). Since this was apparently his first sweep across the area, and since we know his positions were rather error-prone at the time, I think that the object he saw was, in fact, M8. Giving him the benefit of the doubt, however, I think it fair to assign the NGC number, as I said above, to the southeastern section of the complex. See NGC 6533 for more.

Historical observations

William Herschel (c.1784)

Synonyms: H V-9

Discovered in 1784 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "Large, extended, broad, milky, faint." In the Philosophical Transactions, 1811, Herschel describes it as "a large, extended, broad, faint nebula; its nebulosity, like that of the preceding one (which is de la Caille's last but one in the Catalogue des Nebuleuses du Ciel Austral) [NGC 6523] is of the milky kind."

Published comments

[amastro] NGC 6526 reality check

There's been a thread started on s.a.a. about the supposed nebula NGC 6526, occupying the region between M8 and M20. Evidently some star- chart software and maps indicate there's a nebula here. However, if you'll look at _any_ high-contrast color image that reaches to low surface brightness levels (e.g. the Hallas photo on page 128 of the Oct. 1998 (not 1999) S&T, you'll see that there is no such nebulosity there. This misattribution in various places (for instance the MSA and the new edition of Sky Atlas 2000) arises from a bad position by Herschel. See Harold Corwin's discussion under: http://ngcic.org/corwin/DataFiles/ngcbugs_6.txt ...where he attributes NGC 6526 to the southeastern portion of M8 because of a 1-deg Dec error in the position reduction by Caroline Herschel.

In any case, the challenge remains to find an image showing any sort of nebulosity in the putative 6526 place between the two well known nebulae, and also to figure out who/how/when some nebulosity was assigned to that location---cuz there isn't any there! \Brian

------------------------------------------------------------------------

Hi Folks

NGC 6526 can be identified with the emission nebula Sharpless 2-28 or Gum 74b. Colin Gum noted Gum 74a and 74b to be "Very faint. This may be linking nebulosity between Stromlo 72 (=M8) and Stromlo 76 (=M20)." Gum 74a is Sharpless 2-26.

Trying to identify this nebulosity turned up a few errors in the catalogues. Rodgers, Campbell and Whiteoak identify NGC 6526 as RCW 151 at 18 13.5, -20 25 (1950.0) and cross reference the nebula with Gum 77b. The "NGC 2000.0" gives the position of NGC 6526 as 18 02.6, -23 35. There is NO nebulosity at this position and the immediate area is relatively free of stars. Sinnott's position is centered between Sh2-26 and Sh2-28.

A good image of this area is in Neckel and Vehrenberg's " Atlas of Galactic Nebulae " ( Vol.II, pg. 24). I bought a print from Tony Hallas at the TSP a few years ago that weakly shows a faint red tint in the positions of Sh2-26 and 28. I doubt that the magazine reproductions of this photograph would show the weak glows.

NGC 6526 is included in the Astronomical League's "Herschel II" list. I remain skeptical of the observation published in this manual as it was made with an 8" telescope and a deep sky filter. I think this particular observer was deceived by the bright starcloud that the faint nebulosities are superimposed upon.

The NASA publication "An Emission Line Survey of the Milky Way" has a nice image of the Sharpless nebulae. It suggests that a Hydrogen Beta filter may be useful when trying to detect these elusive patches. For what it is worth, I've never had any luck with these nebulae using my 18" reflector, a 48mm [OIII] filter and a 35mm Panoptic eyepiece under a reasonably dark sky. I don't have the original "discovery" notes to refer to and I'm assuming that John Herschel found NGC 6526 from South Africa. Can anyone provide these? Dave Riddle

------------------------------------------------------------------------

It was Bill Herschel from England---the Sharpless objects certainly aren't what he was describing. The Corwin URL under the NGC/IC site that I mentioned previously contains WH's original observations. Looks like this is another "nonexistent" case in the AL list, as with NGC 1990. Lots of people have "seen" it, however! \Brian

------------------------------------------------------------------------

Brian Skiff posted:

] There's been a thread started on s.a.a. about the supposed nebula

] NGC 6526, occupying the region between M8 and M20. Evidently some

] star- chart software and maps indicate there's a nebula here. However,

:

] assigned to that location---cuz there isn't any there!

Gee, and to think some people have claimed that no one posts anything about observing on sci.astro.amateur :-)! Challenge accepted Brian!

Unlike the NGC 1990 case, I did in fact manage to see something real this time at the plotted location, and it was definitely *not* scattered light, although whether it is a nebula or not is debatable. I examined the area with my UHC a week or so ago while doing my Herschel II search, which brought my response to the question about NGC 6526. The image I recalled seeing of the area was the one wide-field shot in Burnham's Vol. 3, page 1589, so I didn't give the idea of the nebula being questionable a serious thought at the time. I found some slight enhancement of a faint diffuse somewhat patchy glow around a large faint star cloud using the UHC, but this was with the region quite low. One possible explanation may have been the effect of the UHC reducing the ambient skyglow and allowing some of the unresolved starlight to be more noticable (and to think some people have said the UHC doesn't help clusters ;-)). Hence, I logged it as being seen as a nebulous cluster. It appears that the Herschel II list once again has apparently included an object which has some real questions about it. The wide-field image of the Sagittarius Milky Way in Burnham's Vol. 3, p. 1589 does indeed show a glow at that location of the "correct" shape with some imbedded stars in the shape of fat "U", but the scale is too small to tell on that image if the glow is nebular or not.

I think the main reason NGC 6526 is shown as a nebula in some of the various charts like Uranometria and the "new" Sky Atlas 2000.0 is due to that large somewhat faint star cloud bordered by a little dark nebulosity at the plotted location. This cloud of faint stars sits between two very prominent nebulae as well, so mistaking it for a nebula might be understandable, especially if those charting the region have not looked in detail at the area. The star-cloud is quite real, but now, after examining the photographs, I don't know about the nebulosity. The wide-field image of M8 and M20 in Malin's book A VIEW OF THE UNIVERSE, page 124-5 does show hints of faint nebulosity in fine red filament-like tendrils near the center of the cloud, with just a suggestion of faint diffuse nebulosity on the east and northwest sides, but they are almost overwhelmed by the number of faint stars in the cloud. The nebulae to the east (NGC 6559, IC 1274-5, IC 4678) are much more prominent, so perhaps the NGC 6526 nebulosity is out of range visually, even in a filter. Vehrenberg's shot of the M8-M20 region shows the cloud nicely, and hints at some very faint reddish patches, although they do appear granular, and may be merely faint reddened stars instead of nebulosity. Perhaps NGC 6526 can be reclassified as an open cluster, but in that case, we substitute one controversy for another (ie: is it a star cloud/Milky-Way enhancement, or is it a cluster?). Clear skies to you. --

David Knisely KA0CZC@alltel.net Re: position of NGC 6526. Well, the SIMBAD database seems to have another error, probably coordinates which have not been precessed. The correct position is 18h 4.8m Dec. -23 deg. 35'. -- David Knisely KA0CZC@alltel.net

------------------------------------------------------------------------

My one observation of something at the position of NGC 6526, on August 6-7, 1997, with a 6-inch Intes Maksutov, reads "Found NGC 6526 at 58x, better view at 47x with Orion Ultrablock LPR filter." I was using _Uranometria_2000.0_ at the time. I did not record how the size and shape of what I saw compared with what was plotted on the charts. -- Jay Freeman

------------------------------------------------------------------------

[amastro] More on NGC 6526 Since no one has read Harold Corwin's commentary, I'll paste it in below. There is a +1 degree error in Herschel's position, so the starcloud between M8 and M20 _is_not_ NGC 6526 as folks continue to insist. more. This text is from: http://ngcic.org/corwin/DataFiles/ngcbugs_6.txt \Brian

------------------------------------------------------------------------

I'll stick to my high-horse on this one since:

a) there is no visual nebula at the location where many atlases plot one;

b) Herschel's position does not correspond to the starcloud

c) the present atlas identification is not historically correct.

d) essentially no literature on the object

The SIMBAD biblio entry that mentions NGC 6526, curiously, identifies two stars belonging to the "reflection nebula" here as HD 167263 (16 Sgr) and BD-20 5043. Have a look at an atlas to see where 16 Sgr is....so the single article in the literature mentioning the NGC number doesn't apply to that object either! \Brian

------------------------------------------------------------------------

Brian posts:

] Since no one has read Harold Corwin's commentary, I'll paste it in

] below. There is a +1 degree error in Herschel's position, so the

] starcloud between M8 and M20 _is_not_ NGC 6526 as folks continue to

] insist.

]

Uh, Brian, I did read the notes Corwin made on NGC 6526. No one is really "insisting" anything, other than the fact that a visible DSO does indeed exist at the position shown on some of the most prominent star atlases used by amateurs: Megastar, the 2nd Edition of S.A. 2000.0, and Uranometria. It may look like a nebula, but the glow appears to be due to a rich but fairly faint star cloud with a few brighter stars superimposed on it. Those who made up the Astronomical League's Herschel II list obviously made the mistake of observing the cloud, and putting it on the list as NGC 6526 which, according to the sources they were using, was a nebula. As far as the observing amateur community is concerned, the issue of whether the cloud is in fact "the real" NGC 6526 that Herschel logged falls into the same category as the missing Messier objects and the assignments of M102 to NGC 5866, M110 to NGC 205, and M91 as NGC 4548. Corwin's commentary "suggests" that NGC 6526 is part of M8 (NGC 6523). At the most extreme, NGC 6526 should then be removed from the catalog, since (if the alleged error in Caroline Herschel's position is removed), it is a duplicate observation of M8. However, since it is already on some of the more prominent atlases and is in a few catalogs, it might be more prudent to simply have the astronomical community reassign NGC 6526 to the star cloud and leave it at that. -- David Knisely KA0CZC@alltel.net

------------------------------------------------------------------------

Let's try this again... Harold uses equinox 1950 coords, so the position he quotes from William Herschel below precesses to: 18 04 18 -24 27.4 (2000), which is in the eastern part of M8. \Brian

===========

from: http://ngcic.org/corwin/DataFiles/ngcbugs_6.txt

NGC 6526 = H V 9 is probably the part of M8 southeast of the dark lane. The nebula sweeps on up to the northeast to encompass NGC 6530, the bright, well-known cluster in M8.

WH found this the 22nd of May 1784, and measured the position with respect to 51 Ophiuchi. When re-reduced using the modern position for that star, WH's position for N6526 falls at 18 01 14, -24 27.6, well within the M8 complex. As Dreyer notes in the Herschel papers, the GC and NGC positions are one degree too far north due to an error by Caroline Herschel in her reduction of the position. WH describes V 9 only as ``Large, extended, broad, milky figure.'' Thus, this could apply to any part of (or even all of) M8 (look at WH's second description of M20 = IV 41 for another almost discrepant description of the same object). Since this was apparently his first sweep across the area, and since we know his positions were rather error-prone at the time, I think that the object he saw was, in fact, M8. Giving him the benefit of the doubt, however, I think it fair to assign the NGC number, as I said above, to the southeastern section of the complex. See NGC 6533 for more.

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a diffuse nebula.

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