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RA: 18h 03m 37s
Dec: −24° 23′ 12″
Ch: MSA:1392, U2:339, SA:22
Type: bright nebula (HII region)
Mag: B=?, V=?
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NGC 6533 = M8 = H V 13. WH's position for this, reduced from the offsets published in the Scientific Papers is in a pretty empty patch of sky roughly 30 arcmin south of M8. He does give a pretty good description in his 1786 first catalogue, however. He observed it only one night, 12 July 1784: "Extensive milky nebulosity divided into 2 parts; the most northern above [larger than] 15 arcmin, the most southern followed by stars." What struck me about this was its uncanny similarity to his description of M8 given in his 1785 paper (which I unfortunately do not have a copy of), quoted by Kenneth Glyn Jones in his fine book on the Messier objects: "An extensive milky Nebulosity divided into two parts; the north being the strongest. Its extent exceeds 15 arcmin; the southern part is followed by a parcel of stars which I suppose to be the 8th of the Connaissance des Temps [i.e. M8]." WH's 1786 description reads like a simple condensation of his 1785 description. Is it therefore possible that H V 13 = N6533 is M8?
WH's position doesn't encourage that interpretation. Both JH (in GC) and Dreyer (in WH's Scientific Papers which he edited in 1912) have notes about WH's problems determining the position -- as I've noted, that position is over 30 arcmin south-southeast of M8 in a barren patch of sky. But if WH was indeed looking at M8, is there any way that his offsets (4m 54s following, 38':: south of 5 Sagittarii) can be made to fit? Well, once I tracked down 5 Sgr (it is SAO 186074, not labeled as "5 Sgr" in Sky Catalogue 2000.0), it was clear that the NGC position was properly reduced (once the earlier bugs found by JH had been cleaned up. He says in GC that the offset as originally published in PT for 1786 -- 39' north -- is wrong.).
Did WH observe any other nebulae that night? In particular, did he use that same comparison star? The answers are "Yes" to both questions. H V 10, H V 11, and H V 12, all = NGC 6514 = M20 = the well-known "Trifid Nebula" have a single position referred to that same star on that same night. When we reduce that position, we find that it is about 30 arcmin south-southeast of M20 in a barren patch of sky.... Yet there is no doubt that these three nebulae constitute M20 (along with IV 41); both JH and Dreyer accept that in GC and NGC. So what's going on?
The short of it: WH may have misidentified his comparison star (but see also NGC 6698, found the same night, referred to a different star). He probably used 4 Sgr = SAO 186061, rather than 5 Sgr as is printed. Once that correction is made, it's clear that NGC 6533 is, in fact, M8. WH's resultant position is about a minute of time following the brightest part of the nebula (N6523), but is more in line with the center of the entire complex as we see it on photographs. However, as Steve and I have noted before, WH's positions from these early runs of 1783 and 1784 have generally larger errors than his later positions -- he was still perfecting his observing techniques.
The mystery here is this: if JH and Dreyer knew that H V 10-12 referred to the Trifid, why then did they not make the connection -- through the comparison star in common -- to the Lagoon as well? I don't see an answer to this in any of the papers I have in my collection. However, if there is any information in WH's 1785 paper that might shed some light on this, we should look at it again.
Synonyms: H V-013
William Herschel observed it in 1784 with his newly completed 18.7-inch f/13 speculum telescope. He called it "an extensive milky Nebulosity divided into two parts; the most north being the strongest. Its extent exceeds 15'; the southern part is followed by a parcel of stars." and also ". . . the most north, above 15'. the most south, followed by stars."
The RNGC (Sulentic and Tifft 1973) notes that this is a nonexistent object. Their coded description reads NF S.
A catalogue of H-II regions. Astrophys.J.Suppl.Ser., 4, 257-279.
Sh 2-25: "M8. Appears to be connected with M20. Part of I Sgr association. Contains cluster NGC 6530"
Instrument:12"Dobsonian Reflector Telescope.
Sky Conditions:The fainter parts of the Milky Way are barely visible.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere stable with little interference.
Chart Number:No.18(Extract taken out of "Atlas of the Night Sky").
Size:26mm Eyepiece:Field Of View:57'/3.5=16.2'.
20mm Eyepiece:Field Of View:50'/3=16.6'.
Size in Arc Minutes:16.4'.
Brightness Profile:High Surface Brightness.
Challenge Rating:A fantastic sight to observe under extremely dark skies.
This nebula has some areas of uneven brightness while other areas are extremely bright.This nebula presents a bright open cluster of stars where star formation is still taking place.This nebula has the appearance of a lagoon when observed through my telescope.Close to the center of this nebula,I have found a dark patch where no stars are visible.
The Messier objects
The Bennett objects
The Caldwell list
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