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RA: 18h 18m 48s
Dec: −18° 33′ 0″
Ch: MSA:1368, U2:339, SA:22
Ref: NED, Steinicke (2009), DAML02, Archinal&Hynes (2003)
Type: star cloud
Mag: B=?, V=2.5
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IC 4715 = M24. Barnard's RA as published in AN 4239 is 10 minutes of time too large. This is either a typo, or a simple digit error. His description of the star cloud is accurate, though, as are his notes about the two dark nebulae on the northern edge. Also, his positions for the dark nebulae are correct. And the star cloud is too large to miss -- even Barnard's incorrect position is within its boundaries. The position I estimate is for the entire elongated cloud of stars, 2 deg by 1 deg. M24 may just be the northern part of this cloud, about a degree across with NGC 6603 near the center (N6603 is often mistakenly equated with M24, but the NGC object is too faint and too small to match Messier's description).
Dreyer copied Barnard's incorrect position into the second IC, so insured that a casual reader of the catalogues would not notice the identity with M24. Brent Archinal was apparently the first to catch the error, and it was pointed out to me in March 2001 by Brian Skiff.
Described by Smythe as "A region of surpassing splendor", this is the star cloud between the Lagoon Nebula and the Swan Nebula in Sagittarius. Also known as the Small Sagittarius Star Cloud, it was catalogued by Messier in June 1764 as "a large nebulosity in which there are many stars of different magnitudes; the light which is spread throughout this cluster is divided into several parts... diam 1 degree 30'."
In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, August 2, 20 feet telescope, considerable stars in great number."
Admiral Smythe wrote of it as "a beautiful field of stars; the whole is faintly resolvable, though there is a gathering spot with much stardust." Webb calls it "A magnificent region.. visible to the naked eye as a kind of protuberance in the Galaxy."Clerke notes that Father Sechi called the cloud "Delle Caustiche" from "the peculiar arrangement of its stars in rays, arches, caustic curves and intertwined spirals." Enclosed in this nebula is the open cluster NGC 6603. It lies in the northern part of the star cloud, and is the "gathering spot" Smythe referred to. It is a faint but very rich group about 5' across, but "not easy to detect in any aperture smaller than 8-inch", reports Burnham. Indeed, the total magnitude of the cluster is 11.0, with the brightest member only of 14th magnitude. It is a member of the Sagittarius OB 4 Association.
Ced 162 (IC 4715)
Position (1900): RA 18 20, Dec :- 18 29
Star: Star cloud
Spectrum of nebula: (not classified)
Classification: Nebulous cluster (Nebulous envelop of intricate structure, eg. NGC 2175)
Size: (not given)
Notes: "IC 4715. Disc. Barnard 1908 (85). (93 Pl 29, Pl 31 Pl 32). R. A remarkable nebulous star-cloud in Sagittarius, which deserves further study."
In Sky & Telescope, July, 1992, Phil Harrington writes: "Some older observing guides suggest that these designations [M24 and NGC 6603] refer to the same object. Yet,when Messier's description of M24 is compared to that for NGC 6603 in the NGC, a different conclusion emerges. Of M24 Messier wrote, 'a large nebulosity in which there are many stars of different magnitudes ... diameter 1 degree 30'.' NGC 6603, however, is called a 'remarkable cluster, very rich and very much compressed, round, stars of [12th] magnitude.' Can these be the same object? The latter description closely matches the visual impression of NGC 6603 through many backyard telescopes. Situated just 1/4 degree east-northeast of a 6th mag field star, NGC 6603 appears as a rich, albeit faint, glow measuring 5' across and accented with a few feeble points of 14th mag light. Although most definitely an open cluster, the misty appearance of NGC 6603 has led more than a few observers to believe it's a globular. Only after closer examination with higher magnifications and larger apertures will the true nature of NGC 6603 be revealed/ It is very unlikely that Messier's crude telescopes could have uncovered NGC 6603. What, then, is M24? The English deepsky expert Kenneth Glyn Jones suggests that it is the large, bright region that surrounds NGC 6603. Also known as the Small Sagittarius starcloud, it fits Messier's description perfectly. Through binoculars the whole area explodes with countless stars of many magnitudes. Highlighting the scene are a pair of dark nebulae silhouetted in front of the starcloud. Barnard 92 and Barnard 93 are near the northern edge of M24, with the former being the more prominent.
Coe, observing with a 13" f/5.6, notes: "(M 24) Extremely bright, extremely large, elongated 2X1 in PA 45, very rich, very compressed at 60X. The Small Sagittarius Star Cloud does not quite fit in the field of view on my widest eyepiece for the 13". M 24 has been mistaken for the small cluster NGC 6603 for many years. Actually, the entire Star Cloud is M 24. This is made obvious by Messier's notes. Many good views can be had with binoculars. My best view was with a 6" f/5 and a 24mm eyepiece. The dense starfield sparkles and shimmers, several dark lanes and patches stand out from the gathering of stars. The most prominent dark area is B 92 on the NW edge."
Tom Lorenzin, in the electronic version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "2.2 degree x 1.3 degree extent! axis oriented E-W; many dark nebulae include B-92 in NW segment; beautiful, if somewhat subtle OPN CL N6603 (5' diameter; 100-plus 12M and dimmer members) in lower central portion centered at the coordinates listed to the left; includes a straight, 1'-long line of 8 to 10 equidistant 12.5M stars running NNW-SSE, and comprising the "WALL"; use high-x and as much aperture as you can bring to bear; PL NEB N6567 in SW sector; talk about having your head in the stars! naked eye, binoculars, scope...use 'em all!."
"The Small Sagittarius Star Cloud. This large and milky patch of light is easily seen with the naked eye as a disconnected portion of the Milky Way. Its large size makes it best seen in binoculars. Dark streaks running its length are seen against a backdrop of countless stars. Within its boundaries lies"
Location: Jonkershoek Nature Reserve, Assegaaibosch Station
Date: 1998 July 31 / August 01, 01:00-02:40 SAST
11x80 tripod-mounted binoculars (9.5 mag stars at times not easy)
Sky conditions: Mediocre (transp. low, seeing average, dew) The skies are showing the effects of the combination of pollution (mainly from a nearby wood-processing plant) and a stable inversion layer, turning daytime skies grey-blue, and night skies ashen.
Notes: No doubt about the reality of this one! Sweeping from the triangle 15-16-17 Sgr northwards brings on a field of large and small stars, scattered over an area 2 degrees long and 45' wide, making an ill-defined but recognisable kidney-shaped grouping.
2008 May 02, 23:55
Southwell, Timm's farm.
8x21 binoculars, 2.5-inch f/7.6 refractor (EP: 25mm 28x 45arcmin fov)
Conditions: Conditions stable, good.
Size=95x35arcmin. V=3.1. Appearing almost as a separate brighter section in the Milky Way. 2.5-inch: Immensely populated, about 60 stars, many being prominent stars of M4.5-M5.2 intermingled with coarse stars of 7th-8th magnitude with some dark patches spread throughout. 8x21: very large with a noticeable uniform haze of unresolved stars belonging to the Milky Way, forming a clear boundary; also noticeable with the naked eye as a separate slightly isolated brighter patch. There are three prominent field stars: 45arcmin southwest M4.2, one degree south M4.5, and 56arcmin east M4.6. IC 4715 is easy to find and contains the open cluster NGC 6603.
Sky Conditions:The fainter parts of the Milky Way are barely visible.Haziness only visible on the horizon.Atmosphere stable with little interference.
This open cluster is an extremely star rich region which is almost four times the diameter of the Full Moon.This very large cluster is bedridden with plenty of starless patches and dark regions where there is plenty of stars glowing like lit up candles in the dark and that most of the stars in this open cluster are nearly the same brightness as each other.In overall the stars in M24 is not at all separated.This open cluster measures 30.9'x 23.7'.Chart No.317,NSOG Vol.2.
Telescope:12"-inch Dobsonian Reflector Telescope.
Eyepieces:26mm super wide field eyepiece.
20mm ultra wide angle eyepiece.
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.
Object Name:Sagittarius Star Cloud.
First Impression:Open Cluster.
Chart Number:No.12(Extract taken out of the "Atlas of the Night Sky"0.
Size:26mm Eyepiece:Field of View:57'/1=57'.
20mm Eyepiece:Field of View:50'/0.9=56'.
Size in Arc Minutes:56.5'.
Open Cluster is 56.5'*4.0'.
Brightness Profile:High Surface Brightness.
Challenge Rating:A stunning sight to observe this cluster under an extremely dark sky.
This is a large cluster which has an irregular appearance of bright stars.This cluster is also well detached which consists of over 4000 stars.All the bright and faint stars are mixed and they are not all concentrated towards each other.I have also noticed that there is a vast area of starless patches.I have found within a group of numerous bright stars there is a striking pair of bright stars.Around the bright stars in this cloud I have found a slight glow of unresolved nebulosity.
The Messier objects
The Bennett objects
The Caldwell list
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