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RA: 18h 20m 0s
Dec: −17° 06′ 0″
Ch: MSA:1368, U2:339, SA:15
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 23pn
Mag: B=7.24, V=6.9
This open cluster was discovered by Messier in June 1764, and he wrote of it as "a cluster of small stars, a little below M17; surrounded by a slight nebulosity, Easier to see than M16... Appears nebulous in a 3.5 foot telescope; with a better telescope the stars can be seen... diam 5'"
In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, July 31, 20 feet telescope, About 20 large and several small stars irregularly scattered. 1784, June 22. A cluster of coarsely scattered large stars, not rich."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 7' and the class as 2 3 p.
Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part IV. M.N.R.A.S., 36(2), 58.
Burnham says that M 18 is "among the minor objects in Sagittarius... little more than a loose grouping of about a dozen stars, the brighter members arranged in several coarse pairs, on a background flecked with countless faint star-points. Admiral Smythe says the field contains "a long and straggling assemblage of stars... The whole vicinity is very rich and there are several splendid fields about a degree to the south." This latter reference is to M24. Burnham notes that Messier's impression of nebulosity "was doubtless due to the unresolved background of faint stars.. photographs do show a faint nebulosity enveloping the cluster, but this cannot be detected visually..."
The RNGC (Sulentic and Tifft 1973) notes that this is a 8.0 mag open cluster.
Vorontsov-Velyaminov, B. "Integral magnitudes of south star clusters", Astron. Nach. 228, 325. Comparing the brightness of the cluster with the extrafocal images of stars, he estimates the magnitudes as 7.33
(IAAC) Obj: M18 (NGC 6613) - Inst: 12.5" f/6.5 Cave equatorial Newtonian
Observer: Dave Mitsky
Your skills: Intermediate (some years)
Date/time of observation: 6/18/98 06:10 UT
Location of site: Naylor Observatory http://www.msd.org/obs.htm (Lat 40.1d N, 76.9d W, Elev 570')
Site classification: Exurban
Sky darkness: ~5.0 Limiting magnitude
Seeing: 8 1-10 Seeing Scale (10 best)
Moon presence: None - moon not in sky
Instrument: 12.5" f/6.5 Cave equatorial Newtonian
Object(s): M18 (NGC 6613)
Category: Open cluster.
Class: II 3 p
Data: mag 6.9 size 9.0'
Position: RA 18:19.9 DEC -17:08
M18 is a sparsely populated open cluster about a degree south of M17. It consists of about 20 stars of varying brightness that span about 10 light years. This 32 million year old cluster is some 4900 light years away.
At the eyepiece M18 appeared as a large but scattered stellar group.
Steve Coe, observing with a 13" f/5.6, notes: "(M 18) Bright, pretty large, not rich, not compressed at 60X. This coarse cluster is seen in the 11X80 finder and I can resolve 15 pretty bright stars."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8M; 7' diameter; small and sparse; 12-plus 9 thru 10M members; raspberries!."
As seen with a 15.5-inch working at 220 power, this poor open cluster has about 11 prominent members and is very well spread out - it appears better in a finder. Three of these stars form a curved bough on the northern edge of the cluster.
30/04/93: Observing with a 6-inch f/8.6 Newtonian, this cluster has about 20 stars in all, the three brightest lying in a short bow NE-SW. This row marks the edge of the cluster, the other, fainter members lying to the south and east. The surrounding field is very busy and has several 8-9th mag stars. The cluster is pB, pS, iF and I estimate the Trumpler class to be III 3 m.
1995-05-30: 11x80.Technopark. 23:00 SAST. Hazy sky. Small round knot with a few stars. Milky Way here very rich, especially M24 to the south.
1997 April 21. Jonkershoek. 11x80 tripod-mounted. No moon. Looks like a round globular cluster with several stars; compact cluster, small but bright.
1997 July 7, Monday, 21:00 - 24:00 Jonkershoek. 11x80's tripod-mounted. With a spectacular field to its south, this bright, roundish cluster looks like a globular cluster, with some stars resolved.
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.
Clearly noticed as a small, lumpy, irregularly-round mottled patch.
Instrument:12-inch Dobsonian Reflector Telescope.
Sky Conditions:The fainter parts of the Milky Way are barely visible on the horizon.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere stable with little interference.
Object Type:Open Cluster.
First Impression:This object looks like an open cluster.
Chart Number:No.12(Extract taken out of "Atlas of the Night Sky").
Size:26mm Eyepiece:Field Of View:57'/8= 7.1'.
20mm Eyepiece:Field Of View:50'/7= 7.1'.
7.1'+ 7.1'= 14.2'.
Size in Arc Minutes:7.1'.
Open Cluster is 7.1'* 2.3'.
Brightness Profile:The whole cluster is equally bright all over.
Challenge Rating:Very Easy.
This open cluster's stars are not separated. I have in overall counted 25 stars within a fixed diameter. Most of the stars in this cluster however are nearly the same brightness as each other.The stars range from 8th to 9th magnitude and that the stars are not at all concentrated. On a final note within this cluster there are plenty of starless patches.
The Messier objects
The Bennett objects
The Caldwell list
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