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RA: 11h 36m 13.3s
Dec: −61° 36′ 55″
Ch: MSA:990, U2:450, SA:25
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 13r
Mag: B=5.66, V=5.3
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This cluster was discovered by Lacaille and included in his 1755 catalogue as Class III No. 7. In his half-an-inch 8x telescope he saw it as "three faint stars in nebulosity."
James Dunlop observed it from Paramatta, New South Wales, and included it as No. 289 in his catalogue of 1827. Using a 9-inch f/12 telescope, he described it as "A pretty large cluster of stars of mixt magnitudes, about 10' diameter. The greater number of the stars are of a pale white colour. There is a red star near the preceding side; another of the same size and colour near the following side; another small red star near the centre; and a yellow star near the south following extremity, all in the cluster."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded the position of "the preceding of two chief stars of a fine, large, loose, round cluster of stars 8..12th mag; gradually pretty much brighter in the middle, fills field; 150..200 stars." His second observation recorded "A very fine cluster class VII; nearly round, 8' diameter, slightly compressed in the middle, stars of 9..15th magnitude; place that of an orange star 9..10th mag following the centre."
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
Journal BAA, 36(3), Dec, p91
Contains a considerable number of brighter stars
"! cluster, condensed."
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
"Cat. of Open Cl. south of -45� Decl.", Mem. 17 Mnt Stromlo Obs.
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 12' and the class as 1 2 r.
Burnham calls it a pretty large, pretty rich compact cluster, 12' across, containing about 60 stars of 8..13th magnitude, including Innes 421, a double star of 7th and 10th mag components 1.5 arcseconds distant in PA 117 .
"A Catalogue of Estimated Parallaxes of 112 Nebulae, Open clusters and Star Groups", Vol 36 (4), p 107-115.
"fairly dense; bright stars." He gives the approx. diameter as
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Sanford writes that the cluster contains about 60 stars, is some 12' in diameter, has a number of highly coloured stars, and is "good for small aperture telescopes."
Sher, D. () "Structure of the Milky Way in Carina" QJRAS, v 6, p 299-320. "a bright and moderately compact cluster in the field of which OB and emission line stars and M supergiants have been fdound.
by Jim Lucyk: Vehrenberg's Atlas of DS Splendors (3ed) p111, Ast.Obj.for South.Tel. (Hartung, 1984).
(Sulentic and Tifft 1973) notes that this is a 4.5 mag open cluster.
Hartung calls it a "fine scattered cluster with broad central condensation [which] is effective even for small apertures. It is at least 15' across, merging into a rich field with a pattern of star loops giving a lobed appearance and containing orange, yellow, white and bluish stars."
Harrington writes that "smaller instruments show this open cluster as a very pretty, compact swarm of stars buried within the nebulous glow of fainter suns. A 6-inch telescope resolves about 80 stars of 7th to 13th mag set in an arrowhead. The brighter stars - some appearing golden, others bluish - form an arc across the cluster's northern perimeter."
Glen Cozens calls it a "fine sprinkling of colorful stars."
Gerd Bahr-Vollrath (Noosa Heads, Queensland, Australia) writes in The Webb Society Nebulae and Clusters Section Report No. 11, January 1993: "Fairly large, rich and bright. Set in a dense and rich region of the southern Milky Way, it is difficult to tell how many stars actually belong to the cluster. My guess is about 60, these ranging from mag 8 downwards. Many apparent chains and loops of stars make NGC 3766 a very attractive cluster. (8-inch f/12 SCT)"
Sher: pair in N side = H8.
15cm - vbr & rich cl well detached from rich fld. 140x: 12' diam w/200 *s well
concen. *s m8+. m11 = pair isolated on N side of core. exquisite cl
@ 30x. BS, 23Feb1990, LCO.
10x50 binocular mounted. 1997-07-08 Dew, no moon. "small bright triangle, very distict from background, 7 brighter members can be resolved in the cluster, surrounding star field makes a number 2 asterism (including 3766)." [Rui Henriques]
A beautiful tight grouping of stars, 11x80 binoculars show three primary stars, reddish in colour, forming a long triangle. The rest of the cluster members, much fainter, are distributed reasonably evenly amongst these luminaries. The cluster lies in a four-degree long snaking chain of bright stars. Due north of the cluster is a nice triangle of 7th mag stars.
A wonderful tight compact grouping in a rich field filled with bright stars. The cluster sports two red beacons on opposite sides. In a 10-inch at 30x, direct vision shows about thirty stars, but averted vision brings out the fainter members, filling in the empty spaces, resulting in a fine sprinkling of large and small stars making up this very rich cluster. At 120x the cluster is framed beautifully, showing it to be very rich in small 10th mag and fainter stars.
1994-02-06, Die Boord, 11x80's tripod-mounted. This cluster has three bright members, forming an elongated (ratio 2.5 to 1) slightly skewed triangle, pointing northwest. The two stars forming the northern edge of the triangle are a remarkable deep red. A short chain of about five stars, starting north of the cluster, crosses the northern leg of the triangle and then, inside the triangle, appears to curve back, forming a stretched S-shaped chain. This cluster needs sketching in the four-inch!
1997-03-24, Monday. Jonkershoek. 11x80 tripod. Full Moon. Magnificent. Grander version of nearby 4103. Lies in a rich field, along a 2-degree chain of bright stars. The triangular cluster appears like a lop-sided Jewelbox. Three bright stars in a narrow triangle, with stars scattered between them irregularly. At least 15 stars can be make out as fine points of light, and averted vision shows surrounding glow of many more. The 2� chain mentioned can be easily extended to form a long snake-like curve. Rui pointed out that it can even be made to look quite like a "2" - this asterism lies between 11:48 and 11:46, -61 and -63.5; contains Lambda Cen in the tip.
1998-01-30/31. Unitron 4-inch f/14.7 refractor. Die Boord. A beautiful large (9.5 arcmin, 1/4 sweeper fov) scattering of bright and faint stars, well resolved in the sweeper eyepiece. Shares this wide field with 4 very bright stars (two west-ish, one east)
The 25 arcmin field of the K18 frames the cluster perfectly (D=1/3 fov = 8 arcmin) , showing a wonderfully intricate grouping of large and small stars - sketch made. The cluster has a vague oblong shape, north-east to south-west. Very rich, and large brightness range - six or so bright stars in the grouping, filled in by a host of smaller ones. There are two distinct orange stars - one on north-east and the other on south-west edge of cluster.
Stellenbosch (Paradyskloof Rifle Range)
11x80 tripod-mounted binoculars (12.5-mm aperture mask)
Conditions: Dark moon. Slight easterly breeze. NELM approx 5.5 at the pole. Dew.
Four stars (or three, one being a close double) arranged to form a sharp triangle, lying northwest-southeast (the close double marks the northwest tip). In between these stars, and surrounding them, is a rich dim clustering of stars. With the 11x12.5mm, I cant convincingly see the three/four stars.
11x80 Laser Optics binoculars
Conditions: Clear, dark.
"Hilda's Cluster", "The Rich Man's Jewel Box".
Date: 2007 02 16, 21:00
Instrument: 8-inch Dobsonian, 25mm eyepiece
Sky: Clear; light pollution
Notes: Open cluster. Almost shape of an eye - central grouping [figure] could be seen as pupil. Two bright orange stars in the group. Further research: my 'pupil' is commonly seen as a pearl, hence the name Pearl Cluster. In light polluted skies, jewels are more difficult to see.
2007 July 04, 18:45 SAST
Walmer, Port Elizabeth
2.5-inch f/7.6 refractor (EP: 25mm 28x 45arcmin fov)
Scorpius Australis. NGC 3766 consists of 14 bright to faint stars M8-M11, with a brightness of M5.3 and a visual magnitude of M7.2. Cluster was found with a northerly sweep from theta Carinae cluster. Surrounded by many coarse stars M11, prominent stars well concentrated into a semi-circle M10-10.5, tapering concentration towards centre with two faint stars M10. NGC 3766 is bordered by contrasting dark empty spaces. Three prominent outer stars ("forehead and pincers") running NW to SE in the eastern part of the cluster vary in brightness with a relatively bright M8 red star NW, the remaining two M9-9.4 SE. Fainter semi-circle stars vary from M10-10.5. Cluster well concentrated to a large densely concentrated clump of coarse stars NE. NGC 3766 is relatively large, 40arcminutes. Cluster does appear to be of a low nebulosity.
2008 April 13, 19:20
Walmer, Port Elizabeth
2.5-inch f/7.6 refractor (EP: 25mm 28x 45arcmin fov)
Conditions: Clear, moderate.
"Pearl Cluster". Highly visible cluster with pronounced surface brightness, visible in 8x21 binoculars as an unresolved splurge. Very visible in 16x32s, placed next to many field stars. At 28x, well concentrated cluster with four stars easily resolved M7.6-8.4 (many more stars seen at a darker location � Southwell). Averted vision reveals hiudden stars better at 28x, cluster is made of coarse stars with two chains, one running southeast-to-northwest consisting of resolvable stars, the other running northeast-to-southwest consisting of stars revealed with averted vision M91-10. The Pearl Cluster is well attached to field stars southwest M6.1 12arcmin orange/red, south-southwest M6.6 13arcmin and west M6.8 9arcmin and north 5.9 12arcmin.
Location: Bonnievale SSP (Night Sky Caravan Park)
Telescope: Skywatcher 200-mm f/5, Delos 8-mm (0.57-deg fov)
Binoculars: Canon 12x36 IS (5-deg fov)
Sky conditions: Good (8.5/10)
Quality of observation: Good
Open cluster with an intricate "B" shape, backed by a bar and topped by a row of 5 stars. Pretty! Star magnitudes about 5. Size about 15-arcmin.
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