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RA: 16h 55m 36s
Dec: −39° 28′ 0″
Ch: MSA:1460, U2:407, SA:22
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 13m
Mag: B=7.09, V=6.4
This open cluster was discovered by Lacaille and included in his 1755 catalogue as Class I No. 10. In his half-an-inch 8x telescope he saw it as an "elongated faint oval spot."
James Dunlop observed this object from Paramatta, New South Wales, and included it as No. 520 in his catalogue of 1827. Using a 9-inch f/12 telescope, he described it as "a cluster or group of small stars, about 4' diameter, with branches extending S.p. and N.f., with considerable compression of the stars towards the centre of the group. This answers to the place of 155 Scorpii, but there is no nebula."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "cluster VI class, B, L, rich, discrete, 12', irregular figure, vlbM, fine object; place of a red star 9th mag, rest 11th mag, white." On a second occassion he called it "a fine large rich cluster, class VII, stars 9..12th mag, fills field, place of a red star 8-9th mag in centre." His third observation was recorded as "a p rich brillaint cluster of stars 10...12th mag, with one 7-8th mag near middle."
The NGC records it as "Bright, large, rich, composed of stars 8-11th magnitude."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 11' and the class as 1 3 m.
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
The RNGC (Sulentic and Tifft 1973) notes that this is a 8.0 mag open cluster.
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
Photo Index by Jim Lucyk: Burnhams V3 p1681, Vehrenberg's Atlas of DS Splendors (3ed) p171.
Vogt. N. & Moffat, AFJ (1973), "Southern Open Star Clusters III." Astron.Astrophys.Suppl., 10, 135-193. [image, table]
"Only stars which lie in the most obvious core with diameter 5' of this medium populated cluster have been measured. d = 1.03 kpc, earliest Sp = B5.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Hartung calls it "a scattered star group about 10' across, somewhat extended north-south with a bright orange leader 2.5' S.f. ... merges into a fine field."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "10' diameter; bright, large and rich; 40-plus 8 thru 11M members; 1.5 degrees SSE of 3M star pair, Mu 2 & 1 SCO."
Steve Coe, observing with a 17.5" f/4.5 at 100X, notes: "Bright, pretty large, rich, elongated 'V' shape, 12 bright stars and several dozen fainter stars at 100X.
10x50 tripod-mounted. 1997-07-06. "many faint stars resolved, small, rich and condensed" [Rui Henriques]
The cluster is easy in 7x50 binoculars as a roughly triangular nebulous patch with 1 bright reddish star.
1995 May 24, 21:00 Jonkershoek. 11x80. Very easy as a round puff of light with a bright star attached.
Observing from Stellenbosch, 1983, I saw this cluster in a 2-inch refractor as a star-poor object, only three stars easily visible, whilst another 2 were suspected. The 3 stars give it a triangular shape, although I get the impression that it has a sausage-shape. It is easily possible to go right over this object with low powers and not recognize it as a cluster.
A two-inch refractor shows a small, compact open cluster; at first glance, triangular in shape, with a bright star on its eastern edge. DIrect vision shows 5 other cluster members, not counting the eastern leader, whilst averted shows a whole sea of small stars. Casual impression is of an elongated nebulosity with a very bright tip. Two of these brighter stars in the cluster form a small isoceles triangle with the eastern leader.
With a 15.5-inch reflector the cluster has 8 or so bright stars and numerous fainter one, arranged so as to appear elongated north-south. In fact, the cluster appears to be divided along a north-south line by a starless strip, the western half containing the brighter stars (including the red one), whereas the eastern strip has only one pretty bright star, situated on its northern tip. At this magnification, the cluster is not well separated from the background stars, and is mostly notable for its striking red member on the southern tip.
The Messier objects
The Bennett objects
The Caldwell list
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