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RA: 18h 10m 18.38s
Dec: −31° 45′ 48.6″
Ch: MSA:1415, U2:377, SA:22
Ref: SIMBAD, Collinder (1931), SEDS
Type: globular cluster
Mag: B=12.39, V=11.29
Discovered by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "globular, pB, irreg R, gmbM, composed of stars 16m, on a milky way ground of mixed L and vS stars." On a second occassion he called it "globular, pB, R, gbM, 80 arcseconds, resolvable or resolved in a field very full of milky way stars." His third observation was recorded as "globular, not vB, R, glbM, 2', resolved. The stars barely discernible."
Hinks, A. R. (1911) On the galactic distribution of gaseous nebulae and of star clusters. MNRAS, 71(8), 693-701.
List 6: "NGC numbers of clusters classed as globular, not in Bailey's catalogue"
Bailey, S.I. A catalogue of bright clusters and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
RA 18 10 18.4 (2000) Dec -31 45 49 Integrated V magnitude 9.26 Central surface brightness, V magnitudes per square arcsecond 17.08 Integrated spectral type F7- Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 2.50c: Core radius in arcmin .03. ["Catalog Of Parameters For Milky Way Globular Clusters", compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487) ]
The RNGC (Sulentic and Tifft 1973) notes that this is a globular cluster.
vS cl. about 1'x1'; probably should be classed as globular.
Confirms HOB 21.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Steve Coe, observing with a 13" f/5.6, notes: "Pretty bright, pretty small, round and has a very bright middle. I could resolve 10 members at 220X."
30/04/93: I searched in vain for this cluster with the 6-inch f/8.6 Newtonian at 52x and 72x.
Observing site: Pinnacles overlook
[18h 10m 18s, -31° 46' 0"] A small, very faint cluster, not granulated. It was not easy at 163x, and was almost invisible at 36x
The Messier objects
The Bennett objects
The Caldwell list
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