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RA: 18h 30m 56.25s
Dec: −25° 29′ 47.1″
Ch: MSA:1391, U2:340, SA:22
Ref: SIMBAD, SEDS
Type: globular cluster
Mag: B=10.81, V=9.68
Synonyms: H I-051
Discovered in 1784 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "cL R vBM easily resolvable." In the Philosophical Transactions, 1814, Herschel wrote "I.51 and Connoissance des Temps [NGC 6637] are second miniatures of the 53d [NGC 5024]."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "B, S, R, psbM, diam in RA = 4.5 seconds, barely resolved, a very delicate object, doubtless a globular."
The NGC description reads: "bright, small, round, partially resolved, some stars seen."
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.
A study by W. W. Morgan of Yerkes Observatory indicates this globular cluster to have a spectral type of G0-G2.
RA 18 30 56.2 (2000) Dec -25 29 47 Integrated V magnitude 9.02 Central surface brightness, V magnitudes per square arcsecond 17.27 Integrated spectral type G0 Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.40 Core radius in arcmin .26. ["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) ]
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 7.57
(Sulentic and Tifft 1973) notes that this is a 10.0 mag globular cluster.
Notes that it has a "broad centre." It's catalogued concentration rating, however, is 6. Hartung also notes that "this globular cluster is not easy to resolve; however faint stars may be glimpsed in it with a 12-inch...It is a conspicuous object about 70 arc seconds across...a 4-inch telescope shows it clearly but faintly."
Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "9.5M; 2' diameter; round, unresolved glow with brighter center 40' E of Lambda SGR; planetary N6644 (2" diameter; 12.2M) 30' NE; TOUGH!."
Coe, observing with a 13" f/5.6, notes: "Pretty bright, pretty small, round and much brighter in the middle. I could it to look very grainy, but it would not resolve at any magnification up to 320X."
In a 15.5-inch telescope, this moderately bright cluster lies at the end of a chain of three bright stars. The globular seems all core, with no clear division into core and fringe stars.
30/04/93: Observing with a 6-inch f/8.6 Newtonian at 52x, this cluster is pS, pF. It lies less than 1 degree E of Lambda Sgr.
12-inch Meade, 40mm eyepiece, 53' fov. 1997-08-05, fair sky conditions "Very small globular cluster just about a patch of light. Starlike with an uneven colour and elliptical shape. Embedded in a medium starfield." [Magda Streicher, Pietersburg 23-53S, 29-28E]
12-inch f/10 SCT (EP: 2-inch 32mm SW 95x 42' fov; 2-inch 14mm UW 218x 23' fov; 2-inch 8.8mm UW 346x 15' fov)
Very small globular cluster, just merely a patch of light. Star-like core and a very evidently elliptical shape in a east west direction. The bright white 2.8 magnitude star "Kaus" can be seen 40' arc minutes to the west, which underline the globular together with another three stars to the south.
Location:Night Sky Caravan Park,Bonnievale.
Sky Conditions:Whole Milky Way is visible.The sky is clean.
Atmosphere stable with little interference.
In this globular cluster the stars are not resolved and no stars are seen in this cluster and that this globular cluster has the resemblance of a very small round snowball.This globular cluster has a slightly condensed nucleus.The nucleus of this cluster is slightly brighter compared to the stars on the far outskirts of this globular cluster.This globular cluster measures 2.7'*2.7'.Chart:No.326,NSOG Vol.2.
The Messier objects
The Bennett objects
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