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RA: 18h 04m 30s
Dec: −24° 22′ 0″
Ch: MSA:1392, U2:339, SA:22
Ref: SIMBAD, Collinder (1931), DAML02
Type: open cluster
Mag: B=4.74, V=4.6
It is a member of the Sagittarius OB 1 Association.
Discovered by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "cluster large and bright, just fills field, a double star taken. The cluster follows M8." On a second occassion he called it "a bright, pretty rich irregularly round cluster, place that of a double star in the following part of the cluster, which is itself in the great nebula M8."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 14' and the class as 2 2 mN.
Duncan, J. C. (1920) Bright nebulae and star clusters in Sagittarius and Scutum photographed with the 60-inch reflector. Astrophys. J., 51, 4-12.
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 6.97.
The RNGC (Sulentic and Tifft 1973) notes that this is a 7.5 mag cluster associated with nebulosity.
John Bortle (Webb Society Quarterly Journal, January 1976) using 10x50 binoculars, estimates the visual magnitude of NGC 6523+6530 as 3.7.
Observer: Lew Gramer; Your skills: Intermediate; Date and UT of Observation: 1997-07-4/5, 03:45 UT; Location: Savoy, MA, USA (42N, elev 700m); Site classification: rural; Limiting magnitude: 7.1 (zenith); Seeing: 5 of 10 - mediocre, near total cumulus; Moon up: no; Instrument: Naked eye, 50mm Simmons binoculars; Magnification: 1x, 7x; Filters used: None; Object: M8, M20, ngc6530 (globular ngc6544 not seen); Category: Reflection nebulae and open cluster; Constellation: Sgr; Data: mags 4.6, 6.3, 4.6 sizes 80'x40', 28', 15'; RA/DE: 18h00m -24o;
Description: Two bino fields (8o) due N of Sgr's teapot spout, and arrayed about the line formed by the mag 4-6 stars 4, 7 and 9 Sgr and the blurry open cluster n6530. M8 and M20 fall easily in the same field, forming between them a beautiful complex of BRIGHT haze and intermingled field stars. The dark bar running through the center of M8 was readily apparent even at 7x, as was some mottling whenever averted, concentrated vision was used. M20 to the N appeared much fainter, and (more suprisingly) larger than M8, and often required averted vision to see. No detail could be discerned in the extended haze. Interestingly, back- ground stars in this field were so thick and poorly resolved that they seemed to form a "bridge" of nebulous light between and among the two brighter (M8) & fainter (M20) true nebulae. No note of globular ngc 6544, SE of M8, was made at the time.
Observing site: Pinnacles overlook
[18h 4m 48s, -24° 20' 0"] This is the cluster associated with M8. About 20 bright stars, embedded in the nebula. A spectacular sight, overall.
The Messier objects
The Bennett objects
The Caldwell list
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