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NGC 6531 (14,896 of 18,816)

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Messier 21

NGC 6531, Cl Collinder 363, C 1801-225, OCISM 3, Ocl 26.0, COCD 427, Messier 21, h 1993, GC 4367

RA: 18h 04m 12s
Dec: −22° 29′ 0″

Con: Sagittarius
Ch: MSA:1392, U2:339, SA:22

Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)

(reference key)

Type: open cluster, 13r

Mag: B=6.02, V=5.9

Size: 14′
PA: ?

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Photos  (1)

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Historical observations

Messier, Charles

Messier discovered the cluster in June 1764 whilst examining the Trifid, and thought he saw some hint of nebulosity.

William Herschel (c.1784)

In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1786, May 26, 20 feet telescope, a rich cluster of large stars."

Published comments

Doig, P. (1925)

Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part IV. M.N.R.A.S., 36(2), 58.

Trumpler, R.J. (1928)

Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 12' and the class as 1 3 p.

Burnham's Celestial Handbook

Burnham describes it as "a fairly compact group highlighted by about six 8th mag stars in a tight knot, surrounded by several dozen more scattered members." Admiral Smythe described it as "a coarse cluster of telescopic stars in a rich gathering galaxy region... About the middle is a conspicuous pair, A, 9 mag yellowish; B, 10 mag ash coloured; PA 317 , d. 30.9" (1875). Messier included some outliers in his description and what he mentions as nebulosity must have been the grouping of minute stars in view..."

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 7.0 mag open cluster.

Vorontsov-Velyaminov, B. (1925/1926)

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 6.54

Raab, S. (1922)

Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.

Discussed, based of F-A plates.

Bailey, S.I. (1908)

"cluster, fairly condensed"

Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.

Doig, P. (1926)

Doig, P. (1926) "A Catalogue of Estimated Parallaxes of 112 Nebulae, Open clusters and Star Groups", Vol 36 (4), p 107-115.

"irregaulr; condensed." He gives the approx. diameter as 15 arcmin.

Melotte, P.J. (1915)

A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.

Modern observations

Harrington, Phil

Harrington writes: "Since I first viewed this group through a 2.4-inch refractor more than two decades ago, M21 has become a favorite of mine on a midsummer's night ... M21 is one of those rare open clusters that look good in just about all amateur telescopes. Through small instruments, it is easily resolved into about two dozen stars highlighted by a string of stars near its centre. Larger scopes increase the count to about 60, with several close pairs seen within the groups 13' span."

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "7M; 10' diameter; 50-plus 9 thru 12M members; bright center star; unimpressive at first glance; close study yields the smaller members which flesh this beast out; in NE quadrant is an almost perfect circle of 11 thru 13M stars with center void! very, very pretty! 45' NE of M-20's center."

Steve Coe

Steve Coe, observing with a 13" f/5.6, notes: "(M 21) Bright, pretty large, pretty rich, somewhat compressed, several bright members at 100X. I counted 48 stars at 135X."

Contemporary observations

Auke Slotegraaf

1983

Observing from Stellenbosch, 1983, I saw this cluster in a 2-inch refractor as well spread out, few bright stars except for the brightest, which is double. Appears stellar in 8x40 binoculars.

1998 July 31

Location: Jonkershoek Nature Reserve, Assegaaibosch Station

Date: 1998 July 31 / August 01, 01:00-02:40 SAST

11x80 tripod-mounted binoculars (9.5 mag stars at times not easy)

Sky conditions: Mediocre (transp. low, seeing average, dew) The skies are showing the effects of the combination of pollution (mainly from a nearby wood-processing plant) and a stable inversion layer, turning daytime skies grey-blue, and night skies ashen.

Casually sweeping over this area, didn't notice this knot at first, seeing it only as a mB star. Once I fixed it in the view, I saw it as a small bright knot of stars. In the centre is a bright star (or very small knot of stars) which swells wonderfully with averted vision into a host of stars. A subtle object tonight.

Tom Bryant

2010 6 11 3:4:23

Observing site: Little Tycho Observatory

Telescope: C-8

[18h 4m 36s, -22 30' 0"] A group of around 20 stars, as per Dyer's description.

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