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NGC 6656
Bennett 114
Messier 22, GCL-99
RA 18:36:23
Dec -23°54.4'
Globular
cluster 
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Burnham: Discovery of this superb globular
cluster is “usually credited to the obscure German astronomer Abraham Ihle
in 1665; virtually nothing appears to be known about Ihle, however, and it has
even been suggested that the name is a misprint for Hill”, according to
Burnham. He goes on to quote Smythe: “.. Hevelius, however, appears to have
noticed it previous to 1665.”
Halley mentions it in 1716, and LeGentil
observed it in 1747 in a telescope of 18-foot focal length, wherein it was seen
as “very irregular, long-haired, and spreading some kind of rays of light
all around its diameter”.
Lacaille included it in his 1755 catalogue as
Class I No. 12. In his half-an-inch 8x telescope he saw it as “a small
comet nucleus.” Messier in 1764 described it as a “round nebula
without stars, near 25 Sagittarii”. Sir John Herschel called it “a
magnificent globular star cluster; gradually brighter in the middle but not to a
nucleus. All the stars of two sizes: 10 and 11 mag, and 15 mag., as if one shell
over another... larger ones ruddy.” Admiral Smythe spoke of it as “a
fine globular cluster: consists of very minute and thickly condensed particles
of light with a group of small stars preceding by 3 min. somewhat in a crucial
form.”
Burnham's following comment deserves to be
quoted in full:
“It has always seemed to the author of this book that
J.R.R. Tolkein, in his delightful fantasy The Hobbit, unwittingly created an
exquisite description of M22 when he spoke of the fabulous jewel called the “Arkenstone
of Thrain”: 'It was as if a globe had been filled with moonlight and hung
before them in a net woven of the glint of frosty stars...”
In the Philosophical Transactions, 1818,
William Herschel wrote: “7 feet telescope. 460 has not light enough to show
it; with 227 I see it very imperfectly. 1801, 10 feet telescope, with 600 it is
a cluster of stars. 1783, small 20 feet telescope, with 350 power all resolved
into stars. 1784, 20 feet telescope, an extensive cluster of stars. 1810, large
10 feet telescope, the stars are condensed in the middle, the diameter is 8',
the greatest condensation is about 4'.”
h: “globular, vB, vL, vm comp, vgvmbM, 7'
diameter. The stars are of two magnitudes, viz 15..16m and 12m; and what is very
remarkable, the largest of these latter are visibly reddish; one in particular,
the largest of all (= 12-11m) S.f. the middle, is decidedly a ruddy star, and so
I think are all the other large ones.” On a second occassion he called it “globular,
fine, v rich, vm comp, gmbM, but not to a nucleus; diam in RA = 35.5 seconds;
consists of stars of two sizes, 11 and 15m, with none intermediate, as if
consisted of 2 layers, or one shell over another. A noble object.”
M22 is one of the easiest globulars to resolve
as the brightest members are 11th mag and within easy reach of an 8”.
Indeed, Hartung says a “3-inch shows the brighter ones quite plainly”.
Shapley found that this globular had an elliptical elongation in PA 25 degrees;
there were 30% more stars along the major axis than along the minor axis.
Hartung says the clusters “broad centre is about 7' across and the
scattered outliers extend to 15'; it is a most beautiful object in a fine starry
field...” On long-exposure photographs the overexposed nucleus appears
irregularly round, and quite distinct from the outliers.
Stewart Moore (Fleet, Hampshire, UK),
observing with a 12-inch f/5, writes in the The Webb Society Nebulae and
Clusters Section Report No. 10, July 1992: “A superb object and easily
resolved to the centre. Gives the appearance of having three faint fingers of
stars protruding from the W edge of the cluster.”
John Bortle (Webb Society Quarterly Journal,
January 1976) using 10x50 binoculars, estimates the visual magnitude as 5.9.
Marilyn Head writes: “... the stars are
more easily resolved. Indeed on one memorable night, the red and yellow stars in
M22 reminded me far more of a jewel box than John Herschel’s choice for
that epithet - it was absolutely beautiful!”
Tom Lorenzin: “6M; 18' diameter; very
open and uncondensed for a GLOB; well resolved 12M thru 13M stars against
unresolved, diffuse glow”
Donald J. Ware:”Probably the finest
globular cluster easily seen by observers at temperate northern latitudes. This
cluster is about 15-20' in diameter, and is fairly easily resolved to its
center. Many stars are seen in this rather loosely gathered globular cluster.
This is a”must see" object.”
Rich Jakiel posted comments on the planetary
nebula IRAS 18333-2357 to s.a.a, and provided the following references:
Borkowski and Harrington, 1991. AJ 379 168; Gillet et al.,1989. ApJ 338 862.
Steve Coe, observing with a 13” f/5.6,
notes: “(M 22) Very bright, very large, very rich, very compressed and much
brighter in the middle at 135X. This magnificent globular has a blazing core and
many streamers and coils of stars which make their way outward through the
cluster. It is second only to Omega Centauri and 47 Tucana when it comes to
globulars. I only got a short glimpse at 47 Tuc when I was in Austalia, but we
got to study Omega Centauri near the zenith with a 12.5" f/6. M22 isn't
quite that spectacular, but it is a close second.”
11x80: “A brilliant
cluster, flanked neatly by a right-angled triangle (containing a brilliant
orange star 24 Sgr) and a parallelogram. With averted vision, the cluster
appears to span about 10’ of sky. (suburban skies) [AS]
6-inch f/8.6 Newtonian: At
52x, this cluster is vL, vB and has a broad centre. The cluster has a drift time
of at least 22 seconds and appears mottled. At 104x small stars appear scattered
across the main unresolved mass. I estimated its diameter at about 8'. (suburban
skies) [AS]
8-inch Meade, 18mm eyepiece, 36.2’
fov: Very large, round, beautiful, full field very bright star cluster.
Spread out in trails and curves to a more elongated fringy edge. Faint
compressed core, with some dark lane structure. [MS]
15.5-inch f/9 Newtonian: A
15.5-inch at 220 power clearly shows this; a dark starless lane, starting at the
north-eastern edge of the nucleus, cuts across the northern quarter of the
nucleus, creating the impression that the nucleus is roughly triangular in
shape, with one apex pointing due west. Just south-east from this apex, to the
south-west, extends a brief chain of stars, and a more spread-out chain radiates
outwards from the southern point of the triangular nucleus. The angular
appearance of the nucleus has been glimpsed with a 4.5-inch reflector at 45
power. The 15.5-inch at 220 power resolves this spectacular, large, pretty
bright cluster easily, showing an unresolved central triangular nucleus
containing some red stars. The apex of the triangular nucleus points inwards to
the heart of the cluster, with the base of the triangle making up the edge. The
two prominent star chains radiate out from the centre, parallel to the two sides
of the triangle. With a 260x eyepiece, the appearance of a triangular core and
two arms is still evident, but the 220x eyepiece showed a dark lane between the
arms and the body of the triangle, whereas with the 260x eyepiece this
impression is lost as stars are now seen in these lanes. The cluster bears
magnification well, but at 260x I would describe it as a dense open cluster
since it now appears very spread out, the stars scattered around the field. At
650x the arms dissolve into little clumpings of stars, but the triangular core
defies resolution. (suburban skies) [AS] |
