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NGC 6121
Bennett 75
Messier 4, GCL-41
RA 16:23:33
Dec -26°31.6'
Globular
cluster 
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This beautiful globular cluster was probably
discovered by Cheseaux in 1746. He described it as “close to
Antares...white, round and smaller than the preceding ones. I do not think it
has been found before...”
Lacaille included it in his 1755 catalogue as
Class I No. 9. In his half-an-inch 8x telescope he saw it as “a small comet
nucleus.”
Messier observed in it May 1764, describing it
as a “cluster of very small stars; with an inferior telescope it appears
more like a nebula; this cluster is situated near Antares and on the same
parallel. Observed by M. de la Caille and reported in his catalogue... Diameter
2.5'“
In the Philosophical Transactions, 1814,
Herschel described it as “a rich cluster of considerably compressed small
stars surrounded by many straggling ones. It contains a ridge of stars running
through the middle from sp to nf. The ridge contains 8 or 10 pretty bright
stars. All the stars are red. The curious construction of this cluster is
sufficiently accounted for by the bright stars in what is called a ridge; the
small stars accumulated about it having somewhat the appearance of the shelving
sides of the ridge. The observed red colour was probably owing to the low
situation of the object.” In the Philosophical Transactions, 1818, William
Herschel wrote: “.”1783, 10 feet telescope. All resolved into stars. I
can count a great number of them, while others escape the eye by their
minuteness. 1783, small 20 feet telescope. All resolved into stars. 1784, 20
feet telescope. The cluster contains a ridge of stars in the middle, running
from sp to nf.”
Admiral Smyth called it “a compressed
mass of small stars with outliers and a few small stellar companions in the
field. It is elongated N-S and has the aspect of a large pale granulated nebula,
running up to a blaze in the centre.”
Burnham calls it “a rather loose cluster,
showing no great central condensation, and begins to show resolution into stars
in a good 4-inch refractor. The brightest detail and first to appear is a
curious central 'bar' or chain of 11th magnitude stars, some 2.6' in length,
oriented toward PA 12 .” This bar was noted by Herschel in 1783, who
described it as a “ridge of 8 or 10 pretty bright stars running from the
middle to N.f.” The NGC description reads “Cluster, 8 or 10 stars in
line, with 5 stars, clearly resolved into stars.” Burnham adds that “with
larger apertures, many other chains of stars are discerned around the edges of
the cluster, flowing outward in the form of curved loops and streams. The
central bar makes the cluster appear quite oblate in small instruments, but long
exposure photographs show that the outer distribution is very nearly
spherical...In a star count reaching to magnitude 19.3, slightly over 10,000
star images have been detected in M4; the number of fainter stars will probably
never be known.”
Hartung writes “it is crowded with stars
running to a broad haze at the centre, across which is a bar of brighter stars.
The outliers are in curved arms, forming a marked concentric pattern reaching to
12' across and visible with care even with a 3-inch.” Because of its
proximity to Antares, M 4 is very easy to find since both objects are visible in
the same binocular field.
Bennett observed it with a 5-inch short-focus
refractor, including it in his list of cometary objects as number 75. His coded
description describes it as a circular or slightly elliptical extended object,
fully or partially resolved into stars under a higher magnification, with a
larger aperture or in very good conditions.
William P. Clarke (San Diego, California, USA)
writes in the The Webb Society Nebulae and Clusters Section Report No. 11,
January 1993: “Observed [NGC 6121 & NGC 6144] with 20x80 binoculars.
NGC 6121 (M4) just shows some traces of resolution. NGC 6144, its smaller
neighbour, is a small hazy spot north of the line joining Antares and M4. The
field is an excellent binocular subject.”
Tom Lorenzin: “5.9M; 26' diameter; like
M-13 but less so; bright belt of stars girds cluster through center in a N-S
direction”
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 very impressive cluster.
Easily resolved and stands magnification well. One particularly bright star
visible on the S.W. edge. Unlike other reported observations, I found that M.4
appeared perfectly round. A pleasing starfield. Due to low haze this cluster was
not a naked eye object.”
Steve Coe, observing with a 17.5” f/4.5
at 100X, notes: “(M 4) Very bright, very large, irregularly round and much
brighter in the middle at 135X. There is a central bar that can be seen even in
the 8X50 finderscope and long chains of stars surround this cluster and wind
their way out into the Milky Way.; 36" f/5 TSP 96 S+T 7/10 20mm Nagler
Amazing spray of stars it is spectacular, as you would expect. Several of the
chains are curved, but are parallel to each other, so they appear like old Roman
stadium seats. Little pairs all over the place, several stars are light orange
in color. I counted 76 stars in NE quad. A terrific view of a real showpiece.; 6"
f/6 Dugas Rd. 5/10 seeing, a little twinkly; 6/10 contrast. with 8.8mm EP very
bright, very large, irregularly round, the "bar" feature obvious, with
several chains of stars, even at just 100X. The curved chains form a "stadium"
row of seats. In moments of good seeing, there is a silvery background of stars
seen. I counted 32 stars resolved.”
Donald J. Ware:”As this fine globular
cluster lies only about 1.5 degrees due west of the bright star Antares, this is
one of the easiest of the Messier objects to find. This loosely concentrated
globular is about 15' in diameter, and is easily resolved. Visually, this object
is distinctive in that about 8-10 relatively bright stars appear to form a bar
right through its center. These stars help in giving the appearance that the
cluster is slightly elongated.”
John Callender, 8-inch Dobsonian: “Easy
at 49x, a diffuse glow with a sprinkling of foreground stars, especially a
north/south line of stars that seemed to bound the east side of the cluster,
giving it a semi-circular look. The north/south line of stars was even more
prominent at 122x and 244x, since the background glow disappeared at those
magnifications.”
8-inch Meade, 18mm eyepiece:
“Showpiece globular cluster of well resolved bright stars. Stars bright and
faint forming lines round and about that lies scattered towards the edge. Very
gaseous field.” [MS] |
