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M7 is a large and brilliant group, easily
seen with the naked eye, and “one of the few clusters which can be
thoroughly appreciated in a good pair of field glasses” writes Burnham. He
continues: “It is mentioned in the catalogue of Ptolemy, and in the 16th
century Latin translation of the Almagest appears as “Girus ille nebulosus”,
the reference probably including both M7 and M6. The Arabian name “Tali al
Shaulah” is the equivalent of the Latin translation of Ulug Beg's title: “Stella
nebulosa quae sequitur aculeum Scorpionis [The Cloudy Ones which Follow the
Sting]” Hevelius includes M7 in a list published in 1690, and it appears
again in W. Derham's short catalogue of “nebulous stars” in 1730.
Lacaille observed it at the Cape of Good Hope
in 1751, and included it in his 1755 catalogue as Class II No. 14. In his
half-an-inch 8x telescope he saw it as a “group of 15 to 20 stars in a
square.”
Messier, in May 1764, described M7 as “a
cluster considerably larger than the preceding [M6]. It appears to the naked eye
as a nebulosity; it is situated a short distance from the preceding, between the
bow of Sagittarius and the tail of Scorpius. Diameter 30'.”
In the Appendix to the 1912 'Scientific
Papers of Sir William Herschel' this object is described as “1783, July 30.
About 20 small stars (Only seen once).”
h: “a brilliant coarse cluster class
VIII of about 60 stars 7-8 .. 12m, which fills field. Irregular figure. A star
8m taken.” On a second occassion he called it “Cluster VIII Very fine
and brilliant; stars of very large and mixed magnitudes. Fills field.”
The cluster is seen projected on a background
of numerous faint and distance Milky Way stars, while the bright stars of the
group are close to naked-eye visibility. On Lowell Observatory 13-inch telescope
plates the bright central portion of the cluster just fills a 30' field; the
total apparent diameter is possibly about 50'....M7 contains 80 stars brighter
than 10th magnitude in a field 1.2 in diameter. The group as a whole resembles
Praesepe (M44) in Cancer, though somewhat smaller, and would certainly be as
well known if it were further north. Incidentally, this is the southernmost
object in the catalogue of Messier.”
Harrington writes that Messier's description “seems
a bit sterile for such a magnificent group. My 7x50 wide-angle binoculars create
a three-dimensional effect as many of the brighter stars appear to float in
front of fainter points of light. Colors abound in M7, with several stars tinted
yellow and blue. The brightest is a G-type star of 6th mag lying close to the
group's centre.”
Hartung notes that M7 is “a remarkable
sight in a large field with its structure of quadrant and straight lines. With
outliers it is more than 40' wide, and very effective for small telescopes. The
fine orange star S.p. is the very close pair Struve 342. This close double was
discovered in 1897 by T.J.J. See, and Hartung notes that it seems elongated but
not truly resolved with a 12-inch telescope.
Tom Lorenzin, in the e-version of “1000+
The Amateur Astronomers’ Field Guide to Deep Sky Observing”, notes: “5M;
1 degree diameter! 50-plus 7 thru 11M members; easy naked-eye object; GLOB N6453
(11M; 1' diameter) 30' to WNW from N6475's center; M-7 looks like M-6 but twice
as large; each's brightest star is deep orange.”
Donald J. Ware:”This beautiful open
cluster is almost a degree in diameter, so either very low powers or binoculars
should be used to optimally view it. This loosely concentrated cluster is easily
visible to the naked eye, but should you observe it through a telescope, be sure
to look for NGC 6453. This is a small, faint globular cluster seemingly imbedded
in M-7's western edge.”
John Bortle (Webb Society Quarterly Journal,
January 1976) using 10x50 binoculars, estimates the visual magnitude as 2.8.
Steve Coe, observing with a 17.5” f/4.5
at 100X, notes: “ (M 7) Very, very bright, very, very large, not
compressed, many faint members at 60X. This huge cluster is easily naked eye,
even on poor nights. I can resolve 8 to 10 of the brightest members in 10X50
binoculars. It is at its best in an RFT. Using a 4.25" f/4 at 16X there is
enough room around the cluster to frame it in the Milky Way and there are 40
stars resolved with this modest scope.; 6" f/6 Dugas Very, very bright,
very, very large, not compressed, 49 stars counted, including a nice orange 9th
mag star on the SW edge of the cluster. Several delicate pairs and groupings,
best view ever of this cluster.
10x50: Triangle-shaped glow
with a distict x shaped pattern of stars in middle; wide field of stars 60';
stars more or less the same size; nebulosity of unresolved stars in the
background; two darker patches each side of the x formation (OXO) (suburban
skies) [RH]
11x80: Glorious cluster. One
orange star. Dark nebulae near, esp. Barnard 283. (suburban skies, moderate
conditions) [AS] | 