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NGC 6397
Lac3-11, Dun 366
RA 17:40:41.3
Dec -53°40' 25''
Globular
cluster |
This globular cluster was discovered by
Lacaille and included in his 1755 catalogue as Class III No. 11. In his
half-an-inch 8x telescope he saw it as a "faint star in nebulosity."
James Dunlop observed it from Paramatta, New
South Wales, and included it as No. 366 in his catalogue of 1827. Using a 9-inch
f/12 telescope, he described it as "a pretty large nebula, extended nearly
in the parallel of the equator, brightest and broadest in the middle; a group of
very small stars in the middle give it the appearance of a nucleus, but they are
not connected with the nebula, but are similar to other small stars in this
place which are arranged in groups. The nebula is resolvable into stars."
Sir John Herschel observed it at the Cape of
Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "globular
cluster; fine; large; bright; round; gradually brighter to the middle; not very
compressed; 5' diameter, but stragglers extend a great way. In the middle is a
more compact group of much smaller stars. The stars at circumference are larger
than in the middle; at N.f. border is a double star." On the second
occassion, Herschel "viewed past meridian, a fine, large rich cluster; not
very compressed, stars of 13th magnitude. In the S.f. part is a delicate double
star." His last observation recorded it as a "Beautiful globular
cluster; large; rich, somewhat coarse, 10' diam. taking in all outliers; rather
irregularly round, very much compressed in the middle where, however, the stars
are very small, while every where else they are 13th magnitude."
Burnham notes that it is "not one of the
richer globulars, but has a rather loose, scattered structure which permits easy
resolution in relatively small telescopes. The extreme diameter is close to 20',
and the total magnitude is 7.3. The two dozen brightest stars (10..12th mag)
show no evident concentration towards the cluster centre, but seem to be
distributed in random groups and curving rows across the background of fainter
members. The cluster closely resembles the better-known M4 in Scorpius."
Burnham also notes that it is possibly the nearest globular cluster, and and
also one of the oldest.
Hartung writes that "on a clear dark
night this cluster makes a wonderful sight, with bright scattered outliers over
an area 20' wide round the well-condensed centre 3' across; there are orange
stars in it and some of the outliers are in arcs and sprays. This is one of the
best globular clusters for small telescopes, 3-inch resolving it well."
Sanford notes that "visually it resembles
M4 in Scorpius, with the brightest stars scattered across the 'face' of the
grouping, with no concentration in the centre."
Phil Harrington (1990, Touring the Universe
through Binoculars) writes that it "presents an attractive face through
binoculars. Although no separate stars are visible, a definite 'grainy' surface
texture is seen through large glasses, as if stellar resolution were imminent
... due to its far southern declination .. it is destined to remain a
little-observed beauty."
ASV Journal Vol 24 No 3 June 1971: "beautiful
object - many stars in 6-inch 64x."
"10 Easy Globs!" by Marilyn Head
(105 Owen Street, Newton, Wellington, Aotearoa / New Zealand;
[email protected]) "NGC 6397 lies about halfway between and down from
Alpha and Beta Arae. Because it is close (7200 l.y.), the stars are mostly
resolveable and cover a wide field in a regular, circular pattern , which makes
it exceptionally pretty."
The mean blue magnitude of the 25 brightest
stars, excluding the 5 brightest, is 12.71. The RNGC (Sulentic and Tifft 1973)
notes that this is a 7.5 mag globular cluster. From Harris: Integrated V
magnitude 5.73 Central surface brightness, V magnitudes per square arcsecond
15.65 Integrated spectral type F4 Central concentration, c =
log(r_total/r_core); a 'c' denotes a core-collapsed cluster 2.50c: Core radius
in arcmin .05. ["Catalog Of Parameters For Milky Way Globular Clusters",
compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from
http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487)
]
11x80: As seen in 8x40
binoculars, the cluster lies near two bright stars, forming a right-angled
triangle with them. When near the horizon, it looks like an unfocused star and
can be passed over without recognition, but is nevertheless reasonably easy to
spot. 11x80's show a very bright broad-centered globular. Several field stars
very close by. Rating: very easy. Easy to find, since it shares the picturesque
field with the blue-white Gamma Ara and the orange Beta. [AS]
11x80: “pB, L, R
globular, with three small stars in the outer reaches. Forms a 90° triangle
with two bright stars (one is Pi Ara; cluster at hypotenuse). The cluster grows
gradually brighter to the middle, to a broad dense bright disc, 6' across. With
averted vision, the cluster expands to delicately cover a 14' circle of sky.”
(urban; seeing good; transparency below average; dew) [AS]
15.5-inch f/9 Newtonian: A
15.5-inch reflector resolves the edge of the cluster into stars, but the centre
is not resolved, appearing angular. The cluster is small but pleasant to view,
and it appears rather untidy, with many stars around it which straggle from the
cluster outward. (suburban skies) [AS] |
