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NGC 4755
Jewelbox, Mel 114, Cr 264, Rb 96
RA 12:53:35
Dec -60°19.0'
Ocl |
This cluster was discovered by Lacaille and
included in his 1755 catalogue as Class II No. 12. In his half-an-inch 8x
telescope he saw it as "five to six stars between two mag. 6 stars."
James Dunlop observed it from Paramatta, New
South Wales, and included it as No. 301 in his catalogue of 1827. Using a
9-inch f/12 telescope, he described it as "five stars of the 7th magnitude,
forming a triangular figure, and a star of the 9th magnitude between the second
and third, with a multitude of very small stars on the south side."
It was Sir John Herschel who made it famous by
comparing it to a "superb piece of fancy jewellery." He observed it
during his stay at the Cape of Good Hope with an 18inch f/13 speculum telescope.
He recorded it as "the central star (extremely red) or a most vivid and
beautiful cluster of from 50 to 100 stars. Among the larger there are one or
two evidently greenish; south of the red star is one 13th mag, also red; and
near it is one 12th mag, bluish." His next observation recorded it as "The
same red star taken. Several others laid down, of different shades of green."
In preparation for his meticulous sketch of the cluster, he drew up a catalogue
of 110 stars, accompanied by the following explanation: "Though set down
by Lacaille as nebulous, and on that authority entered as a nebula in Bode's
Catalogue, no nebula is perceptible in any part of the extent of this cluster,
which though neither a large nor a rich one, is yet an extremely brilliant and
beautiful object when viewed through an instrument of sufficient aperture to
show distinctly the very different colour of its constituent stars, which give
it the effect of a superb piece of fancy jewellery. The area occupied by it is
about one-forty-eighth part of a square degree, within which area I have laid
down, partly from micrometric measures (as regards the large stars) and partly
from intertriangulation by the eye (as respects the small ones) the stars (110
in number)...." As regards colour, Herschel listed eight stars in which
the "colour is conspicuous"; these colours are greenish-white (3
stars), green (2 stars), blue green, ruddy and red. Concerning the appearance
of nebulosity mentioned above, Donald McIntyre (FRAS) in a publication of the
Astronomical Society of South Africa ("An Astronomical Bi-Centenary. The
Abbe de Lacaille's Visit to the Cape, 17511753") wrote: "Lacaille
includes Kappa Crucis in his Catalogue. Sir John Herschel during his residence
at the Cape made repeated observations of this famous cluster. but could see no
trace of any nebulosity. Yet curiously, Edward James Stone, Her Majesty's
Astronomer at the Cape from 1870-1879, records of one of the stars in the
cluster: 'Nebula: a red start within it observed.' (Cape Catalogue of 12,441
stars for the Epoch 1880, p316). This red star appears to have altered in
brightness since Sir John Herschel observed it. The whole cluster would have
appeared nebulous to Lacaille through his tiny telescope of half-an-inch
aperture, magnifying only 8 times. Stone, after compiling his own catalogue,
wrote: 'It is impossible, for me at least, to overestimate the advantages which
I have derived from his (Lacaille's) work.' "
Hartung writes: "On a clear dark night
this beautiful cluster has a jewel-like quality; it is rich and bright, about
10' across with very marked geometrical pattern and the stars show delicate
colours accented by the orange red Kappa Cru. It is a good object for small
apertures, and magnificent in large ones."
Simon Tsang notes that "the cluster is
easy in binoculars, but was better resolved in my 6-inch at 60x. I saw a
single, bright red star set among 7 or 8 yellow-white and blue-white stars,
which made a very colourful contrast! At higher powers two dozen fainter stars
were visible."
Harrington calls it a "dazzling 4th
magnitude treasure chest of sparkling suns outside the northern edge of the
Coalsack. Its brightest star, Kappa Crucis, looks like a brilliant orange-red
ruby surrounded by colourful stellar gems sprinkled in the shape of an
arrowhead. While it encompasses only 10 arc minutes, the unparalleled radiance
of NGC 4755 more than makes up for its tiny size."
Gerd Bahr-Vollrath (Noosa Heads, Queensland,
Australia) writes in the The Webb Society Nebulae and Clusters Section Report
No. 11, January 1993: "The famous Jewel Box cluster. About 1 degree SE of
Beta Crucis, this open cluster is easily visible to the naked-eye as a diffuse
star, bordering the northern rim of the Coal Sack dark nebula. Three stars of
about 6th mag mark a prominent triangle about 10' across. Within and around
this triangle are about 50 stars of a wide range of magnitudes, some forming
apparent chains and loops. The compact size, the abundance of bright members
and the fact that a deeo red star of about 7th mag lies right in the heart of
the cluster rightfully earns it the name Jewel Box. A southern showpiece.
(8-inch f/12 SCT)"
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives
the diameter as 12' and the class as 1 3 r. The RNGC (Sulentic and Tifft 1973)
notes that this is a 5.0 mag open cluster. See also Thackeray A D (1949) "The
Kappa Crucis Cluster" MNASSA Vol 8 p30.
11x80: This bright cluster,
sitting on the northern edge of the Coalsack, is visible to the naked eye. In
hand-held 8x40 binoculars, it shows as a triangular grouping with 6 stars easily
visible (a 2-inch refractor ups the count to 11 prominent members). 11x80’s
show a spectacular triangle of stars with a bright knot or row of stars in the
centre. (suburban skies) [AS]
10-inch f/5 Newtonian: The
10-inch at 120x shows this cluster very well; the three brightest stars form a
pointed triangle pointing north-west. In the centre is a bright very red star
with a bluish star next to it. The southernmost star of the triangle lies
within a rich region of many slightly fainter (10th mag) stars. It is a very
rich grouping, small, compact, with the brighter stars clearly coloured.
(suburban skies) [AS]
15.5-inch f/9 Newtonian: At
220x it is an absolutely fantastic sight; three of the brighter cluster members
(brilliant white in colour) form a pointed isoceles triangle, and a further
three bright stars lie in a line bisecting the triangle parallel with its base.
The central star of these three is red and it is flanked by a yellow-white and
orange star. (suburban skies) [AS] |
