This well-known southern cluster was first described as the "hazy star" Kappa by Bayer in his Uranometria (1603). Later in 1752, Abbe Lacaillé listed it in his second listing of "Nebulous Stars and Clusters" and list the object as Lac-II-12 (12h 39.2m. -59°01'; Jan 1752) by stating ;
5 or 6 small stars between two of sixth magnitude.
James Dunlop made one of the first detailed observations, by listing the cluster as Δ301 in his 629 object catalogue of 1827. Dunlop states ;
(Chi Crucis, Bode) [Herschel's Brackets] is five stars of the 7th magnitude, forming a triangular figure, and a star of 9th magnitude between the second and third, with a multitude of very small stars on the south side.
In this paper is a drawn figure of Δ301 that Dunlop
has, for some reason, seems oddly distorted. Some of the
stars do differ very slightly from how it appears today -
even though Dunlop emphatically says "Figure 13, is a
very correct representation." Also in this text is an
interesting note that, according to Dunlop, the star is
marked as 'Chi'(χ) and not Bayer's 'Kappa' (κ).
Furthermore, this nomenclature continued until the
beginning of the 20th century, as several paper published
by F. Abbott, G. Ellery and H.C. Russell, for example,
still used the Greek letter Chi. Of course, Crux was at one
time counted as is counted as part of Centaurus, which it
was prior to the mid-17th Century and well after Dunlop's
time. However, Chi Centauri (14060-4111) is the 4.5
magnitude star placed some 19oN of β
Centauri near the north-western corner of Lupus. John
Dunlop here seems clearly wrong, who has made a simple
error that persisted for some time. It seems the Innes
might have been the first to correct it. In the literature
search for this article he was the first to refer to Kappa
Crucis in 1903. At present lowest Greek letter in use
within Crux is the wide pair μ Crucis.
Sir John Herschel was next view this cluster, giving his catalogue number HJ 3435. There is little doubt that he was very impressed with what he saw - mainly seen in the copious amounts of written text on NGC 4755. During his four-year stay in South Africa, and using his 50cm (18") f/13 reflector,all observations were made from residence "Feldhausen", which is near the township of Claremont at the Cape of Good Hope. He once said of the Jewel Box;
The central star (extremely red) and a most vivid and beautiful duster of from 50 to 100 stars. Among the larger there are one or two evidently greenish [stars.]; south of the red star is one 13th magnitude [Argenlander magnitudes], also red; and near it/s one 12th magnitude, bluish... The same red star taken. Several others laid down, of different shades of green.
This similar quote has also been duplicated in several sources, like Rev. Thomas W. Webb's Celestial Objects for Common Telescopes Vol. 2, p.278 (1917).
Herschel found the cluster very conspicuous, noting with clear admiration the extent of the colours. Oddly, he described many stars as greenish. This is immediately strange for an astronomical description, as green is both hard to see, and rare, even among some bright and prominent colour-contrasting double stars. I can only conclude that Herschel is describing an overall impression of the stars rather than the individual stars actually being greenish. It is quite possible that Herschel is describing what he sees, and what likely may cause this is that speculum mirrors might quite differently absorb some wavelengths in reflecting the light.
Like Dunlop, John Herschel also made detailed sketches of the cluster showing some one-hundred and ten stars. Here he says:
Though set down by Lacaillé 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 or rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly very different colours of its constituent stars, which give it the effect of a superb piece of fancy jewellery. The area occupied by it is about 148th part of a square degree, within which area have laid down, partly from micrometric measures [from the brighter stars] and partly from inter triangulation by the eye [for the fainter stars] the stars....
According to Auke Slotegraaf's Deepsky Observers Companion, an was article published by Donald McIntyre in the Journal of the Astronomical Society of South Africa "An Astronomical Bi-Centenary. The Abbe de Lacaillé's visit to South Africa; 1751 / 1753" says ;
Lacaillé includes Kappa Crucis in his Catalogue. Sir John Herschel during his residence at the Cape made repeated observations of his famous cluster, but could not see any trace of nebulosity. Yet curiously, Edward James Stone, Her Majesty's Astronomer at the Cape from 1870-1879, records of one of the stars in the duster; 'Nebula: a red star within it observed.' (Cape Catalogue of 12,441 stars for Epoch 1880, p316.) This red star appears to have altered brightness since John Herschel observed it, and has been speculated to have done so by others like Victorian amateur astronomer Francis Abbott in 1862. The whole cluster would have appeared nebulous to Lacaille through his tiny [12mm]... magnifying 8x. Stone, after compiling his own catalogue, wrote; 'if is impossible, for me at least, to overestimate the advantages which I have derived from his [Lacaillé's] work.
Richard Allen, in Star Names: Their Lore and Meaning (1899), says:
Around the 6th magnitude is the celebrated cluster of coloured stars .... the central and principal one being a deep red, ...others green, blue and of various shades.
Nevertheless, its colourful visage has not impressed everyone. An example is another quotation from Allen who quotes (p.191) from the English astronomy author Miss Agnes M. Clerke (taken from "System of the Stars" (1881) and "The Herschels and Modern Astronomy" (1895)) who states:
It must be confessed that, with moderate aperture, it fails to realise the effect of colour implied by Sir John Herschel's [its discoverer] comparison to "a gorgeous piece of fancy jewellery." A few reddish stars catch the eye at once; but the blues, greens and yellows belonging to their companions are pale tints, more than half drowned in white light.
Allen correctly immediately puts down this comment by saying with some abeyance:
"Gould, however, called it exquisitely beautiful."
However, name has persisted to today, and it is possibly
the most famous quotes about any southern object in the
sky, and I counted some thirty-five sources with the same,
or slightly varied, words. The Jewel Box name
has conjured up much to the cluster's mythology, and in
turn, much interest in the southern skies.
The next recorded observations were made by Francis Abbott in 1861 from his observatory in Victoria entitled On the Cluster χ Crucis, R.A. 12h 43m 36s, N.P.D. 149o 25' 31" (3435, H.) Lac. 1110 (Neb.) MNRAS, 23, p.32 Aug (1862). Here he described by comparing Herschel' and Dunlop's drawings and descriptions that many of the stars had change in relative position, number and colour. This suggested that the stars were undergoing evolutionary changes and hence making it an important deep-sky object for study. The evolution of stars and the beginning of the emergence of astrophysics was a fairly new concept, and during this time there were those who believed in the literal Biblical account of the creation. Many assumed the postulate of Archbishop Usher that the Earth was created in 4004 BC, and when applied to the stars meant that changes seen in them were scientific proof of the recent formation of the Universe.
In 1872, H.C. Russell catalogued some 130 stars, publishing this in the MNRAS, 33, 66 (1873). During the photographic surveys Gould listed and positioned 129 stars from two suitable plates of the cluster, among the eighteen-odd plates in all taken. The latter images were both photographed in 1881 that were later privately published by Gould in 1897. Subsequent analysis of the plates give the plate magnitude limit as around 12 to 12.5 magnitude. A further description of NGC 4755 and another of the first photographs by George Ellery also appears in MNRAS, 45, 395 (1883). H.C. Russell also achieved photographic results in 1890, whose magnitude limit is around 14th. The result and image was presented by Russell to the Royal Society of New South Wales, July 1, 1891. Entitled "Preparations Now Being Made in Sydney Observatory for the Photographic Chart of the Heavens" (1891), it showed the usefulness of the application of astrophotography for celestial objects in the southern hemisphere. (A full version of this article also appeared in the ASNSWI "Universe",April (1984)). Appearing as Plate XI in the original. I have reproduced a scanned version as Figure A.
In 1888 Dreyer published the New General Catalogue and
classified the Jewel Box as "cl, vl, stvb,
- Cluster, very large, stars very bright, surrounding Kappa Crucis.
R.T.A. Innes (1908) text in the updated classic Celestial Objects for Common Telescopesby Rev. Thomas Webb says of the Jewel Box:
"Vivid and beautiful cluster, fifty to one hundred [stars] of various colours, some greenish, round Kappa Crucis, extremely red."
Southern Astronomical Delights © Andrew James (2002) Sydney, Australia