Kappa Crucis or the Kappa Crucis Cluster
Δ301 / M e l 1 1 4 / C 1 2 5 0 - 6 0 0
Position : 12 h 53.6 m -60° 20' (U 420)
NGC 4755 is the archetype of an open star cluster, which
has been labelled since the Kappa Crucis or the more
popular name of the "Jewel Box". Some modern
writers have even compared this cluster to a set of
"Traffic Lights", and for children, perhaps this
name is a bit more realistic and easier to understand.
The cluster lies some 28'E from the Centaurus-Crux border,
or alternatively, either 0.7° SE (40') of 2nd mag Beta
Crucis or 42'NNW (PA 340°) from the edge of the Coal
Sack. NGC 4755 is likely one of the best known, and
arguably the most beautiful, of all deep-sky objects. It is
so bright, we can see it with the naked-eye as a 4.2 mag
“hazy star”. Although the 'A' shaped asterism of
bright stars dominates NGC 4755, the entire cluster
reveals, in most amateur telescopes, between fifty and
about one hundred stars, all compacted within its tight 10
arc minute size. Yet, the majority of stars are actually
outside this asterism, with most of the remaining multitude
of fainter stars lying towards the cluster's south-western
boundary.
Stellar magnitude distributions play significantly with
the appearance of this and other star clusters. There are
five stars brighter than 7th magnitude, eight above 8th,
fifteen above 9th, thirty-five above 10th, but between 9.5
and 12.0 magnitude finds a significant pause in star
numbers, before they again rise exponentially. In some
ways, this cut-off is near the magnitude limit of 10.5cm
and this is no illusion. It is no wonder that small
apertures only see the bright stars of the cluster!
Increasing the aperture to either 20cm or 25cm, suddenly
reveals significant additional stars, and so about eighty
stars will be seen. A 30cm sees about ninety stars. Larger
apertures, revealing stars down to 16th magnitude, will see
many more stars, perhaps as many as 135 to 145 stars.
The common name of the "The Jewel Box" was first
introduced by Sir John Herschel from a casual quip he made
in the mid-1830's. NGC 4755 contains handfuls of bright
coloured stars, and even binoculars or small telescopes
show these obvious star colours. As E.J. Hartung rightly
says in his classic "Astronomical Objects for Southern
Telescopes" (AOST1) ;
“...a good object for small apertures... magnificent with large ones.”
NGC 4755 is also one of the few far southern clusters, or for that matter any other far southern deep-sky objects, mentioned in the detailed notes within Burnham's ("Burnham's Celestial Handbook." Vol.2. p.730-733 (1966)) and the Dover Edition (1978). In this three-volume tome, he says of the Jewel Box:
“...a brilliant and beautiful galactic duster ranking among the finest and most spectacular objects of the southern Milky Way... The cluster lies in a rich and remarkable region in the Heavens, well worth exploring with low power telescopes and instruments of the rich-field type.”
The namesake of the cluster is its brightest star Kappa Crucis, and this is surrounded by another seven 6th to 7th magnitude stars making the customary 'A' shape. These stars are mainly blue to white in colour, with one rich yellow star, and the prominent orange-red component nearer the centre of the telescopic asterism. An apt quote is from John Stanford's Book "Observing the Constellations : The Mitchell-Beazley Guide to the Stars."(p.76) where he says:
“The telescopic wonder of Crux Australis must be the Kappa Crucis open cluster NGC 4755... This concentration of stars which is a good object in any instrument from binoculars upwards. The brighter stars are blue and red giants, giving beautiful - if not brilliant - colours to the points of light.”
Even some of the professional astronomers have been impressed by the Jewel Box. On particular description, for example, is by the paper produced by Perry, C.L. et.al “ Multicolour photometry of the open star cluster NGC 4755.” ; Astron. J., 81, 8, 632-638 (1976) who said in their introduction to their paper;
“MANY NORTHERN hemisphere observers encounter several astronomical surprises as he or she views the southern skies for the very first time. Not the least of these experiences is the open star cluster NGC 4755... As this aggregate of stars is dominated by one red and four B-type supergiants, the cluster richly deserves the name "Jewel Box" bestowed on it by John Herschel. [NGC 4755 is a] highly concentrated and populous group of stars with a very large difference in luminosity between its members.”
My own first look at the Jewel Box in my teenage years from Sydney Observatory using the 29.8cm (11.75-inch) Refractor. I was immediately stunned at its gorgeous richness and simple beauty - especially with the colour gems against the royal velvet blue field with the faint 'twinkling' of an occasional star. This view is one of the very few astronomical impressions which remain permanently imprinted in my mind.
My own long description from my Observation Book from 21st May 1981
“What a magnificent creature! The "A" shaped asterism is so gaspingly striking. There are stars strewn everywhere across the field, and this is easily seen in binoculars. Sitting on the edge of the inky blackness of the Coal Sack, wide field seemed to me to leave the impression of a giant black leviathan emerging from the Milky Way, like the "Angel of Death" ready to consume the sparkle and life of an innocent child. However, realistically, modern science of astronomy now shows that the Coal Sack is much closer to the Earth by a factor of five or six - so this is just chance alignment. It is compelling to think that in untold hundreds of thousands of years from now the Coal Sack might intersect between us and the duster, hiding its bright luminaries from our wanting eyes. Perhaps we should be grateful that we live just at the right time (and the right place) to be able just to see it magnificent light.
Binoculars see the compact group unmistakably as an open star duster. What a sight! The C-8 in dark skies] shows the "A"- shape immediately, and one tends to forget there are many more stars, especially southwest of the main star - Kappa Crucis. One of the most prominent features, at least to me, is the pervasive blueness of the duster, and even more so than the generalised statement of the distribution of the star colours. The most centralised star, of around seventh magnitude, has an obvious orangey-red colouration, and from the scattering of observations over the years, it doesn't at all to me seem as red when compared to earlier observations.
I have trouble seeing any green or greenish colouration in any of the stars. The exception is perhaps the star alongside the bright red star, about 1' arc min towards the SSW. This is likely a similar problem to the colour contrast with stars like Beta Cygni (Albireo) or the companion to the first magnitude star Antares. Perhaps these effects are just “tricks on the eye”, and looking at the general spectral class of the cluster, and the narrow range of surface temperatures of the component stars, make me think this must be so. Furthermore, this might just also account for the appearance of a few yellowish stars with the fainter duster members next to a few of the brighter components.
For a bit of fun, I enhanced the cluster with coloured filters that I have been using for Venus observations, and used them to some effect. Using the 81B Wratten blue filter confirms the overall brightness, and few stars reduced in their overall magnitude - except perhaps the reddish central star, and a few of those south-west and beyond the main asterism. The red 25A Wratten filter had the most dramatic effect, consuming the blue star's brightness. Unfortunately, the light loss here is tremendous (some three to four magnitudes), so I could only see the brighter components. The reddish star is now prominent, and so is the star some 5' towards the south. The blue stars drop in intensity by a factor of three magnitudes at least. NGC 4755 appears quite strange. The last experiment that I tried was using a “star filter”, which is an etched pattern on a photographic filter. The interference produces six spectacular spikes which I set slightly east of north. What a beautiful sight! If reminds me of the impression of golden stars near the horizon drawn in my childhood books. It makes me recall my first view through Michael Harrip's 8" (20cm) Newtonian on its first light in his backyard in Five Dock [Michael is a close personal friend of mine, who was a past Vice President of the ASNSW in 1979-1980], or looking through Joe Cauchi's new 10" (25cm) Newtonian, where the spider vanes produce a similar effect.
How can one ever forget this beautiful cluster. If certainly made me so pleased that I bought the C-8 (or any other telescope) in the first place.
Wonderful, exquisite - Stun - Wah.”
“Southern Astronomical Delights” © Andrew James (2002) Sydney, Australia