NGC 4755 : THE JEWEL BOX
Although discovered by Abbe Lacaillé in 1752, the “Jewel Box” did not receive its common name until it was found by Sir John Herschel in 1834. The name apparently came from a casual quip that he made in the mid-1830s, and eventually this name stuck. The name is appropriate as NGC 4755 contains a handful of bright coloured stars, and even binoculars or small telescopes show the obvious star colours.
Figure 9. Outer Regions of The Jewel
Show the star colours which have been slightly exaggerated to show
the general colours. The central star in the ‘Bar’ often
shows a greenish tint in moderately large apertures, likely caused
by colour contrast with the obvious red and blue stars either side
of it. The Jewel Box is therefore quite aptly named! Also Note:
NORTH is towards the LEFT
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 cluster 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.”
As E.J. Hartung rightly says in his classic “Astronomical Objects for Southern Telescopes” (AOST1) ;
“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. …a good object for small apertures… magnificent with large ones.”
The namesake of the cluster is its brightest star Kappa Crucis and this is surrounded by seven or eight 6th to 7th magnitude stars making the customary ‘A’ or ‘V’ shape. Oddly to me, some have described the shape as triangular which my mind has never pictured. Another description is that it looks like a Greek capital Delta. I.e. (Δ). To me this gives a the wrong impression mainly as a triangle shape has three point and not five or seven!
These vivid 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;
“A NORTHERN hemisphere observers encounters 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, also known as the κ Crucis cluster.”
They go on to say;
“…As this aggregate of stars is dominated by one red and four B-type supergiants, the cluster richly deserves the name “Jewel Box” [as] 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.”
Northern observers are very impressed and taken with the cluster’s beauty, but I’ve always found many to be hesitant to claim it is the best in the entire sky. An example of this appears in the recent book “Star Cluster” by B.A. Archinal and S.J. Hynes (2004) who states on pg.174; “One of the most striking and spectacular open clusters in the southern sky”
My own first look at the Jewel Box in my teenage years from Sydney Observatory using the 29.8cm (11¾-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.
Mike Inglis in his book “Astronomy of the Milky Way : Observer’s Guide to the Southern Sky” Patrick Moore’s Practical Astronomy Series; pg.17 (2004), aptly says;
“Let’s now look at that object for which Crux is justifiably famous, the Jewel Box Cluster… (Caldwell 94). …show[ing] a selection of bright and delicate colors). Many amateurs who have had the lucky opportunity to observe clusters in both hemispheres often say that this one is the finest. However, it is a surprisingly small cluster, only 10 arcminutes across. The colors are not vivid except for one bright orange-red supergiant star Kappa (κ), but more of a pale and delicate hue, but color they defiantly do exhibit. However, the use of larger apertures will enhance the colors of the stars. The stars are in a marked geometric pattern with its three brightest members in a line stretching from the northeast to the southwest. These three stars have often been referred to as the traffic lights due to their contrasting colors. The cluster stars range in magnitude from 6th to 10th. It is just visible to the naked eye and in binoculars the cluster is great; in large apertures it is wonderful.”
My own long description taken from my Observation Book appears below. It was observed under good seeing on the 21st May 1981 from the Blue Mountains about 80 kilometres west of Sydney, Australia.
“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 cluster, 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 cluster. What a sight! The 20cm in dark skies] shows the ‘A’-shape immediately, and that one tends to forget there are many more stars, especially towards the southwest of the main star — Kappa Crucis. One of the most prominent features, at least to me, is the pervasive blueness of the cluster, 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 arcmin 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 several yellowish stars with the fainter cluster 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 brightness of the blue star. 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! It 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 20cm. (8-inch) Newtonian on its first light in his backyard in Five Dock [Michael is a close personal friend of mine, who was also a past Vice President of the ASNSW (Astronomical Society of New South Wales) in 1979-1980], or looking through ASNSW member’s Joe Cauchi’s then new 25cm. (10-inch) Newtonian where the spider vanes holding the secondary mirror produce a very similar effect.
How can one ever forget this beautiful cluster. If certainly made me so pleased that I bought the 20cm C8 (or any other telescope) in the first place.
Wonderful, exquisite — Stun-Wah.”
Last Update : 17th July 2012
Southern Astronomical Delights © (2014)
For any problems with this Website or Document please e-mail me.