NGC 4755, also known as the Jewel Box or Kappa Crucis Cluster, is one of the finest open clusters in the sky. It's located in the small southern constellation of Crux and at mag. +4.2 is bright enough to be seen with the naked eye. It contains over 100 stars, mostly blue or red, spread over 10 arc minutes of apparent sky. The cluster is one of a handful of night time objects that offers something for all observers of all telescope sizes.
NGC 4755 is located 6,440 light years distant and is best seen from southern latitudes during the months of March, April and May. It's circumpolar from locations south of 30S and can also be seen from the tropics, although for many Northern Hemisphere observers it never rises above the southern horizon.
Nicolas Louis de Lacaille discovered NGC 4755 on March 25, 1752. The name "the Jewel Box" was originally coined in 1830 by John Herschel, who described it as beautiful multi-coloured cluster resembling a superb piece of fancy jewellery.
Although Crux is the smallest of the modern 88 constellations, it's one of the most recognisable due to the prominent cross shape of its four brightest stars. NGC 4755 is centred on star Kappa Crucis (κ Cru - mag. +6.0), which is a cluster constituent and also bears the cluster's name. Situated close by is the huge dark nebula known as "the Coal Sack".
To the naked eye, NGC 4755 appears as a hazy star one degree southeast of first magnitude Beta Crucis (β Cru - mag. +1.25). A pair of 7x50 or 10x50 binoculars resolves the brightest six stars surrounded by a sprinkling of fainter members. A small 80mm (3.1 inch) scope reveals a dozen more stars of various subtle colours, mostly reds or blues, that appear in two distinct groups with one noticeably brighter than the other. Due to its compact size and high surface brightness, NGC 4755 takes magnification well. Larger amateur scopes hint at or reveal some associated nebulosity.
NGC 4755 is one of the finest open clusters in the southern sky. In total, it contains at least 100 stars including many hot B types. With an estimated age of between 7 and 10 Million years, it's one of the youngest known open clusters and is listed as number 94 in the Caldwell catalogue.
NGC 4755 Data Table
|Name||Jewel Box Cluster|
|Object Type||Open Cluster|
|RA (J2000)||12h 53m 42s|
|DEC (J2000)||-60d 22m 00s|
|Apparent Size (arc mins)||10 x 10|
|Age (years)||7 to 10 Million|
|Number of Stars||>100|
|Other Name||Kappa Crucis Cluster|
|Notable Feature||One of the youngest known clusters|