NGC 4755 is stunning open cluster in the constellation Crux, the Southern Cross. It is known as both the Jewel Box and the Kappa Crucis cluster (after its most prominent member).
This cluster is one of the finest objects discovered by Nicolas Louis de Lacaille during his 1751-1752 visit to South Africa. John Herschel described this famous group of bright young stars as "a casket of variously coloured precious stones" in his telescope; the name "Jewel Box" derives from this description.
The Jewel Box is only visible from southern latitudes, and appears close to Beta Crucis, the easternmost star of the Southern Cross. It has an apparent magnitude of 4.2, and looks like a single star to the unaided eye. The cluster contains around 100 stars within a 10' diameter area. Most are blue giants; the brightest is κ Cru, a blue supergiant of visual magnitude 5.9 and spectral type B5 Ia. The fourth-brightest is a magnitude 7.6 red supergiant of class M2 Iab that contrasts strongly against its hot, blue companions.
This cluster is one of the youngest known, with an estimated age of only 7.1 million years. Distance estimates for the Jewel Box range from 6,400 to 8,150 light years. The Coal Sack, a huge dark area of the sky within the band of the Milky Way, appears close to NGC 4755 in the sky. This huge dark nebula is probably much closer than the cluster, at only 500 to 600 light years distant.