The Jewel Box (also known as NGC 4755, Herschel’s Jewel Box, the Kappa Crucis Cluster and Caldwell 94) is an open cluster about 20 light-years across. It contains over 100 stars and is located 6,440 light-years away in the southern constellation of Crux. The cluster is one of the youngest known, with an estimated age of only 14 million years.
This group of young bright stars was named the Jewel Box from its description by Sir John Herschel as “a casket of variously coloured precious stones,” which refers to its appearance in the telescope.
The brightest stars in the Jewel Box cluster are supergiants, and include some of the brightest stars in our Milky Way, like the supergiant star Kappa Crucis, from which the cluster takes its name. This is a very luminous young star in its red supergiant stage, meaning that the star is at the end of its life.
This bright orange star contrasts strongly against its predominantly blue, very hot companions. These bright blue stars are indicating that this is an incredibly young cluster, full of short-lived stars that haven’t run out of fuel yet, some of which will go supernova during the next few million years. The Jewel Box is not as bright as one may think because some of the cluster’s light is obscured by the Coalsack Nebula.
Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. Stars in an open cluster make excellent laboratories in the study of the evolution of stars. Born at the same time and from the same stellar gas cloud, they all move in the same direction. Although they may have different masses, they have the same “star DNA”.