Cosmic Jewel Box Photographed in Detail
The FORS1 instrument on the ESO Very Large Telescope (VLT) at ESO’s Paranal Observatory was used to take this exquisitely sharp close up view of the colourful Jewel Box cluster, NGC 4755. The telescope’s huge mirror allowed very short exposure times.
Credit: ESO/Y. Beletsky

A new set of images made with three different telescopes are some of the best ever taken of the sparkling ?Jewel Box? star cluster, showcasing the evolution of the stars within.

The Kappa Crucis Cluster, also known as NGC 4755 or simply the ?Jewel Box,? is just bright enough to be seen with the unaided eye. It lies about 6,400 light-years away in the southern skies near the Southern Cross in the constellation of Crux.

It was given its nickname by the English astronomer John Herschel in the 1830s because the striking color contrasts of its pale blue and orange stars seen through a telescope reminded Herschel of a piece of exotic jewelry.

Open clusters such as NGC 4755 typically contain anything from a few to thousands of stars that are loosely bound together by gravity. Because the stars all formed together from the same cloud of gas and dust their ages and chemical makeup are similar, which makes them ideal laboratories for studying how stars evolve.

The Jewel Box cluster can be seen in a combination of images from the European Southern Observatory's Very Large Telescope on Cerro Paranal, Chile, the MPG/ESO 2.2-metre telescope at ESO?s La Silla, Chile, observatory and the NASA/ESA Hubble Space Telescope.

The image taken with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope has a large field of view that shows a vast number of stars, many located behind the dusty clouds of the Milky Way and therefore appear red.

The FORS1 instrument on the ESO Very Large Telescope (VLT) allows a much closer look at the cluster itself. The telescope?s huge mirror and image quality give a very sharp view despite a total exposure time of just 5 seconds.

Observing the cluster from space allows the NASA/ESA Hubble Space Telescope to capture light of shorter wavelengths than can not be seen by telescopes on the ground. The new Hubble image (taken near the end of the long life of the Wide Field Planetary Camera 2) of the core of the cluster represents the first comprehensive far ultraviolet to near-infrared image of an open galactic cluster.

Several very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly colored stars are visible in the Hubble image, as well as many much fainter ones. The different colors result from the differing intensities of the stars at different ultraviolet wavelengths.

The huge variety in brightness of the stars in the cluster exists because the brighter stars are 15 to 20 times the mass of the Sun, while the dimmest stars in the Hubble image are less than half the mass of the Sun. More massive stars shine much more brilliantly. They also age faster and make the transition to giant stars much more quickly than their faint, less-massive siblings.