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Opening the lid on a
cosmic jewel box

Posted: October 29, 2009

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Combined images from ESO's Very Large Telescope, MPG/ESO 2.2 metre telescope at La Silla, and the NASA/ESA Hubble Space Telescope have revealed the Jewel Box cluster in a new light.

Wide field image of the Jewel Box cluster as seen by the MPG/ESO 2.2 metre telescope at La Silla. The field of view is 20 arcminutes across. Image: ESO.

The Jewel Box, more formally known as NGC 4755 or the Kappa Crucis Cluster, is bright enough to be seen with the unaided eye nestled deep in the southern skies near the Southern Cross. With contrasting coloured jewels from orange to blue, NGC 4755 is an open cluster, a collection of stars that all formed together from the same cloud of dust and gas, loosely bound together by gravity.

By taking advantage of variously equipped telescopes, both on the ground and in space, astronomers have built up a truly three-dimensional view of the Jewel Box in glorious detail.

Hubble's view reveals very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly coloured stars. Image: NASA/ESA and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain).

Data from the Digitized Sky Survey 2 shows the cluster located amongst the rich star fields and dust clouds in the southern Milky Way, alongside part of the vast dark cloud known as the Coal Sack. A new image taken with the Wide Field Imager (WFI) on the MPG/ESO 2.2 metre telecope at ESO's La Silla Observatory also takes in the vast number of stars and rich surroundings in this location.

Zooming in on the cluster with the FORS1 instrument on ESO's Very Large Telescope (VLT) provides a new-look sharp view of the cluster, the best of its kind with a total exposure time of just five seconds.

This composite image puts the Jewel Box in context of its Southern Hemisphere location, zooming all the way in to reveal individual inhabitants of this colourful cluster. Image: ESO, NASA/ESA, Digitized Sky Survey 2, and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain).

The Hubble Space Telescope rummaged deep into the jewel box, providing the first comprehensive far ultraviolet to near-infrared image of an open galactic cluster. It was taken using the Wide Field Planetary Camera 2 using seven filters to bring previously hidden details into the light. The gems include very bright, pale blue supergiant stars, a solitary ruby-red supergiant, and handfuls of other brilliantly coloured stars.

The spectrum of colours result from their differing intensities at different ultraviolet wavelengths – the brightest stars are some 15 to 20 times the mass of our Sun and hence shine much brighter than the dimmest inhabitants, which are less than half that mass.

The Jewel Box cluster is about 6400 light-years away and is approximately 16 million years old.