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.
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