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Jewel Box (star cluster)
The Jewel Box, Kappa Crucis Cluster, NGC 4755, is an open cluster in the constellation Crux, originally discovered by Nicolas Louis de Lacaille during 1751–1752. This famous cluster was later named the Jewel Box by Sir John Herschel when he described its telescopic appearance as "a casket of variously coloured precious stones." It is easily visible to the naked eye as a hazy star some 1.0° southeast of the first-magnitude star Beta Crucis. This hazy star (the core of the cluster) was assigned the Bayer star designation Kappa Crucis, from which the cluster takes one of its common names. The modern designation Kappa Crucis has been assigned to one of the stars in the central A-shaped asterism of the cluster, the third brightest cluster star (HD 111973, HIP 62931, HR 4890), located at the base of the right leg of the A. This is a blue supergiant of spectral type B3Iae, shining at the apparent visual magnitude of 5.94. Two other cluster stars are brighter than κ Cru, namely, 5.77 magnitude star HD 111904 (HIP 62894), at the tip of the A, and HD 111613 (HIP 62732) at 5.75, which is 15 arc minutes beyond the asterism toward Beta Crucis.〔http://www.southastrodel.com/Page002.htm, NGC 4755 : The Jewel Box "The Jewel of the South" : Introduction〕
This cluster is one of the youngest known, with an estimated age of 14 million years. It has a total integrated magnitude of 4.2, is located 6,440 light years from Earth, and contains around 100 stars.
==Discovery and observation==
The Jewel Box as a star cluster was first determined by Nicolas Louis de Lacaille while he was doing his South African astrometric observations for his star catalogue. He saw the object as a nebulous cluster due to the small aperture of his telescope, but was the first to recognize it as consisting of many stars.〔http://www.docdb.net/show_object.php?id=ngc_4755, DOCdb : Jewel Box〕〔http://www.southastrodel.com/Page004.htm, History of the Jewel Box Part One : 1752-1834〕 The name "Jewel Box" comes from John Herschel's description of it: "...this cluster, though neither a large nor a rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly the very different colour of its constituent stars, which give it the effect of a superb piece of fancy jewellery"〔
Herschel recorded the positions of 100 members of the cluster in 1834–1838.
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