| Jewel Box | |
|---|---|
NGC 4755 taken by the VLT Credit: ESO |
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| Observation data (J2000 epoch) | |
| Constellation | Crux |
| Right ascension | 12h 53m 42s[1] |
| Declination | -60° 22.0′[1] |
| Distance | 6.4 kly[2] (1.98 kpc) |
| Physical characteristics | |
| Mass | - M☉ |
| Radius | - |
| Estimated age | 14±2Myr[3] |
| Notable features | - |
| Other designations |
NGC 4755,[1] Herschel's Jewel Box,[1] |
| See also: Open cluster, List of open clusters | |
The Jewel Box (also known as NGC 4755, the Kappa Crucis Cluster and Caldwell 94) is an open cluster in the constellation of Crux. As Kappa Crucis, it has a Bayer designation despite the fact that it is a cluster rather than an individual star.
It is one of the finest open clusters discovered by Nicolas Louis de Lacaille when he was in South Africa during 1751–1752. This cluster is one of the youngest known, with an estimated age of only 14 million years. It has an apparent magnitude of 4.2, and is located 6,440 light years from Earth and contains around 100 stars.
This famous 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 bright orange star Kappa Crucis contrasts strongly against its predominantly blue, hot companions. Kappa Crucis is a very large (hence very luminous) young star in its red supergiant stage, which paradoxically indicates that its life is drawing to a close. The cluster looks like a star to the unaided eye and appears close to the easternmost star of the Southern Cross, (Beta Crucis), so is only visible from southern latitudes.
Another cluster, NGC 457, has also a jewel-box-like appearance and lies virtually diametrally opposite this one in the sky in the constellation Cassiopeia.
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The Jewel Box cluster was discovered by Nicolas Louis de Lacaille when he was in the Southern Hemisphere from 1751 to 1753. 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 multiple stars.[4] The name "Jewel Box" comes from John Herschel's description of it:
"this cluster, though neither a large nor a rich one, is yet and 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.[5]
The Jewel Box cluster is one of the youngest known open clusters.[6] It contains the M-type supergiant Kappa Crucis.
The mean radial velocity of the Jewel Box cluster is −17.9 kilometres per second (−11.1 mi/s).[7]
The brightest stars in the Jewel Box cluster are supergiants, and include some of the brightest stars in the Milky Way galaxy.[8]
Calculating its distance is difficult due to the proximity of the Coalsack Nebula, which obscures some of its light.[9]
The Jewel Box cluster is regarded as one of the finest objects in the southern sky.[10] It exhibits subtle colours, aside from the vivid red supergiant Kappa Crucis from which the cluster takes its name.[11] It is visible to the naked eye as a hazy object of the fourth magnitude.[12] The Jewel Box cluster can be easily located using the star Beta Crucis as a guide, and appears as a fourth magnitude object.[13] The three brightest members of the cluster lie in a straight line known as the 'traffic lights' due to their varying colours.[11]
The Jewel Box cluster is impressive when viewed with binoculars[11] or a small telescope, but even more so in a large telescope because the background nebulosity becomes more prominent.[9]
Coordinates: 12h 53m 42s, −60° 22′ 00″
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