| Jewel Box | |
|---|---|
 | |
| Observation data (J2000epoch) | |
| Right ascension | 12h 53m 42s[1] |
| Declination | −60° 22.0′[1] |
| Distance | 7.06kly[1] (2.16kpc[1]) |
| Physical characteristics | |
| Mass | – M☉ |
| Radius | – |
| Estimated age | 16 Myr[2] |
| – | |
| Other designations | Herschel's Jewel Box,[1]κ Crucis cluster,[1]NGC 4755,[1]Cr 264,Caldwell 94[3] |
| Associations | |
| Constellation | Crux |
| See also:Open cluster,List of open clusters | |
TheJewel Box (also known as the Kappa Crucis cluster, NGC 4755, or Caldwell 94) is anopen cluster in theconstellationCrux, originally discovered byNicolas Louis de Lacaille in 1751–1752.[4] This cluster was later named the Jewel Box byJohn Herschel when he described its telescopic appearance as "... a superb piece of fancy jewellery". It is easily visible to the naked eye as a hazy star some 1.0° southeast of the first-magnitude starMimosa (Beta Crucis). This hazy star was given theBayer star designation "Kappa Crucis", from which the cluster takes one of its common names. The modern designationKappa Crucis has been assigned to one of the stars in the base of the A-shapedasterism of the cluster.
This cluster is one of the youngest known, with an estimated age of 14 million years. It has a total integratedmagnitude 4.2, is located 2.16 kpc, or 7,060 light years from Earth,[1] and contains just over 100 stars.
The Jewel Box as a star cluster was first found byNicolas Louis de Lacaille while doing astrometric observations for his 1751–1752 southern star catalogueCœlum Australe Stelliferum at theCape of Good Hope inSouth Africa. He saw this as a nebulous cluster in his small 12 mm( 1 / 2 inch) telescope, but was first to recognise it as a group of many stars.[5][6]The name "Jewel Box" comes fromJohn Herschel's own description of it:
Herschel recorded the positions of just over 100 members of the cluster in 1834–1838.[7]
The central part of the cluster is framed by bright stars making up an A-shapedasterism.[8]The upper tip of this asterism is HD 111904 (HR 4887, HIP 62894), a B9 supergiant and suspected variable star. It is the brightest member of the A asterism at magnitude 5.77. The brightest star in the region of the cluster is the variableDS Cru (HD 111613, HR 4876), which lies well beyond the A asterism. It is a B9.5α Cyg variable supergiant with an average visual brightness of magnitude 5.72, but is thought to be a foreground object.[9]
The bar of the "A" consists of a line of four stars. On the right (south) isBU Cru, a magnitude 6.92 B2 supergiant andeclipsing binary. Next to it is BV Cru, a magnitude 8.662 B0.5giant andBeta Cephei variable. Next in line isDU Cru, an M2red supergiant that varies irregularly between magnitude 7.1 and 7.6 . The last of the four is CC Cru, a magnitude 7.83 B2 giant andellipsoidal variable.[9]
Each leg of the base of the asterism's outline is marked by a blue supergiant star. HD 111990 (HIP 62953) is magnitude 6.77 and B1/2 . The star κ Cru itself is magnitude 5.98 and B3.[9]
The Jewel Box cluster is one of the youngest knownopen clusters. The meanradial velocity of the Jewel Box cluster is −21 kilometres per second (−13 mi/s).[2]The brightest stars in the Jewel Box cluster aresupergiants, and include some of the brightest stars in theMilky Way galaxy.[10]
Calculating its distance is difficult due to the proximity of theCoalsack Nebula, whichobscures some of its light.[11]
The Jewel Box cluster is regarded as one of the finest objects in the southern sky.[12]It is visible to the naked eye as a hazy object of the fourth magnitude.[13]It can be easily located using the starBeta Crucis as a guide, and appears as a fourth magnitude object.[14]It is impressive when viewed with binoculars or a small or large telescope. Three members along the crossbar of the A-shaped asterism lie in a straight line known as the 'traffic lights' due to their varying colours.[15]
.jpg)
Image of NGC 4755 copyrightAnglo-Australian Observatory
Image copyright T. Warner
