The Jewel Box (also known as the Kappa Crucis Cluster, NGC 4755, or Caldwell 94) is an

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

in the constellation

Crux Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for ...

, originally discovered by

Nicolas Louis de Lacaille Abbé Nicolas-Louis de Lacaille (; 15 March 171321 March 1762), formerly sometimes spelled de la Caille, was a French astronomer and geodesist who named 14 out of the 88 constellations. From 1750 to 1754, he studied the sky at the Cape of Goo ...

in 1751–1752. This cluster was later named the Jewel Box by John 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 star

Mimosa ''Mimosa'' is a genus of about 590 species of herbs and shrubs, in the mimosoid clade of the legume family Fabaceae. The generic name is derived from the Greek word (''mimos''), an "actor" or "mime", and the feminine suffix -''osa'', "resemb ...

(Beta Crucis). This hazy star was given 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 base of the A-shaped asterism of the cluster This cluster is one of the youngest known, with an estimated age of 14 million years. It has a total integrated

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of 4.2, is located 1.95 kpc. or 6,440 light years from Earth, and contains just over 100 stars.

Discovery and observation

The Jewel Box as a star cluster was first found by

while doing astrometric observations for his 1751–1752 southern star catalogue '' Cœlum Australe Stelliferum'' at the Cape of Good Hope in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...

. He saw this as a nebulous cluster in his small 12mm. (½-inch) telescope, but was first to recognise it as a group of many stars.http://www.docdb.net/show_object.php?id=ngc_4755, DOCdb : Jewel Box The name "Jewel Box" comes from John Herschel's own 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 just over 100 members of the cluster in 1834–1838.

Prominent members

The central part of the cluster is framed by bright stars making up an "A"-shaped asterism. The upper tip of this asterism is HD 111904 (

HR 4887 HR 4887 (HD 111904) is a suspected variable star in the open cluster NGC 4755, which is also known as the Kappa Crucis Cluster or Jewel Box Cluster. Location HR 4887 is one of the brightest members of the NGC 4775 open cluster, better kn ...

, 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 variable

DS Cru DS Crucis (HR 4876, HD 111613) is a variable star near the open cluster NGC 4755, which is also known as the Kappa Crucis Cluster or Jewel Box Cluster. It is in the constellation Crux. Location DS Crucis is one of the brightest stars in ...

(HD 111613, HR 4876), which lies well beyond the A asterism. It is a B9.5 α Cyg variable supergiant with an average visual brightness of magnitude 5.72, but is thought to be a foreground object. The bar of the "A" consists of a line of four stars. On the right (south) is BU Cru, a magnitude 6.92 B2 supergiant and

eclipsing binary A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in w ...

. Next to it is BV Cru, a magnitude 8.662 B0.5

giant In folklore, giants (from Ancient Greek: ''gigas'', cognate giga-) are beings of human-like appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''giant'' is first attested in 1297 fr ...

and

Beta Cephei variable Beta Cephei variables, also known as Beta Canis Majoris stars, are variable stars that exhibit small rapid variations in their brightness due to pulsations of the stars' surfaces, thought due to the unusual properties of iron at temperatures of 200, ...

. Next in line is DU Cru, an M2

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Ant ...

that varies irregularly between magnitude 7.1 and 7.6. The last of the four is CC Cru, a magnitude 7.83 B2 giant and ellipsoidal variable. 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.

Physical characteristics

The Jewel Box cluster is one of the youngest known

s. The mean radial velocity of the Jewel Box cluster is . The brightest stars in the Jewel Box cluster are

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...

s, and include some of the brightest stars in the

Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

. Calculating its distance is difficult due to the proximity of the

Coalsack Nebula The Coalsack Nebula (Southern Coalsack, or simply the Coalsack) is a prominent dark nebula in the skies, being easily visible to the naked eye as a dark patch obscuring a brief section of Milky Way stars as they cross their southernmost region of ...

, which obscures some of its light.

Observation

The Jewel Box cluster is regarded as one of the finest objects in the southern sky. It is visible to the naked eye as a hazy object of the fourth magnitude. It can be easily located using the star

Beta Crucis Mimosa is the second-brightest object in the southern constellation of Crux (after Acrux), and the 20th-brightest star in the night sky. It has the Bayer designation β Crucis, which is Latinised to Beta Crucis and abbreviated Beta Cru ...

as a guide, and appears as a fourth magnitude object. 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.

Gallery

File:Wide Field Image of the Jewel Box.jpg, Wide Field Image of the Jewel Box File:Digitized Sky Survey 2 Image of NGC 4755.jpg, Digitized Sky Survey 2 image of the Jewel Box File:A Hubble gem - the Jewel Box.jpg, Hubble image of the Jewel Box File:Putting the Jewel Box in perspective (composite image).jpg, Putting the images in perspective File:NGC 4755 "El Joyero" a través de un pequeño telescopio.jpg, The Jewel Box seen through a small amateur telescope

References

External links

SEDS: NGC 4755
with a Copyright Anglo-Australian Observatory Image of NGC 4755 *
A Copyright T. Warner Amateur Image of NGC4755
{{DEFAULTSORT:Jewel Box (star cluster) Open clusters Crux (constellation) NGC objects 094b