Posted on by Tammy Plotner

The “Jewel Box” by Don Goldman

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Since it was first observed in a half inch diameter spy glass by Abbe Nicholas Louis de Lacaille during his visit to South Africa in 1751-2, the Kappa Crucis star cluster (NGC 4755) has intrigued and and confounded astronomers since. Today let’s open John Herschel’s ‘casket of variously coloured precious stones’ and take a closer look at the “Jewel Box”…

Situated about 7500 light years away near a vast, dark cosmic dust cloud known as the “Coal Sack”, the Kappa Crucis star cluster has a Bayer designation even though it is a cluster instead of an individual star. Just one look at this colorful array is to understand how it came to be known as the Jewel Box. Sprinkled across 20 light years of space and maybe perhaps only 7.1 million years old, it is home to red, white and blue giant stars alike. If its brightest star were at the center of our own solar system, it would shine 83,000 times brighter than Sol!

The bright orange star is Kappa Crucis, a standout amongst its hot, vivid blue members. A very young star gone into its red supergiant stage? During mid-1862 a man named Francis Abbott began studying the Jewel Box and his observing notes say; “Certain changes that are apparently taking place in the number, position, and colour of its component stars.” This was some pretty radical thinking since he was going up against the notes of the likes of John Herschel and George Airy. But, as so often is the case, sometimes one astronomer can spot what another one can’t and some 10 years later H.C. Russell took Abbott’s notes to heart – measuring and cataloging 130 of the cluster’s stars. Despite extreme criticism, another observer named R.T. Innes also claimed color change as noted in the classic work “Celestial Objects for Common Telescopes”.

Of course, study did not end there and it went into the early 1900s with Trumpler and then Harlow Shapley. The first significantly important astrophysical paper on this cluster appeared in 1958 and was published by Halton Arp and Cecil van Sant who were trying to find out more about galactic supergiant stars. “The three brightest stars are supergiants… and the red star, are all members of the cluster, then NGC 4755 must be somewhat like h and χ Persei… Since these types of clusters are rare, observational material sufficient to derive a colour-magnitude diagram was obtained.” However, as more stars were revealed and studied, the more confusing the designations became! The years progressed and NGC 4755 became even more understood – and better cataloged.

According to studies of helium, carbon, nitrogen and oxygen abundances done by G. Mathys (et al) “After consideration of the CN abundances in this sample, there is no clear evidence of internal mixing. Only three stars among the non-supergiants seem to show a nitrogen enhancement. Two of them have a fairly low projected equatorial velocity (admittedly, they may be rapid rotators seen pole-on); the third one is a definite fast rotator. In the lower gravity stars some kind of mixing has apparently occurred. The supergiants do not differ significantly from the other programme stars in their respective helium contents. The mean helium abundance for each cluster is close to the standard value, (He/H).”

Studying variable stars within open clusters is extremely important. They are clues as to distance and evolution! In young clusters like the Jewel box, the brighter stars should be variables and should be blue. They should also have started evolution away from main sequence, unlike the low mass stars who just quietly burn away their hydrogen. As we know, one of the principle variable types are the Beta Cepheid stars and studies done by Stankov (et al) show the detection of four new variable stars in NGC 4755. “We give frequency solutions as indicators of the time-scales and amplitudes of the pulsations. NGC 4755-116 is probably a B2 dwarf with a period of 4.2 d whose variability is caused by a spot or g-mode pulsation. NGC 4755-405 can be considered as a new β Cephei star with two pulsation frequencies. For NGC 4755-215 we found one frequency and for NGC 4755-316 three pulsation frequencies; we suggest that both are new slowly pulsating B stars of short period.” These variations may be caused by radial pulsations from an instable hydrogen core and even more studies are needed.

But is there more? Yes. Very recent studies done by C. Bonatto (et al) show the dynamical state of NGC 4755. “We explore the possibility that, at the cluster age, some main sequence and pre-main sequence stars still present infrared excesses related to dust envelopes and proto-planetary discs. The core is deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halo but they are scarce in the core. Compared to open clusters in different dynamical states studied with similar methods, NGC 4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass.”

Did NGC 4755 form from the same molecular cloud? Is it two overlapping clusters? Does the proximity of the Coal Sack influence its visual properties? No matter what the science is behind it, the light that you see now left about the same time the Great Pyramids of Egypt were being built. Let the words of Burnham ring the loudest: “…a brilliant and beautiful galactic duster ranking among the finest and most spectacular objects of the southern Milky Way… The cluster lies in a rich and remarkable region in the Heavens, well worth exploring with low power telescopes and instruments of the rich-field type.”

This week’s awesome image was done by Don Goldman and taken at Macedon Ranges Observatory. We thank you!

16 Replies to “The “Jewel Box” by Don Goldman”

  2. You’re right, Hans. I had a dyslexic blond moment. It happens every time I try to remember my own age! 😉

    Error corrected and thanks for the heads up!

  4. Ummmmm, last I had checked, the Coal Sack nebula was approximately 600 light years away. So it is highly unlikely to have any influence on the Jewel Box at 7,500 light years away.

  6. Geez… I’m having a rough time getting into the swing of things after vacation! The link is fixed.

    Winchell? Some stellar extinction could possibly have occured in the Jewel Box according to Fresneau, Vaughan, and Argyle; “The weak distant encounter of stars with the cold neutral medium can be explained by a mean mass of about 2500+/-1250 Msolar for the gas and dust content in order to account for the diffusion of stellar orbits. A mean displacement of 0.012″+/-0.001″ yr-1 of the western limb of the Southern Coalsack is estimated in the northwest direction and permits a search for strong close stellar encounters. The transfer of angular momentum between the interstellar medium and the scattering of stellar orbits resulting from the encounters suggests a dissipation rate of (2.0+/-0.7)×10-4 Msolar yr-1, leading to an equipartition time of 12+/-4 Myr. A spin rate of 0.25+/-0.05 km s-1 pc-1 of the diffuse clouds in the Southern Coalsack is measured and interpreted as the result of the perturbation induced by stellar systems during flyby encounters with a crossing time of 3 Myr, suggesting a cloud core radius of 1 pc.”

    I should have been more precise that its not neccessarily interacting with NGC 4755 as much as it is blocking our view.

  7. For my RPG I tried to write a few Tolkienesque riddles and one of those is appropriate here –

    Brightly cold and very old
    Rubies beyond price,
    Diamonds beyond measure,
    And sapphires all comprise
    The bounty of this treasure.

    The answer of course is ‘stars’. A box of precious jewels indeed.

  9. Tammy, thanks for the article. So few southern objects are mentioned in most of the media, especially when they are as interesting as the Jewel Box.

    As a comment, it is a pity you didn’t mention that the main cluster stars are not the only components of the cluster. The star in the upper left-hand corner is also a member, which shares similar proper motions and small parallax as the colourful A-shaped asterism. It listed as HIP 62646 (Hipparcos) and is the brightest component at 5.7v magnitude.

    Secondly, Abbott did suspect there were colour changes in the cluster (as he did with the Eta Carina Nebulae). However, H.C. Russell did not measure the 129 stars in the open cluster to search for colour changes, but did so because he suspected the stars had moved because of the discrepancies in between James Dunlop and John Herschel’s drawing of the stars. Colour to Russell was of minor consequent, who saw the majority of stars as “greenish”. Of course, these motions were undetectable, manly as the proper motions are smaller than this and could only be noted in centuries or even millennia – roughly <5 kms^-1.
    As to the Coal Sack (Richard Allen’s “The Black Magellanic Cloud”) and the cluster there is no really association. The distance of the Coalsack is 300 light years, NGC 4755 is much further away.
    As to the evoution, the other interpretation is that the second branch of the main sequence (Page007.htm in the reference article below.) means that there might be just two stages of
    formation (as stated in Sager and Cannon (1995) article [. Sagar, R., Cannon, R.D., ; A deep CCD photometric study of the moderately young southern open clusterNGC 4755 = κ Crucis” Astro.&Astroph. “Sup.Ser., 111, 75-84 (1995)] ). Here the main massive stars are formed, followed by another star formation period for the smaller components, whose ages differ by about 3 to 6 million years. The best estimation of date is probably Ivan Koenig (1998) 12.6±0.1 M.yr. [Koenig, I.; et.al. “Celebrating the 12589254th birthday of Herschel’s Jewel Box” Astronomische Gesellschaft Meeting (AGM), 14, p.35 Jan (1998)]

    The other cluster NGC 4815 (12h 58.0m -64 deg 58′) within the Coalsack (in Musca), which suffers greatly from interstellar absorption, shows significant evidence of a double main sequence. Its B-V lines are separated by about one B-V magnitude, and is one of the significant of this phenomena.. An excellent article about these dual main sequence clusters, is; “Stellar photometric stability. II. Ages and
    distances for 13 open clusters with time series observations.” by Kjeldsen and Frandsen (A&A.,Sup.Ser., 87, 119 (1991)) Another is good NGC 4815 paper is In 1998, the latest paper (Chen, B. et.al. “The spatial distribution and luminosity function of the open
    cluster NGC 4815.”; A&A., 331, 916-924 (1998)) , which discussed this problem you allude to for the Jewel Box – so-called “mass segregation” and the sudden drop-off of stars near the edges of open clusters. Clearly this is related to the open cluster formation process, and in how the embryonic nebulous condenses to produce stars. The important point is that stars are unlikely to form at precisely one instant or moment, but is a gradual process over million of years. “Isocrones” (start date for the star formation stage) for cluster formation are not necessarily single but maybe two (or even more) bursts of activity.

    Those further interested in the Jewel Box (for background), especially its history, might like to refers to the Southern Astronomical Webpage at
    http://
    homepage.mac.com/andjames/Page001.htm

    Anyway, thanks for the article, especially the updates on investigation of the cluster and its stars..

  10. Tammy, thanks for the article. So few southern objects are mentioned in most of the media, especially when they are as interesting as the Jewel Box.

    As a comment, it is a pity you didn’t mention that the main cluster stars are not the only components of the cluster. The star in the upper left-hand corner is also a member, which shares similar proper motions and small parallax as the colourful A-shaped asterism. It listed as HIP 62646 (Hipparcos) and is the brightest component at 5.7v magnitude.

    Secondly, Abbott did suspect there were colour changes in the cluster (as he did with the Eta Carina Nebulae). However, H.C. Russell did not measure the 129 stars in the open cluster to search for colour changes, but did so because he suspected the stars had moved because of the discrepancies in between James Dunlop and John Herschel’s drawing of the stars. Colour to Russell was of minor consequent, who saw the majority of stars as “greenish”. Of course, these motions were undetectable, manly as the proper motions are smaller than this and could only be noted in centuries or even millennia – roughly <5 kms^-1.
    As to the Coal Sack (Richard Allen’s “The Black Magellanic Cloud”) and the cluster there is no really association. The distance of the Coalsack is 300 light years, NGC 4755 is much further away.
    As to the evoution, the other interpretation is that the second branch of the main sequence (Page007.htm in the reference article below.) means that there might be just two stages of
    formation (as stated in Sager and Cannon (1995) article [. Sagar, R., Cannon, R.D., ; A deep CCD photometric study of the moderately young southern open clusterNGC 4755 = κ Crucis” Astro.&Astroph. “Sup.Ser., 111, 75-84 (1995)] ). Here the main massive stars are formed, followed by another star formation period for the smaller components, whose ages differ by about 3 to 6 million years. The best estimation of date is probably Ivan Koenig (1998) 12.6±0.1 M.yr. [Koenig, I.; et.al. “Celebrating the 12589254th birthday of Herschel’s Jewel Box” Astronomische Gesellschaft Meeting (AGM), 14, p.35 Jan (1998)]

    The other cluster NGC 4815 (12h 58.0m -64 deg 58′) within the Coalsack (in Musca), which suffers greatly from interstellar absorption, shows significant evidence of a double main sequence. Its B-V lines are separated by about one B-V magnitude, and is one of the significant of this phenomena.. An excellent article about these dual main sequence clusters, is; “Stellar photometric stability. II. Ages and
    distances for 13 open clusters with time series observations.” by Kjeldsen and Frandsen (A&A.,Sup.Ser., 87, 119 (1991)) Another is good NGC 4815 paper is In 1998, the latest paper (Chen, B. et.al. “The spatial distribution and luminosity function of the open
    cluster NGC 4815.”; A&A., 331, 916-924 (1998)) , which discussed this problem you allude to for the Jewel Box – so-called “mass segregation” and the sudden drop-off of stars near the edges of open clusters. Clearly this is related to the open cluster formation process, and in how the embryonic nebulous condenses to produce stars. The important point is that stars are unlikely to form at precisely one instant or moment, but is a gradual process over million of years. “Isocrones” (start date for the star formation stage) for cluster formation are not necessarily single but maybe two (or even more) bursts of activity.

    Those further interested in the Jewel Box (for background), especially its history, might like to refers to the Southern Astronomical Webpage at

    Anyway, thanks for the article, especially the updates on investigation of the cluster and its stars..

  12. The missing link should be ;

    NGC 4755 – THE JEWEL BOX “The Jewel of the South”

  14. Hi, AJames! Nice write up. Needless to say I was overwhelmed with the amount of information available on the entire region and equally astounded at the amount of Beta Cephi type stars in that initial area as well. There was so much data available it was difficult to know what direction to take the story! I did feel however, that Abbott’s initial observations shouldn’t be overlooked and probably oversimplified other areas for sake of space and readiblity.

    I love knowing there’s more behind a pretty picture than just what you see!

  15. “I should have been more precise that its not neccessarily interacting with NGC 4755 as much as it is blocking our view.”

    Ah, it makes sense now. I didn’t read your post closely enough. My apologies.

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