Age (M.yr.)	Name and Year	Ref.
1,030	Schmidt, B. (1963)	1
8	Barbaro (1967)	--
34	Abt. H.A (1987)	2
22.4	Ward et al. (1984)	--
8 to 11	Dachs, J., & Kaiser, D., (1984)	3
5 and 8	Frandsen, S., & Kjeldsen, H., (1991)	4
8 to 12	Maeder and Meynet (1991)	5
6 to 7	Sagar, R., Cannon, R.D., (1995)	6
12.6±0.1	Koenig, I.; et al. (1998)	7
12.6±0.1	Koenig, I.; et al. (1998)	7
9.54	Kharchenko, N.V., et al.; A&A. , 438, 1163 (2005) [1]	8
15.8	Pandey, et al., MNRAS., 403, 1491 (2010)	9
19.95	Kharchenko, N.V., et al., A&A., 585, A101 (2016)	10

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Age References

1. Schmidt, B.; Astronomische Nachrichten; 287, 41 (1963)
2. Abt. H.A., AJ, 241, 275 (1980). Note that this particular value is still quoted in the now old “5th Lund Catalogue of Open Clusters′(1987)
3. Dachs, J., & Kaiser, D., A&., 58, 411 (1984)
4. Frandsen, S., & Kjeldsen, H., “Stellar Photometric Stability. II. Ages and distances for 13 open clusters with time series observations.”; A.&A.Sup.Ser., 87, 119 (1991)
5. Maeder, A., Meynet, G., “Tables of isochrones computed from stellar models with mass loss and overshooting.”; A&A., 89, 451 (1991)
6. Sagar, R., Cannon, R.D., “A deep CCD photometric study of the moderately young southern open cluster NGC 4755 = κ Crucis”; A&A.Sup.Ser., 111, 75 (1995)
7. Koenig, I., et al. “Celebrating the 12589254th birthday of Herschel’s Jewel Box”Astronomische Gesellschaft Meeting (AGM), 14, 35 Jan (1998)
8. Kharchenko, N.V., et al., A&A., “MCatalogue of Open Cluster Data (COCD)”, 438, 1163 (2005) [1]
9. Pandey, A.E., et al., “Integrated parameters of star clusters: a comparison of theory and observations”, MNRAS., 403, 1491 (2010)
10. Kharchenko, N.V., et al., “Milky Way global survey of star clusters. V.”, A&A., 585, A101 (2016)

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DISCUSSION

NGC 4755 overall is a very young cluster compared with most of the others in the sky. Looking at the NGC 4755 paper of Cannon and Sagar seems the best modern estimate, though Sagar and Cannon suggest that the brighter components may be some 4 million years older. Several of the later investigations also seem to be confirming this; I.e. Koenig, et al. (1998) and using more recent evolution theory suggest 12.6±0.1 Myr. (Calculating as 12589254 years as appearing in this paper’s title!) Oddly, the figures of the last decade or so have tended to a much higher set of values, which can be attributed to probably to better knowledge of the interstellar absorption betwen us and the cluster. Kharchenko et al. (2016) results seems to be far too high.

Another of the investigations was by Sanner, J., et al., “Photometric and kinematic studies of open star clusters II, NGC 4103, NGC 5281 and NGC 4755.” A&A., 369, 511 (2001) concludes that; “…ages at most of 45Myr.”. This sets the maximum age. There is some evidence that open clusters do not produce their individual stars all at once but that the process occurs over several million years. Such ideas explain the evolutionary variations seen in many colour magnitude diagrams. It is even possible that one (or more) bursts of star formation may occur, giving the appearance that they are seemingly two star clusters. There is much to be said on this particular topic about this open cluster — something which will warrant future investigation and continued observations. In some ways, quoted ages of any young cluster maybe less important than the age range of all its individual stars.

Next we shall compare the Jewel Box to some other bright southern clusters.

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Last Update : 24th April 2016

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