PART FOUR : 1950 — 2008

Development of observational astronomy in this period has changed very much in this short period of just over fifty years. I have divided the period into two parts, being 1950 to 1966, when much of the then new photometry was being undertaken. Unlike the larger star clusters, NGC 4755 was more difficult to ascertain due to its small size — making separation of the components more difficult. After 1966, the number of observations meant that more detail, especially with the deeper magnitude H-R Diagrams, and from this were dedicated papers apply observations to meet the current stellar evolution theory. Derivation of most of the cluster parameters were constructed after 1966, and continue to the present. This page discusses the general information, which is then divided into the next few pages of this site.

Photometric Observations of NGC 4755 and Star Nomenclature

The first significantly important astrophysical paper on this cluster was published by Halton Arp and Cecil van Sant in Southern Hemisphere Photometry IV: — The Galactic Cluster NGC 4755., Astron. J., 63, 341-346. Sept (1958). Data was gained for the brightest stars by photometry for fifteen stars, plus two others with lesser precision. Each of these was listed from A to Q, e.g. NGC 4755 A, and sometimes this stellar nomenclature is still used. These stars, among other southern clusters, were similarly nominated in a series of seven papers that were initiated by Arp. The aim of these investigations was originally to find out about galactic supergiant stars, which were particularly poorly understood at the time. This is due to solitary field supergiants having no standard candles to use as indicators of distance, which were needed to pin down standard derived values, such as luminosity, mass, radius, etc.

TABLE 1 : Brighest Photometric Stars of Jewel Box
[Arp, H. and C. van.Sant (1958) pg.341.]

B-V Spect.
A 5.75 0.325 B8-9 Ib
B 5.94 0.224 B5 II or Ib
C 6.80 0.243 B3-4 II
D 7.85 (7.66) (2.28) K5 giant (M2Iab)
E 8.35 0.118 B5II or III
F 9.09 0.153 B6 III:
G 9.79 0.195 B6 V: (B1V)
H 9.93 0.194 (B1.5V)
H 9.93 0.194 (B1.5V)
I 10.04 0.336 (B1.5Vn)
J 10.58 0.151 (B2V)
K 11.42 0.321 (B3V)
L 11.88 0.302 (B8:V)
M 12.40 0.384 (B8III-V)
N 12.76 0.739
O 13.17 0.494 --
P 13.37 0.254 --
Q 13.38 0.587 --
R* 9.58 0.17 B1.5V
S* 9.59 0.228 B1.5Vnpe
T* 5.75 0.309 A2Iabe


1. These stars do not appear in the article when published but were added later by these observers.
2. Those values given in brackets, have been taken from other sources, especially the spectral classes of the later fainter stars in the list.
3. Star T is DA Cru. and is the last of the lettered components of this cluster.
4. Star D is DU Cru and is the red giant in the Jewel Box. It is certainly redder than the K5 given by Arp.
5. In SIMBAD, the information on these stars can be accessed by typing ”Cl* NGC 4755 AS U”.

It was the observer William P. Bedelman (PASP, 66, 249 (1954)) became the was first to recognise that the principal stars in the Jewel Box must be supergiants. Furthermore as only few clusters have these types of stars, these open clusters provided the most suitable environment for investigation. As Arp and van Sant rightly voiced in their paper :

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 [suitably] obtained.

Later, several different photographic plates were taken using various telescopes from the Radcliffe Observatory in Pretoria. South Africa. These plates assigned magnitudes and B-V values to a further 202 selected stars. These stars are based on the entire field divided into four quadrants and centred on Kappa Crucis at position 12hr 53.0m -60° 22′. Each component star is then designated as NGC 4755 followed by the Roman Numeral I, II, III or IV (the quadrant the star the star is in), and the number of the star. e.g. NGC 4755 I 5, NGC 4755 IV 12, etc.

Sometimes this is written using the more official IAU individual star designations. e.g. CI* NGC 4755 AS I-S or CI* NGC 4755 AS V-12, etc.

Arp and van Sant (abbreviated as AS) placed fifty-seven stars in Quadrant 1 (NW), forty-one in Quadrant 2 (NE). forty-two in Quadrant 3 (SW) and sixty-two in Quadrant 4 (SE). This nomenclature still remains popular in the literature that applies to only to few southern clusters — including NGC 2516 in southern Carina. Confusingly, the numbers used in this system are also now written as; e.g. NGC 4755 105 and NGC 4755 412, etc. instead of the older NGC 4755 I-5 or NGC IV-12.

For the other stars not in this list, the adopted designations become more confusing. The major ones include the star atlas designations, CPD, GSC and HD, while the variables are again separated into different groups. Yet, for some reason, variables are never referred by the Quadrant designations. Stars below about 14th magnitude follow a different system, where the Quadrant System is still used, but the number of stars dramatically increases the numbers, e.g. NGC 4755 4284 — Star 284 in Quadrant 4.

Overall, Arps selection was wisely chosen. Its advantage is mainly so all photometric data could be standardised between all open cluster observers, however, like most advances in astronomy, new designation systems are often required as more detailed data is obtained.

General Data on the Jewel Box

Much of the most fruitful observational work was obtained between the years 1958 and 1965. Astronomers Arp, Helen Hogg and Wallenquist made significant inroads in discovery, by gaining suitable photometric data between 1959 and 1960. Most significant were the photometric observations made by Hogg in 1960, using the 1.82-metre (72-inch) at Mt. Stromlo (Australian Scientist, 1, N4). Spectroscopic and three-colour photometry then quickly followed. The data produced the first accurate estimates of distance, and importantly, the nature of the more important cluster members. All data on galactic clusters, including the Jewel Box, eventually was to be placed in the Catalogue of Galactic Star Clusters and Associations (including Nine Supplements), Prague (1962). (Much of this data on clusters was used, for example, by A. Becvar in the Atlas of the Heavens. Epoch 1950)

Next was the improved paper by M. W. Feast, MNRAS, 126, 11 (1963), who obtained spectroscopic observations of the brighter cluster stars, making inroads into the spectral types and radial velocities. From about 1965, improved techniques started to hone in on the basic cluster dynamics and many of their important parameters. For example, Kennedy (1966) found the improved cluster mean radial velocity of −18kms-1. Later, during this period, studies by A. Hogg (Mem. Mt. Stromlo., N9 (1965)) listed some 237 stars by examining the plates for proper motions. Analysis found 198 stars were likely members, with the remainder being likely field stars.

H. 1966 to PRESENT

After 1966, significant new observational discoveries were made with NGC 4755. I have divided these into different categories to make it easier to read and digest. Much of the text presents knowledge until 2008 and further studies into the cluster are unlikely to change very much in what we presently know. Future observations will likely produce be more accurate astrometry. Hipparcos observations have been presented of late, and it is likely that the next generation of astrometric satellite will vastly improve sorting out cluster members. Furthermore, I have attempted to get some feel for the significant understanding we have about this and other open clusters.

First we will look at is information about the obtained
Colour-Magnitude Diagrams or C-M Diagrams for NGC 4755.

More discoveries and details will appear in future updates as my time permits.


Last Update : 23rd September 2012

Southern Astronomical Delights © (2012)

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