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Part 4

1 9 5 0 - 1 9 6 6

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 NGC4755.", Astron. J., 63, 341-346. Sept (1958). Data was gained for the brightest stars by photometry for fifteen stars, plus two others of 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.

It was the observer Bedelman (PASP, 66, 249 (1954)) who was first to recognise that the principal stars in the Jewel Box must be supergiants. 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 chi 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 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 designation.

e.g. CI* NGC 4755 AS I-S or CI* NGC 4755 AS IV-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 to by their 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 their numbers,

e.g. NGC 4755 4284 - Star 284 in Quadrant 4

Overall, Arp's 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 1.82m (72") 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 landmark 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. e.g. 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.

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Last Update : 05 April 2002

Southern Astronomical Delights”   © Andrew James (2002)          Sydney, Australia

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