C O L O U R - M A G N I T U D E   D I A G R A M

   

Differing colours within any collection of more than one type of stellar aggregate immediately suggests a variety of evolutionary states among the component stars. If all the cluster stars were born simultaneously from their embryonic nebulosity, technically called an isochrone, then as the cluster ages, the individual mass of each star solely determines the order of star evolution. The largest stars evolve first, followed in turn, by the less massive ones. Age is based on the Colour Magnitude Diagram or C-M Diagram, which is used to estimate the cluster's age at only several million years, and for the first time, taking into account the independently derived quantity - the degree of inter-stellar absorption.

Published data for NGC 4755 is now available from twelve different sources. Its nature has now been well studied for at least fifty years. I have adopted some of the results from the "General Catalogue of Photometric Data", which can be easily obtained (via the Internet) from the Swiss Institute of Astronomy at the University of Lausanne. Figure 2 shows some eighty-six stars plotted, of which seventy-one are "high quality" points (first deemed "quality" stars by Arp, H. & Sant, C. T., Astron. J., 63, 341 (1958)).

Similar photometric studies were later obtained by Hagen (1970), Schild (1970) and Schild et.a]. (1976). However, the best improved set of UBV data was obtained from Dach. J. & Kaiser, D., "UBV photometry of the southern galactic cluster NGC 4755 - kappa Crucis", Astron. & Astroph. Sup. Series, 58, 411 (1984), who list some eighty-six "primary" photoelectric UBV observations, and 553 UBV photographic magnitudes. Each reduces the limiting magnitude from 13.4 mag. to 16.4, and some down to about 20.5 magnitude for selected areas of the cluster, I.e. as published by R. Sagar and R. D. Cannon, (Astro. & Astroph. Sup. Ser., 111, 75-84 (1995)).

Figure 2 also directly shows the distribution of stars by brightness and colour. The graph shows the plot of visual magnitude versus the B-V magnitude - the essentials of the Colour-Magnitude Diagram. B-V magnitudes are an indication of colour, where values between -0.2 and +0.5 are blue to white stars, +0.5 to +1.5 tend to be yellow stars, while those around +2.0 are generally red. Each magnitude in Figure 2 was obtained by photometric means. I have added for comparison on the right-hand side of Figure 2 the absolute magnitude scale, which is particularly uncertain, because it needs derived quantities like spectral data and evolution theory. (Note: On this scale the Sun would be below the lowest part on this graph.)

The upper x-axis in any colour magnitude diagrams shows the (B-V)₀ magnitude. However an observer must takes into account the amount of absorption of light, or extinction, of the material or gas between us and the cluster. You may notice that the (B-V)₀ and B-V are slightly different, well this difference is the actual magnitude extinction taken into account. Absorbed light tends to make the observed starlight reddish, where the (B-V)₀ shifts to the right. I have used the mean extinction magnitude of E(B-V) of 0.41, as given by Sagar and Cannon (1995). The scale shifts to the right, suggesting that the light has reddened.

Figure 2 also tells us much about the constitution of the cluster members. Stars above 8th magnitude are the brightest members in the 'A' shaped asterism. The six stars at the very top of the main sequence (the line of stars moving down the left side of the graph) are the blue giant stars, and the single massive red supergiant is at B-V=+2.22. Progressing down the C-M Diagram finds fainter and fainter blue stars, and it is not until about 13th magnitude, do we start to see many of the yellow to red stars. Brightest of the fainter group is the so-called "NGC 4755 104" which is an orange 11.03 mag. star at B-V=+1.57.

Stellar evolution theory tells us that stars lie on the main sequence for 80% of their lives. It is only after the fuel shortages take hold, within each stellar core, that each star moves away from the main sequence. In every case, the biggest and most massive stars are far more voracious with their fuel consumption. So the higher the stellar mass, the higher the placement of the star on the main sequence. Figures 2 and 3 clearly show this, with the largest stars slightly right but off the main sequence. The largest star here has to be the red giant farther to right, as it has already moved away from its siblings and has enlarged to become a red giant. All the stars above this turn-off point are either near or have moved right off the main sequence. Those below the turn-off point have yet to do so, and these simply continue to keep on slowly burning hydrogen.

Amateur Uses of Colour-Magnitude Diagrams

Although these Colour-Magnitude Diagrams seem very technical, they can be practicably used to estimate the cluster's appearance in the telescope. To do this, you will need the edge of a piece of paper, or even better, a clear transparent sheet of plastic.

2. Place the straight edge of the paper along the line of the visual magnitude scale.
3. Next, draw-down the page's straight-edge to the 6th magnitude line. This reveals three stars, which make up the points of the triangle of the 'A'. These are the most luminous stars.
4. Drop the line's edge to 8th magnitude. This shows the principal stars of the 'A', showing seven of the blue stars and one solitary red one.
5. Further down, around 10th magnitude, the limit of 7x50's binoculars, reveals that you should probably see some twenty-three stars, two of which are now yellowish in colour.
6. If we reduce this line down to 14th magnitude, we are principally seeing the main bright stars of the cluster. (I suggest you have a look at a colour photograph of the Jewel Box and do a count of the stars to confirm this. A good one, for example, appears in AOST2 as Plate 18.)
7. You will find that the blue stars counted from Figure 3 are about the same on the AOST2 photograph, or the grey-scale image attached. This also applies to the coloured stars.

As you can see, this method tells us roughly how the cluster will appear, both numerically and colour-wise. Furthermore, one can never claim to have "seen" the Jewel Box, as different apertures show different types of stars - so the overall impressions between individuals are fairly subjective. Such methods also work brilliantly with C-M Diagrams of Globular Clusters. It is interesting to find the magnitude of the turn-off point. This can tell you where the fewer main sequence stars end, and most of the cooler ones start. At this place, the cluster changes in appearance quite rapidly. and it is interesting to apply this to the magnitude limit of your telescope (if the limit of the telescope is say 14th and the turn-off point is about the same). Try looking at a few open clusters and their general appearance, then reduce the aperture with cardboard field-stops. Compare the difference. What you see might surprise you!

One of the lastest observations were gained by Koenig, Ingo et.al. 1998), in which a group of eight astronomers gathered CCD absolute photometry of Kappa Crucis at La Silla in Chile using an 61cm f/15 Cassergrain. There paper (Koenig, Ingoet. al.; AGM, 14, p.35 Jan (1998)) Showed stars down to 14.5V magnitude the resultant colour–magnitude diagram found the main sequence turn–off point at 8.0V magnitude. Furthermore twenty–six stars had their MK spectral classification determined roughly that also produced the mean absorption E(B–V) of 0.344±0.012 magnitude.

One interesting postulate is that Kappa Crucis may be two clusters nearly superimposed on top of each other. (See Figure 3a) This would explain the apparent disparity between the very bright stars against the more numerous fainter ones. Another possibility is that this could be an example of the merging of two open clusters, or even two clusters forming in the same nebula, but during two separate bursts of evolution, possibly some two million years apart. If the latter were true, then these differences would be hard to detect, as the chemical compositions from the initial nebula would make the component stars the same. To seemingly contradict this, it appears that across the face of the cluster, variations exist with the E(B-V) magnitude extinction, which gives the true B-V magnitude values higher errors than expected. Such ideas may explain these differences, as stated by Sagar & Cannon (1994). It is tempting to think that lying near the edge of the nearby Coal Sack may be the cause of these variations. However, no theory or observation is available to prove such speculation.

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