NGC 4755 : COLOUR-MAGNITUDE DIAGRAMS

For a more detailed discussion on this topic, I recommended you read the specific Southern Astronomical Delights page on Colour-Magnitude Diagrams and Hertzsprung Russell (HR) Diagrams and Some Basic Premises About Open Star Clusters.

General parameters for NGC 4755 and its component stars has now been studied in some detail for at least sixty years or so, with published colour and brightness data for NGC 4755 is now available from more than twelve different sources. (2006) 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 al. (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”, A.&A.Sup.Ser., 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, (A&A.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.)

Figure 2. THE COLOUR-MAGNITUDE DIAGRAM of
the BRIGHTEST STARS in JEWEL BOX

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 B-V of +2.22.

Progressing down the CM Diagram finds fainter and fainter bluish 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 of +1.57.

Figure 3. A DEEPER COLOUR-MAGNITUDE DIAGRAM for NGC 4755

Note that around 13th magnitude that the stars curve more sharply inwards — marking the so-called turn-off point. This place is of some astrophysical importance, as it reveals quantifiable information about the age of the cluster. Adapted from Sagar, R., Cannon, R.D.; A&A.Sup.Ser, 111, 75 (1995) (Ref. 26)

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 appearance and magnitude distribution of the cluster 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.

1. Place the straight edge of the paper along the line of the visual magnitude scale.

2. 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.

3. 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.

3. Further down, around 10th magnitude, the limit of 7×50 binoculars, reveals that you should probably see some twenty-three stars, two of which are now yellowish in colour.

4. 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.)

5. 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 in number but more massive main sequence stars end and majority of the lower main-sequence ones start. At this place, the cluster may change 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!

Most Recent Colour Magnitude Diagram

Figure 4. COLOUR-MAGNITUDE DIAGRAM of the JEWEL BOX

Note that the stars are placed in not one but two curves curves superimosed, suggesting the superposition of one cluster over another or possibly two disinct bursts of star formation. The first main-sequence is at “1”, the other at “2”. The Sun is positioned doen the first main sequence showing that many of the brighter components are more luminous. [Adapted from Sanner, J., et al.; A&A, 369, 511 (2001)] (Ref. 27)

One of the lastest observations were gained by Koenig 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. et 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.

Last Update : 17th July 2012