Evolution of Open Star Clusters

We have already seen that clusters are made and formed in great stellar hatcheries within emission nebulae, whose stellar populations are quickly produced roughly at the same time. Once stars join the Main Sequence, the stellar masses determines the rate of evolution. The heavier the star, the quicker it evolves. Most stars sit in their places on the main sequence for 80% of their lifetime, before moving away to become red giants. Therefore, any created Colour-Magnitude Diagram (CMD) is really only a small snap-shot of the evolutionary state of the stellar populations et at the clusters specified age. We simply find from the overall distribution of stellar characteristics found within each cluster CMD, very useful clues finding the cluster age. In this regard, looking at various open cluster CMDs shows that open clusters vary significantly. It therefore becomes easy to distinguishing between, say, some brand-new or aged cluster one.

Visually, the first notable thing on the CMD is the line of stars forming an S-shaped or sting-shaped curve. All the brightest and bluest stars are placed at the top-left moving across the curve to the bottom right corner. This principal line contains mainly similar solar-mass stars and is called the Main Sequence. (White Circle 2 in Figure 1.)

Each component that appears on this line are non-evolved stars that are consuming their hydrogen fuel at a steady rate and are in both gravitational and thermal equilibrium. Any star on this line will spend about 80% of its lifetime more or less near the same place on the CMD. After some period of time eventually the star will have consumed significant portions of its hydrogen which soon affects stellar structures.

On approaching of this looming energy crisis the internal parts of the star begin to react and some reorganisation of the stellar material must take place. Simply the core of the star is slowly crushed inwards by gravity and becomes no longer capable of producing the energy required to keep this force in balance. Consequently the core shrinks and the core temperature rises. Soon the lower density of the outer regions of the star respond by expanding. Here the star begins to swell, evolving into a red giant star. An observer sees this effect as an increase in luminosity compared to its earlier life, but stellar surface area has inflated so much, the surface temperature decreases. Such stars appear as cooler bodies than their former selves, whose colours becoming less blue and more red — seen as an increase in the measured B−V.

Again as seen on the CMD, as the cluster ages, more and more stars will begin to drift away from the main sequence towards the right-hand side. We find that because the heaviest stars evolve first, these are the first to leave the long populated line of the main sequence. As these stars tend to be both hotter and more luminous, they appear in the top left-hand corner. These massive stars are first to peeled-off the main sequence, then are followed in order by the lesser and leaser massed stars as the cluster slowly ages. The place on the CMD where the stars move off the main sequence is called the turn-off point.

By examining any CMD, an estimation of the range of stellar masses can be made. This is achieved by predicting the place of the turn-off point and finding the mass of the stars that lie near there. Using stellar evolution theory, stellar age can be determined which, subsequently implies the total age of the whole cluster. Furthermore, as the cluster gets older the B−V value of the turn-off point slowly decreases over time. For young clusters like the Pleiades, no stars has evolved away from the Main Sequence. Here we see that the B−V turn-off point for the cluster is anywhere between +0.2 and +0.4; being mostly yellowish to yellow stars.

Table 1 below shows roughly the expected B−V at the turning-off point for the corresponding age. It should be noted that some variations in colour are caused by the range of masses of the cluster stars, the chemical composition, etc.

Table 1. Age & B−V for OSC Turn-Off Points
B−V Cluster
5 −0.30 NGC 3324 Car
10 −0.25 NGC 3293 Car
25 −0.20 IC 2602 Car
50 −0.10 IC 2391 Vel
100 −0.05 NGC 2516 Car
500 +0.00 IC 4756 Ser
750 +0.10 M44 Cnc
1000 +0.20 NGC 2660 Vel
2500 +0.30 M67 Cnc
10000 +0.40 -- --
Note: Age is set from the turn-off point.

Sandage Colour Magnitude Diagram
Figure 2. Modified Sandage Composite Colour Magnitude Diagram

This is the Composite CMD of eleven (11) galactic clusters and one globular cluster. The age of each open cluster in question is given by the main sequence termination point, which appears on the right side of the figure.

Modified from original standard textbook example figure originally produced by Allan Sandage
The Color-Magnitude Diagrams of Galactic and Globular Clusters and Their Interpretation as Age Groups; R.A., 5, 41, (1958)

Looking at the respective CMDs, derived simply by counting all stars that have deviated away from the main sequence, you can easily tell which is the oldest of the stellar groups. Age of each cluster is given as 5.6 million years (Myr.) and 3.2 Myr., respectively, while the association is in the order of 7.9 million years.

As for other bright clusters, we have determined their ages with reasonable precision. This has meant we can provide suitable comparisons and have some understand of the general characteristics of the main sequence stars.

Often stars in younger open star clusters are higher in both luminosity and surface temperature. It is from this region that the stars appear at the top left-hand corner of the CMD and then deviate by the curves at the very top end of the turn-off point. Established older clusters like M67 and NGC 752 have most of the stars away from the main sequence. This gap between the open clusters on each side of the CMD (and in the more advanced Hertzsprung-Russell Diagram (HRD)) is called the Hertzsprung Gap. It is produced by stars quickly evolving directly across the CMD towards the red-giant stage. Chances of seeing any stars lying within in the Hetrzsprung gap is likely very small to improbable.

Other Colour-Magnitude Diagrams

The following table shows open star clusters that are listed in increasing ages which has been adopted from WEBDA and Lyngå, G. Lund Catalogue of Open Cluster Data”, 5th Edition (1987). [The previous version of this table data used Gretchen Hagens An Atlas of Open Star Clusters Colour-Magnitude Diagrams” (1970), which is now significantly outdated.]

No. Cluster
Con. Dist.
1 NGC 2362 / τ CMa CMa 1389 I 3 p - 8.2
2 NGC 2467 Pup 1355 I 3 m n:b 12.7
3 NGC 4755 / Jewel Box Cru 1976 I 3 r - 16.3
4 Mel 20 / α Per Per 185 III 3 m - 71
5 NGC 6087 Nor 891 I 2 p - 94
6 M45 / Pleiades Tau 150 I 3 r - 135
7 M11 / NGC 6705 Sct 1877 I 2 r - 200
8 M67/ NGC 2682 Cnc 908 I 3 r - 2560
9 NGC 6791 Lyr 4100 II 3 r - 4315

Note: The discussion on the evolution of these select clusters can be seen in the individual clusters.


Last Update : 19th April 2017

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