Colour-Magnitude Diagrams : Part 2 of 2

Also see detailed example NGC 4755 : Colour-Magnitude Diagram

Some Basic Premises About Open Star Clusters


(1) After this starting stage of evolution as ZAMS, the evolution and lifetime of all the stars will proceed at different rates. This is only depended on the mass of the star, and to a much lesser extent, its chemical composition.

[If this is really true, then any open star cluster is really a ‘snap-shot’ of a collection of stars in various states of development which have started their lives at the same time in the distant past. I.e. All sharing the same birthday.]

(2) The CMD is therefore a predictive and historical reference that tells us some information about the compositions, spectral classes and masses of the component stars.

(3) From the scatter of stars across the CMD, information can be used for a comparison of stellar behaviour and the future evolution of the cluster’s stars.

(4) The common origin is assumed by common proper motions and the compositions of the stellar component stars. This implies some common origin in the same region of space whose stars derive from a common nebula weighing greater than several thousand solar masses.

Although astronomers often state that the every star was formed at the same time within some cluster, in reality the whole stellar formation would have spanned over the minimal period of at least 100,000 years. However, when compared to the cluster’s total age, such short durations are inconsequential; except for perhaps the very youngest of star clusters. I.e. A cluster 100 million years old is one ten-thousandth the age of another cluster that is 10 million years old, finding this initial early transitional period as one-hundredth of its life.

In the formation of clusters, the initial stars produce fractional variations in real cluster’ age. This is believed to be caused by the more gradual mechanism of the star formation and the overall rate of star formation. If we could assume details about the basic formation process, we can deduce much about other open star clusters and also be able to give details of the nature and properties of stars — regardless of the effects of interstellar absorption. This latter point is quite problematic to investigators, because unfortunately, the interstellar absorption varies significantly depending on the observed direction in relation to the Milky Way galaxy.

Light reddening is caused by interstellar dust, which absorbs the blue light of stars and dims the incoming light slightly — the so-called extinction. In some places these absorption values are small, and the observed effects are only minor. More often than not, this applies to clusters well beyond the galactic plane or those that are relatively closer.

When looking towards the more dusty regions of the Milky Way some open cluster are quite heavily obscured. Visually sometimes this can make the stars to appear dramatically reddened along our line of sight. Determination of the interstellar absorption with sufficient accuracy remains the problem, and its value often has to relying on several key assumptions. In respects of the Colour-Magnitude Diagram, the amount of absorption has the real effect of depleting the accuracy of the absolute magnitudes of the stellar components — and hence knowing each cluster’s true distance.

During the 1930’s and for several decades following, the interstellar absorption E(B-V) was accepted as some constant value that applied to all the stars - roughly 0.7 magnitudes per 1000 parsecs. More modern astrophysical papers on open clusters now separately investigate the nature of the absorption using specialised uvbyβ photometry on each of the component stars. Ie. The mean interstellar absorption [E(b-y)] found by Shobbrook (MNRAS, 206, 273 (1984)) was 0.281±0.004 based on the observations of forty-five (45) of the brighter stars in the Jewel Box.

On the CMD, observations that have taken interstellar absorption into account are properly expressed as (B-V)0. This can found normally on the top x-axis of the CMD with the B-V value on the the lower x-axis. In most instances these axes do not coincide in value, and are offset towards the right-hand side. (See Figure 1.)

Sample Colour Magnitude Diagram
Fig. 1. NGC 4755 : Colour Magnitude Diagram


Last Update : 25th December 2010

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