Colour-Magnitude Diagrams : Part 2 of 2

Also see detailed example NGC 4755 : Colour-Magnitude Diagram

Some Basic Assumptions About Open Star Clusters

(1) After the nebulous stages of cluster formation end, and Zero-Aged Main Sequence (ZAMS) stars emerge and shine, stellar evolutionary processes for each star will proceed at different rates and this determines their stellar lifetime. This only depends on either stellar mass, and to a much lesser extent, its chemical composition.

(2) If Point (1) is true, then all open star clusters are really just a snapshot of their evolutionary status. This whole collection of stars are all bound by gravitation, being fixed in various states of development. The stars individually started their lives at once in the distant past and so all share similar birthdays.

(2) Colour Magnitude Diagrams (CMD) are therefore predictive and historical references of star clusters, telling us information about each component star composition, spectral class and mass.

(3) From the scatter of stars across any CMD, information can be used for comparing stellar behaviour and predict future cluster star evolution.

(4) Common origin of open clusters is mostly assumed by tracing the common proper motions and chemical compositions of any component star. This implies some initial formation in the same region of space, and whose stars derive from some ancient nebula exceeding several thousand solar masses.

Details About Colour-Magnitude Diagrams

Often, astronomers will generally say that within some cluster, every star was formed almost simultaneously together. In reality, the whole stellar formation would have spanned over the minimal period of at least ten to fifty thousand years. When this time is compared to the total age of cluster, such short astronomical periods are mostly inconsequential; except perhaps for the very youngest of star clusters. For example, any cluster that was 100 million years old is one ten-thousandth the age of another cluster that is 10 million years old, and finds that this initial early transitional period is merely just one-hundredth of its lifetime.

During open cluster formation, the initially born stars produce fractional variations in real cluster age. This is believed to be caused by the more gradual star formation mechanism and the overall star formation rate. Assuming details about this basic formation process, we could then deduce much about other open star clusters, giving details of their nature and stellar properties — regardless of any interstellar absorption effects. This latter point is quite problematic to investigators, because unfortunately, interstellar absorption varies significantly depending on the direction of starlight in relation to the Milky Way.

Light reddening is caused by intervening interstellar dust, which absorbs blue starlight and slightly dims incoming light — the so-called extinction. In some places, absorption values are small, and its observed effects are only minor. More often than not, this applies to stars and 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 clusters are quite heavily obscured. Visually sometimes stars may appear dramatically reddened along our line of sight. Determining 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 clusters true distance.

During the 1930s, and for several decades following, interstellar absorption E(B−V) was accepted by some constant value that applied to all the stars – roughly 0.7 magnitudes per kiloparsec. More modern astrophysical papers on open clusters now separately investigate the nature of the absorption using specialised uvbyβ photometry on each component star. I.e. 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 wonderful southern gem; The Jewel Box.

On the CMD, photometric observations also have to take interstellar absorption into account; properly expressed as B−V)0. This can be found normally on the top x-axis of the CMD with its B−V value on 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 : 19th April 2017

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