Department of Physics University of Durham   Level One
School of Physics University of New South Wales   General Education

The Hertzsprung-Russell Diagram of a globular cluster

Summary of Results

At this point you should have noted the following information in your notebook:

  • A hard copy of your Hertzsprung-Russell diagram for the globular cluster (GC) NGC 104 with the various different stellar classes marked on it.
  • A list of magnitudes for the stars in the Horizontal Branch (HB), from which you have calculated the mean and the error in the apparent magnitude of the HB in the GC.
  • The absolute magnitude of the HB in a stellar population such as that seen in NGC 104 is M = 1.05±0.05 magnitudes in V (this has been estimated from other globular clusters for which independent estimates of the distance are available). Using the mean apparent magnitude of the HB from your measurements, estimate the distance modulus for NGC 104. Hence find the distance of the cluster in kiloparsecs, and estimate an error for this value. The distance modulus, µ, is the difference between the apparent magnitude (m) and the absolute magnitude (M). It is related to the distance of the source, r (in parsecs), as:

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  • What is your estimate of the distance of NGC 104? What is your best estimate of the error in this measurement?
  • Having measured the Horizontal Branch magnitude in NGC 104, what do you feel are the main drawbacks of this technique as a method for estimating distances to globular clusters?

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    Conclusions

    1. The Hertzsprung-Russell diagram provides a powerful tool for investigating the evolution of stellar populations. Using simple photometric information it is possible to assign stars to different evolutionary phases and investigate the relation between these different phases.
    2. Globular clusters are remarkably homogeneous stellar systems which exhibit the properties expected from simple, single age, single metallicity stellar populations. Their simplicity allows us to estimate distances and ages for these systems from their HR diagrams. Specifically, we used the apparent magnitude of a feature, the Horizontal Branch in the HR diagram for NGC 104, to estimate its distance.

    Further Reading and Information

    The following sections of course text books will provide background information on the astronomy discussed in this exercise.

    1. Colours and Magnitudes, Zeilik & Gregory, Ch 11, p224.
    2. Magnitudes, Zeilik, Ch 14.1, p304.
    3. Magnitudes, Kaufmann & Comins, Foundations III-2.
    4. Colours of Stars, Kaufmann & Comins, Ch 10.1.
    5. Evolution of Stars, Tipler, Ch 42.3, p1400.
    6. Evolution of Stars, Kaufmann & Comins, Ch 11.6-11.10.
    7. Stellar Classification, Zeilik & Gregory, Ch 13, p251.
    8. Stellar Classification, Zeilik, Ch 14.4, p310.
    9. Stellar Classification, Kaufmann & Comins, Ch 10.3.
    10. HR Diagram, Zeilik & Gregory, Ch 13, p251.
    11. HR Diagram, Zeilik, Ch 14.5, p313, Ch 16.2. p350.
    12. HR Diagram, Kaufmann & Comins, Ch 10.4.
    13. Globular clusters, Zeilik, Ch 16.7, p362.
    14. Globular clusters, Kaufmann & Comins, Ch 11.11.

    Acknowledgments

    Thanks to Pat Morris at Leeds University, Davison Soper at Oregon and Harmut Frommert and SEDS for some of the plots and text used in the background of this experiment. Also thanks to the Royal Society and University of New South Wales who paid enough for me to be able to sit and write this listening to the rain in Sydney while drinking good scotch.

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