||Department of Physics
||University of Durham
The Hertzsprung-Russell Diagram of a globular cluster
Summary of Results
At this point you should have noted the following information in
- 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 and hence the distance of the cluster in
kiloparsecs, as well as an error on this distance. The difference
between the apparent magnitude (m) and the absolute magnitude (M),
termed the distance modulus (µ), is related to the distance of the
source (r, in parsecs) as:
- 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?
- 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.
- 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 H-R diagrams. Specifically, we used the apparent magnitude
of a feature, the Horizontal Branch, in the H-R 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.
- Colours and Magnitudes, Zeilik & Gregory, Ch 11, p224.
- Magnitudes, Zeilik, Ch 14.1, p304.
- Evolution of Stars, Tipler, Ch 42.3, p1400.
- Stellar Classification, Zeilik & Gregory, Ch 13, p251.
- Stellar Classification, Zeilik, Ch 14.4, p310.
- H-R Diagram, Zeilik & Gregory, Ch 13, p251.
- H-R Diagram, Zeilik, Ch 14.5, p313, Ch 16.2. p350.
- Globular clusters, Zeilik, Ch 16.7, p362.
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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