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

 Overview - By selecting stars with a range of colours and brightnesses off this true colour image of a small region in the globular cluster NGC104 (47 Tuc) you can investigate the structure of the cluster's Hertzsprung-Russell (HR) diagram resulting from stellar evolution. The image is a composite made up of three separate images taken in the ultraviolet (3800Å, U), blue (4500Å, B) and visual (5500Å, V). The aim is to populate the HR diagram sufficiently to answer the following questions:

1. Identify the main stellar evolution phases on the HR diagram for NGC 104. Specifically you should be able to mark the RGB, AGB, HB and the turn off at the top of the MS on a hardcopy of your HR diagram (see below).
    
   
2. As you select stars notice the way in which the structure builds up in the HR diagram. Try selecting stars with similar colours, or with similar brightness, or all the stars in a patch of the image going as faint as you can. Which of these techniques gives the 'fairest' representation of the star population? Which is the most efficient for defining the Horizontal Branch in the cluster? Can you detect the MS turn off? You'll have to select the faintest stars to be able to do this.
    
   
3. Estimate from your HR diagram the apparent magnitude of the Horizontal Branch for NGC 104. What is the uncertainty in your estimate? Note that NGC 104 shows both blue and red Horizontal Branches, with few stars in the instability Gap.

Measuring the Colours and Magnitudes of Stars - The CCD image shown below is 750 x 450 pixels in size, with each pixel being 0.5 arcsec. When you click the cursor on the position of a star the program performs the following sequence of actions:

• Centroids on that star (ie works out its x and y co-ordinates), and draws a circle around the star selected,
• Integrates the flux within the same aperture on the B and V images of the field (ie adds up the flux within the circle),
• Estimates the background level in the region around the star,
• Subtracts off this background from the aperture fluxes, giving the total flux in the B and V passbands,
• Converts these into magnitudes and calculates the colour from the difference (B - V).

When a star lies in the Horizontal Branch region (marked by the box) of the HR diagram note down the magnitude and colour of the star. By averaging together the magnitudes of all the stars which you select in this region you should be able to estimate not only the apparent magnitude of the HB in NGC 104, but also the statistical error in your estimate.

When you decide you have a sufficient number of measurements to answer the questions listed above you can capture the HR plot (ie save the graph). How you might do this depends on which particular computer you are using (eg unix workstation, PC or Macintosh) and there are some notes about how to do this on the news page. If you find this too difficult to do, simply enter all the numbers in a spreadsheet program (like Excel) and use the plotting features in it to graph the numbers. Mark on your figure the regions associated with the different stellar phases discussed above.

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