Department of Physics University of Durham Level One

### 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 (H-R) 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 H-R diagram sufficiently to answer the following questions:

1. Identify the main stellar evolution phases on the H-R 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 H-R 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 H-R 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 750×450 pixels in size, with each pixel being 0.5 arcsec. When you click the cursor on the position of a star the program will centroid on that star (and draw a circle around the star selected) and then integrate the flux within the same aperture on the B and V images of the field, estimate the background in the region around the star and subtract this off the aperture fluxes to give the total flux in the B and V passbands. Converted into magnitudes these two fluxes will give the colour and apparent magnitude of the star, which are written to the output. These values are written both at the top of the image (for the current star) and should also all be listed on the Java console of your browser. The point will also be plotted on the figure in the attached window. Note, some of the stars in the frame are too faint for the program to centroid on.

When a star lies in the Horizontal Branch region (marked by the box) of the H-R 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 H-R plot using XV (or similar), to make a hardcopy for yourself. Mark on this figure the regions associated with the different stellar phases discussed above.

Globular Cluster H-R diagram