The Evolution of Stars and Galaxies

In stellar astrophysics it is often useful to look at a large number of similar objects simultaneously. Globular clusters have been used for a long time to constrain the theory of stellar evolution; now in Oxford we are developing techniques to use the integrated light from populations of stars to learn about galaxy evolution.

On the right of the page is a Hertzsprung-Russell diagram (or colour-magnitude diagram) showing the different stars in the globular cluster M55. Each point represents one star, and different kinds of stars can be found in different positions on the diagram. Detailed stellar models can be calibrated to the observed populations in local globular clusters. There are still intriguing problems in the understanding of globular cluster evolution - such as the origin of the extreme horizontal branch, self enrichment and the second parameter problem (see Sukyoung Yi and S.J. Yoon).

A more complex problem is how to interpret the light from stars when the cannot be individually resolved. The integrated light from a distant galaxy should contain considerable information about the stars within the galaxy, and in turn the formation of the galaxy itself. Extracting that information is extremely non-trivial (see Daniel Thomas and Claudia Maraston), and requires a comprehensive understanding of spectral synthesis and analysis as well as detailed stellar evolution models.

One specific problem is how to break the age-metallicity degeneracy, i.e. altering the age and initial composition of a stellar system can often produce similar effects. Breaking that age-metallicity degeneracy is a vital issue. The problem becomes extremely knotted when one considers that galaxies may have had several different epochs of star formation - i.e. the galaxy is likely to contain more than one population of stars, each with a different age and composition.

The technique of evolutionary population synthesis used in this work should not be confused with binary population synthesis: they are still largely separate fields. However our collaborator Zhanwen Han has been involved in including binary evolution in these galaxy models, notably with reference to explaining the UV upturn seen in elliptical galaxies.