Overview
This page gives an overview of the areas of astronomy that I work in. You might also find my publications interesting.
The objects that I work on can be, broadly speaking, split up into three groups:
- Single stars
- Binary stars, pairs of stars orbiting one another
- Clusters of many stars, such as globular clusters or the Galactic Centre Cluster
My work consists of trying to understand some of the physical processes that go on in and between stars. Instead of taking observations of the stars myself I try to understand observations that other people have made, using theoretical and computational tools. In particular I work on three closely-related topics:
Stellar evolution
By stellar evolution scientists mean the study of how stars change and evolve from one type of object to another over their lifetimes. A star such as the sun starts its life as a cold, diffuse cloud of gas, that condenses to form a warm yellow dwarf star. Later in its life it will become a cold, luminous giant star, then a very hot, luminous white dwarf which will cool until it becomes extinguished. Because stars live for a very long time, we cannot in general observe these processes in action. Instead, we can observe a range of different stars in the sky, and hypothesise that one type of object might change, with time, into another. Alternatively we can follow the life of a star in a model within a computer. By making a few simple assumptions, such that the star is spherical (which they mostly are) and not changing on short timescales (which again, they mostly are not) we can construct a model for the internal workings of the star, based entirely on the laws of physics as we observed them in a laboratory on Earth. The amazing thing is that such a model fits the stars that we see in the sky extremely well.
The aspect of the evolution of single stars that I have worked on is the production of elements heavier than iron in the late stages of the evolution of intermediate-mass stars, those with mass roughly between one and ten times that of our Sun. In the very late stages of these stars' evolution hydrogen- and helium-burning can interact in a complex cyclical process that, amongst other things, produces a surplus of neutrons. These neutrons can then be absorbed by heavy nuclei, in particular iron, to produce heavy elements such as, for example, lead. In collaboration with colleagues in Italy and Australia I am using a highly-efficient algorithm to measure the rate at which these neutrons are absorbed and the pattern of elements that results in stars with a range of masses and compositions.
Binary star systems
Our Sun is unusual in that it is a single star. The majority of stars, other possibly than the very lowest mass ones, exist in binaries, pairs of stars orbiting their common centre of mass under the force of their mutual self-gravity. Or, in other words, going round one another in circles. These binaries can be very close, with separations of just a few times the radius of the stars or very wide, larger than our entire Solar System. The closest binaries are the ones that are most interesting from a stellar physics point of view, as there the stars can interact. If a star becomes larger than about one third of its orbital separation matter will flow from its surface onto its companion star. What happens next is complicated, and not entirely understood. If the donor star is a giant, it swells further as it loses mass, accelerating the process. Eventually runaway mass transfer occurs, and the star's outer envelope will be completely shredded, forming a large diffuse cloud surrounding both stars. The stars spiral together and the cloud is ejected, often forming a planetary nebula. The two stars may merge, or end up as a very close binary indeed. This process can form a binary where two neutron stars, old stellar remnants that are so dense that their constituent atoms have all be squeezed together into neutrons, orbit with a period of about a day. Their orbit gets smaller and smaller as they emit gravitational waves, eventually merging in an explosion known as a gamma-ray burst, one of the most energetic events in the Universe.
The objects that can be formed through the interactions of binary stars and the processes involved are complex, poorly-understood and extremely variable. As well as working on a computer code that attempts to model the evolution of any binary star self-consistently, a difficult and not-quite-yet-achieved goal, I have worked on a specific problem -- the onset of mass transfer in non-circular binaries. The usual theory that we use to predict when one of the stars in a binary starts to transfer mass its companion was developed by Édouard Roche in the 19th century, but only works if the star's orbit is circular. We used the technique of smoothed particle hydrodynamics, which represents moving fluid by particles, to measure the onset and rate of mass transfer in a binary with an eliptical orbit.
Stellar clusters
The average star in the Galaxy, such as our own Sun, never encounters another star at close quarters. Other than its companion (if it is a in a binary) its chances of coming close enough to another star to interact with it are very small indeed. However, in globular clusters this is not the case. In the very central core of a globular cluster the stellar densities are high enough that stars come close enough to each other to interact. The most significant effect is not on single stars, however, but on binaries. On average, in encounters between binaries and single stars, the closer binaries will become even closer still, imparting energy to the single star, whereas the wider binaries will tend to be broken up. The principal effect, therefore, is to harden the binary population, making the closer binaries closer and the wider binaries wider. This means that clusters contain a larger number of binaries where the pair of stars interact with each other in interesting ways.
The main research thrust of the work I am currently doing in Lund is to measure the effect of the cluster environment on interacting binaries. Using a code that combines an N-body integerator that can follow the paths of the stars in a cluster with a stellar and binary evolution code to follow the life of each star and any interactions that occur if it is in a binary, I am looking at the effects on the stellar population. In particular I am interested in the formation of compact binaries, which contain neutron stars, black holes and/or white dwarfs. These binaries are responsible for some of the more exotic stellar objects such as X-ray binaries, millisecond pulsars and short gamma-ray bursts. We hope, by making some of the most detailed models of stellar clusters yet, to understand better the production of these objects.