What are stellar oscillations?
The study of stellar oscillations is tied to the beautiful idea that the oscillations of a system can teach us about its physical properties. A star is a sphere of gas that, if excited, can oscillate in many different modes each with a slightly different frequency like a ringing bell. The frequencies depend on the internal sound speed which in turn depends on physical properties of the stellar interior like density, temperature and composition. Each mode carries information about the stellar interior that is different from that of any other mode. Hence by measuring frequencies from many different modes we can reconstruct the sound speed profile of the star to learn about physical properties that we otherwise cannot measure. Our field of research is "seismology of stars" or asteroseismology, and it is analogous to the seismological study of the Earth's interior.
Animation of an l=2 m=2 oscillation, produced by students Alexandra Chambers and Darran Baker as part of a course on Scientific Computing in the School of Physics at the University of Sydney
Helioseismology has been used to study the interior of our own sun with great success. Our group is carrying out asteroseismology, particularly for solar-like stars. In January 2007 we organized an observational campaign lasting three weeks and using 10 different telescopes around the globe to measure the oscillations in the star Procyon. This large number of telescopes was needed to get 24-hour coverage of the oscillations. Procyon is slightly more massive than the Sun and is also more evolved, which means that it has used most of its hydrogen fuel in the hot core and is on its way to becoming a red giant star.
Kepler is a NASA space telescope that was launched in March 2009. Its main aim is to search for Earth-sized planets around distant stars. Kepler is also providing data to measure oscillations in over 100,000 stars, and we are involved in the target selection and the analysis of the data.
Our group has a particular interest in K giant stars. These stars have evolved from normal solar-like stars and grown to enormous sizes. Some of them are comparable in size to our solar-system. These dramatic evolutionary changes are difficult to model, and constraints from oscillation frequencies are of high significance for our understanding of this late stage in the life of stars. It is important to remember that it is the ultimate destiny of our own sun. Furthermore, giant stars are the most numerous in the sky.
Our group also study stars that are warmer and more massive than the sun; examples are delta Scuti, gamma Doradus and beta Cepheid stars. These stars show relatively high amplitude oscillations, but the comparison of observed frequencies with models is difficult. As opposed to solar-like stars only certain oscillation modes are excited in more massive stars. Our use of high-precision photometric measurements from space, combined with ground-based observations, is the next step to get a better understanding of these stars.
Asteroseismology group at the University of Sydney. From left to right: Daniel Huber, Tim Bedding, Dennis Stello, Tim White and Rasmus Handberg
For more information contact Tim Bedding