Stellar Oscillations
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
Current research
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
More Information
For more information contact Tim Bedding