Music of the Sun: Helioseismology

Solar physicists have discovered that our Sun is effectively a huge musical instrument. Dr Bill Chaplin from Birmingham University likened it to a woodwind instrument powered by a build up of pressure in a thin solar region only 100km thick. He leads a team of 15 researchers under the project name BiSON, which stands for Birmingham Solar Oscillations Network. The team runs a network of instrumentation at six sites dotted about the Earth with 4 of the 6 sites operating autonomously. Their speciality is in observing waves that cause the Sun´s surface to rise and fall at a rate of a few centimetres each second over a period of about 5 minutes.

The pressure waves are generated in a thin shell at the top of the Sun´s convective zone where plasma bubbles up like a saucepan of boiling water. These undulations are replicated in the Sun´s surface and are transferred on to the light waves leaving the surface. The result is that photons of light escaping the Sun will have their wavelength `doppler-shifted´ and so the light´s colour will change by a small amount. BiSON´s sensitive instruments pick up these changes so that an overall map of surface patterns can be constructed.

It turns out that the Sun has many different modes of oscillation - over 10 million in fact. Some patterns look like a three-dimensional checkerboard whereas simpler resonances mimic the inflation and deflation of a spherical balloon. Just as seismic data from terrestrial earthquakes has allowed geologists to investigate the Earth´s interior, solar oscillations have allowed scientists to model the interior of the Sun.

Solar physicists have discovered that the convective zone has jet streams which surface at the solar equator before breaking up into smaller spiralling storms at higher solar latitudes. They can even `see´ sunspots hidden on the far side of the Sun and so provide prior warning of potentially dangerous solar eruptions that can affect satellites and astronauts in space.

In a logical progression, the study of sound waves in the Sun has been extended to stars further afield and is known as asteroseismology. A nearby star that sounds similar to our Sun is alpha Centauri but other stars have their own vocal range, so to speak. A small white dwarf such as GD358 has the higher frequencies of a choirboy whereas a large red giant star such as Xi (pronounced `zai´) Hydrae is the bass of the group. However, even the highest pitch of the stars is approximately 15 octaves lower than Middle A!

To discover yet more stellar harmony a French spacecraft named COROT, launched last December, will soon begin recording the changes in luminosity of distant stars. Although its main mission is to discover large Earth-type planets, the brightness variations it returns will allow researchers to calculate a star's precise mass, age and chemical composition which will aid in comparisons between our Sun and other stars.