A new kind of singing star

http://plus.maths.org/latestnews/sep-dec08/stellarchoir/EtoileAnimSon_E-Michel.mov

There are three new singing stars on the block, but you're unlikely to hear them on Radio 1. The musical talents of HD49933, HD181420 and HD181906, three nearby stars which are hotter and larger than our Sun, were discovered by a group of scientists, led by Eric Michel, using data from the CoRoT space-based telescope. Michel and his colleagues accurately measured accoustic vibrations in these stars that not only make for eerie listening, but also could reveal important information about how all stars evolve



Scientists have been listening to our own Sun since the 1970s when research into helioseismology began. Just as seismology (the study of how waves caused by earthquakes move though the planet) has revealed important information about the structure of the Earth, helioseismology uses the vibrations of the Sun to understand the solar interior. Stars are giant balls of gas held together in a delicate balance of gravity pulling the gas in, and the nuclear energy created in the star's centre pushing the gas out. This energy is transmitted through the star by radiation in the inner layer, and convection (like heating a pot of water on the stove) in the outer layer. Some of the energy leaves the sun in the form of light, but some is reflected from the surface back into the stellar interior. These reflected energy waves cause the convective layer to vibrate, and helioseismologists observing the oscillations of the surface of the star can understand how the waves are moving through the star's interior.
The first few vibrations modes of a vibrating string

The first few vibrations modes of a vibrating string

When most of us think of sound waves we might visualise the waveforms created by plucking a guitar string. A standing wave is created when the initial wave is reflected off the fixed ends of the string: some points on the string will appear unmoved by the standing wave (called the nodes), while the rest of the string vibrates between these points in the familar shape of a sine-function. Standing waves can occur in different modes corresponding to the resonant frequencies of the vibrating object.

Standing waves also occur in two dimensions, when vibrations move over a surface. The movie below uses grains of rice resting on a sheet of paper on a speaker to show how the modes of vibration change as the freqency of the sound changes — the rice is pushed away from the oscillating regions towards the nodes. Just as the sine function describes standing waves in one dimension, Bessel functions describe the modes of vibration in two dimensions.