Music from the Stars

The pulsations in stars, commonly called “starquakes”, are similar to earthquakes and send ripples across the star’s surface. These vibrations are actually low frequency sound waves out of the range of human hearing, but by boosting these frequencies into the audible range, Kurtz has created sounds that have even been used in nightclubs! When sound waves travel through a star, they cause it to get hotter and cooler, brighter and dimmer, bigger and smaller and to change shape. For experimental purposes, instruments usually detect the corresponding changes in brightness of a star.

Inside a star

But how does this all relate to a star’s internal structure? A star is fundamentally a large body of gas. The gas molecules within a star are continually banging into each other and the frequency with which the molecules collide determines the speed of the sound, or pressure waves, produced. A wave resulting from these collisions, which can cause a “starquake”, moves more quickly the more often the gas molecules collide. Therefore, by measuring the speed of sound – or associated changes in brightness - from various places on a star, astronomers can work backwards to obtain information about what was going on inside it.

Mathematical techniques are used to deconstruct the complex interactions between the various waves produced inside a star. This is not a straightforward procedure: a star is a three-dimensional object that can oscillate in three different directions and is affected by forces like gravity, the Coriolis force and pressure. A star vibrates with specific modes, similar to the behaviour of a musical instrument.

By looking at the different frequencies of vibration in a star, astronomers can get information about the density of the regions where the waves originated from and travelled through. The spectrum (or range of frequencies) obtained relates to the star’s chemical composition. As with ultrasound techniques, these sound waves can be interpreted as an image, with various colours corresponding to various components and their location within the internal structure.