Song of the Sun

4 March 2014

There’s a song of the Sun. It is produced by acoustic waves in the Sun’s interior, and the study of these waves is known as helioseismology. It turns out to be a very useful tool. While light travels slowly through the Sun’s interior, taking thousands of years to travel from the Sun’s core to its surface, the solar interior is relatively transparent to acoustic waves, which means they can travel through the Sun at the speed of sound.

A computer plot of one oscillation mode of the Sun. NOAO
A computer plot of one oscillation mode of the Sun.

Because of this, sound waves in the Sun can be used to study the Sun’s interior, similar to the way ultrasound is used to see inside the human body. The difference is that we can’t send sound waves through the Sun, but instead have to measure the natural sound waves that occur within the Sun. We do this by measuring the oscillations of the Sun’s surface using the Doppler shift of spectral lines.

By analyzing these oscillations, we know that the interior of the Sun (known as the radiation zone because heat transfers radiantly in the core) rotates uniformly, while the outer layers (the convection zone) rotate differentially by latitude. Acoustic analysis also gives us a measure of how the density and pressure of the Sun varies by depth, since the speed of sound within the Sun is affected by density and pressure.

There are even audio recordings of the Sun, where the solar oscillations have been speed up a bit and converted to sounds we can hear. You can hear several of them here.