Paraphrase the basic description of how the sun resonates.


Apart from its role in creating the interplanetary medium, the Sun is itself a humming ball of sound waves. The Sun is a star - not a solid body, but a large sphere of turbulent hot plasma. Nuclear fusion reactions deep in its core generate phenomenal amounts of energy. Material lying above the core is heated, and then rises steadily up through the cooler outer layers of the Sun as large streams of hot plasma. As these rivers reach the Sun’s surface they cool, sinking back down to form a convection cell, in the same way that water boiling in a saucepan moves energy from where the heat is applied at the bottom of the pot to the cooler surface. In the Sun, however, these cells are thousands of kilometres across.

The continual turbulent convection motions inside the star set up vibrations and thus sound waves. When these strike the surface of the Sun, they make it oscillate in and out, in the same way that striking a bell or gong will cause it to vibrate and ring. Here, however, the vibrations are not caused by a single strike, but continuously occurring small strikes, and the difference to a bell or gong is that these sound waves are generated internally rather than externally. Reflected by the surface of the sun, most of them remain trapped inside the star. Under the influence of these continual small strikes, the surface of the Sun moves to and fro in a periodic fashion. The velocities of this motion are small, only about 15-20 cm/s, but can still be tracked by the Doppler effect, whereby a slight change in the spectrum of the light can reveal how fast the emitter is moving towards or away from you. Astronomers use precise measurements of these surface motions as a powerful diagnostic of the internal structure of the Sun. The sound waves start close to the surface and propagate downwards into the Solar interior; but deeper in the Sun, the physical conditions change. Most obviously, the temperature increases, which in turn increases the sound speed. Thus waves will move faster the deeper they penetrate, and the resulting refraction of sound waves slowly bends their path around so that they turn back to head back out to the surface before reaching the centre of the Sun. At the surface the sound wave is reflected as if by a mirror to be redirected back down. Hence small variations in the original direction of propagation direct different sound waves to penetrate to different depths in the Sun.

There are many possible curved paths that the sound waves can follow through the Solar interior. The longest wavelength/period tones sample the region deep down towards the core, while higher frequency tones rattle around the layers closer to the surface. The resonant frequencies detected are thus determined by the way that physical conditions change in the interior of the Sun. For example, if you have two oscillations of slightly different frequencies, the lower frequency penetrates a little deeper into the Sun on its journey than the higher frequency waves. The difference in frequency between the two oscillations therefore says something about that region of the Sun that only the lower frequency wave passed through. Scientists examine the vibration of the sun’s entire surface at once, sorting out the individual patterns and amplitudes of the all the resonant vibrations. This is not a trivial problem, as there are over 100,000 oscillations all happening at the same time! But by determining which frequency of sound wave went through which part of the Sun, it is possible to establish what the sound speed is in every part of the Sun that the sound waves have passed through, and so assemble a detailed picture of the physical conditions in the solar interior. By monitoring the Sun's vibrating surface, ‘helioseismologists’ can probe the stellar interior in much the same way that geologists use seismic waves generated by earthquakes to probe the inside of our planet. It is considered the best method for verifying theories of stellar structure and evolution of the Sun.

Although these vibrations of the Sun are genuine sounds, their frequencies are much too low for the human ear, and they can be made audible only by speeding them up some 42,000 times; 40 day’s worth of vibrations (monitored by the SOHO satellite) need to be compressed into a few seconds.