STARS ARE PLAYING A COSMIC CONCERTO THAT CAN REVEAL THE DEEP SECRETS OF THEIR INTERIORS

Reported by Robert Lea

The Sun may appear to be a stable and monolithic body sitting at the heart of the Solar System, but that isn’t the case. Our T star is rocking with ‘starquakes’ that have become an area of intense study for solar physicists. Beneath the Sun’s surface is a roiling and violent sea of plasma that transfers energy from its core, where it’s generated by the nuclear fusion of hydrogen to helium, to the Sun’s surface, the photosphere, where it can be radiated out to rain sunlight down on Earth and the other planets. This process causes our star to shake, a phenomenon generally referred to as a starquake. Though only the surface of the Sun is visible to us, humanity has learned to deduce what lies beneath using these starquakes, just as seismic waves from earthquakes have been used to decode the interior of our planet. Now, this practice is being used to understand the interiors of more distant stars of all shapes, sizes and ages.

Though the name starquakes is reminiscent of earthquakes because stars are essentially superheated balls of gas, the causes of the two phenomena are very different. “I know it’s a good way of catching people’s attention to call them starquakes, and there are some analogies with earthquakes, but we generally talk about starquakes as the oscillations of stars,” University of Toulouse scientist Sébastien Deheuvels tells All About Space. “An earthquake is something that very locally pushes the Earth to oscillate, but what we see for a star is the whole star oscillating, not just a part of it. So perhaps the most common analogy that we make is with musical instruments – when you pluck a guitar string, the whole string of the guitar oscillates.”

On our planet, earthquakes are caused by sudden movements along faults within the Earth that release stored-up elastic energy as seismic waves. These waves propagate through the Earth and cause the ground surface to shake. Starquakes, on the other hand, happen because the Sun gently vibrates due to sound waves trapped in its interior. These sound waves are produced by fluctuations in pressure in the turbulent convective motions of the Sun’s interior, and they rise outward, reflecting off the surface of our star. This trapped sound is excited when hot blobs of superheated gas called plasma rise on convective currents to the surface of the Sun, while plasma that has previously made this journey cools and falls through these so-called convective zones.

The trapped sound waves set the Sun vibrating in millions of single sound pitches, or ‘modes’, and since this is produced by pressure, these modes of vibration are called p-modes. “What we give as an example is usually boiling water, which generates convection, with hot water rising and then falling when it cools,” Deheuvels says. “You have these big movements in the external part of the Sun, equal to about 30 per cent of its radius, which is convective and generates these motions. When these motions get near the Sun’s surface it can excite the oscillation modes. That’s what causes the oscillations in the Sun.”