Stories from Physics for 11-14 14-16
Sound waves can cause the brightness of stars to vary over periods of 5-15 minutes. Astroseismology is the study of such internal vibrations of stars. Just as seismologists can use data about the travel of seismic waves to learn about the structure of the Earth, astroseismologists develop models of stars’ internal structures from the passage of seismic waves.
For example, astroseismology has been used to develop models of the magnetic fields present in red giant stars. Observations of vibrational data made using the Kepler Space Telescope revealed that the cores of red giant stars rotate ten times faster than their surfaces. At the boundary of the dense core of a red giant, seismic waves travelling in the outer layers of the star are transformed into gravity waves. The propagation of gravity waves can be influenced by the presence of strong magnetic fields. A ‘magnetic greenhouse effect’ occurs in the cores of red giant stars in an analogous way to the manner in which the Earth’s atmosphere traps radiation. The strong magnetic field of the core of a red giant causes gravity waves to become trapped in the star’s core. Observations of the oscillation modes of red giant cores have led to astronomers inferring the presence of magnetic fields of up to 1000 T.