Published: Feb. 12, 2019

Stars are filled with sound. Turbulent convection within a star makes a broadband hum, but certain frequencies get amplified because they are resonant. For example, the Sun’s surface throbs and rings under the combined influence of several million resonant acoustic modes. Thus, stars act like really bad-sounding musical instruments. Helioseismology is the field of study where the sound waves observed at the surface of the Sun are used to image the Sun’s interior, where we view directly. In asteroseismology, the same techniques are applied to more distant stars. Both fields are analogous to how sound waves are used to image a baby, view a bum knee, or monitor blood flow in a heart during an ultrasound. In star’s we have been able to use the frequencies of the resonant acoustic oscillations to measure the radial profiles of mass density and temperature throughout a star’s interior. Further, we have been able to measure flow fields in the interior, including the star’s rotation rate, meridional circulation, and subsurface convective motions. In this seminar, I will present an overview of the topic. In particular, I’ll discuss how the sound waves are generated and why they are resonant. Further, I will examine the integral inversion problem that must be performed in order to obtain estimates of the star’s interior properties from measurements of the resonant frequencies. Finally, I will present a brief historical survey of the major discoveries of helioseismology (and asteroseismology) and how those discoveries have changed our understanding of the fluid dynamics that is going on inside stars.