Pulsation & Rotation
As massive stars evolve across the main sequence, they inevitably must pass through one or more instability strips. These are regions of the Hertzsprung-Russell diagram where global oscillations are excited in stars' interiors, by a naturally occurring heat engine that converts thermal energy into mechanical energy. The oscillations — which are akin to terrestrial earthquakes — perturb the surface geometry and brightness distribution of a star, leading to distinctive periodic variations in the star's observable properties (brightness, line profiles).
By comparing the measured frequencies of these variations against theoretical predictions, it becomes possible to leverage the oscillations to probe the interior of a star, much as an terrestrial geologist studies earthquakes to determine the structure of the Earth deep beneath our feet. This technique of asteroseismology has already proven remarkably successful in revealing the internal structure of the Sun. With the advent of space-based seismic facilities such as MOSTand CoRoT, and most recently with the launch of Kepler, we are arguably now enjoying the golden age of asteroseismology.