The study of the internal structure of stars through the interpretation of their pulsation periods (stellar pulsation). The radial pulsations are the result of sound waves resonating in the stars interior. Different pulsation modes penetrate to different depths inside a star. If a large number of pulsation modes occurs, then the stellar interior, which is not directly observable, can be probed from oscillation studies because the modes penetrate to various depths inside the star. Using a complex mathematical analysis, very detailed investigations of the structure of the star's interior can be carried out. Applied to the Sun, it is called helioseismology.