The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. The surface of the Sun vibrates much like a bell. A piano has 88 keys or musical tones, whereas the Sun has millions of notes. These vibrations are the result of internal pressure waves that reflect off the photosphere and repeatedly cross the solar interior. They are detected through the Doppler shift of absorption lines formed in the photosphere. Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun's surface. The mean period of the vibrations is about five minutes, which corresponds to a frequency of about 3 mHz.