Using telescopes, scientists can observe the light and radiation emission from the surface of stars. But how do we peek inside a star? The answer is seismic waves.
On a daily basis, seismic waves are generated by movements within the earth's crust—earthquakes, violent or mild. They travel either along or near the earth's surface (Rayleigh and Love waves) or through the earth's interior (P and S waves).
In helioseismology, scientists study the solar interior by observing waves on the Sun's surface. Likewise, asteroseismology is the science that studies the internal structure of other stars by the interpretation of their frequency spectra.
Different oscillation modes or "harmonies" can reach different depths of the stell interior. These oscillations can inform astronomers about the internal structure of a star, which cannot otherwise be observed.
Source: PBS Space Time via Youtube