How does asteroseismology determine internal changes related to stellar age?

By analyzing the frequencies of internal oscillations (sound waves) in brightness changes.

Asteroseismology is a powerful technique that relies on studying stellar pulsations, analogous to seismologists studying the Earth. Astronomers observe subtle, periodic changes in a star's observed brightness, which are caused by internal sound waves, or oscillations, propagating throughout the star's interior. These internal sound waves are sensitive to the physical conditions within the star—specifically the internal density profile, temperature gradient, and the ongoing chemical changes as hydrogen converts to helium in the core. As the star ages and its core composition shifts, the way these internal sound waves travel and resonate changes predictably. By precisely measuring the frequencies of these detected vibrations, scientists can effectively map these internal structural changes, providing a highly accurate measure of the age, often with less than ten percent error, especially for stars similar to the Sun.