What specific internal feature provides a direct age signature when using asteroseismology on a hydrogen-fusing star?

Answer

The size of the helium-rich core

By analyzing the frequencies of internal oscillations, scientists can precisely map the interior structure; specifically, the size of the helium-rich core directly indicates how much time has passed since the star began fusing hydrogen in its core.

What specific internal feature provides a direct age signature when using asteroseismology on a hydrogen-fusing star?
ageSpaceastronomysciencestarsstellar evolutionage determination