Asteroseismology
The study of stellar oscillations, or asteroseismology, is the gold standard for characterizing fundamental stellar properties like radii, masses, densities and ages with exquisite precision and accuracy. For stars similar to the Sun, oscillations are driven by turbulent, near-surface processes (i.e convection), causing acoustic sound waves to propagate deep within the stellar interior. A physical analogy that I like to imagine is that the star is breathing, where the fluctuations from inhaling and exhaling can be observed through brightness (i.e. photometry) and intensity (i.e. spectroscopy) variations. Asteroseismology is the only technique that is directly sensitive to the interiors of stars and thus provides critical constraints for much of what we know about stellar astrophysics.
Alf Men: The coolest star with an asteroseismic detection from TESS