Stellar Surfaces as Interface Physics: Boundary Conditions, Mode Coupling, and Effective Vacuum Response

This short perspective paper argues that stellar surfaces should be understood as dynamical interfaces rather than passive boundaries. Across helioseismology, neutron-star seismology, magnetohydrodynamics, and quantum field theory in curved spacetime, stellar and compact-object boundaries support modes, transmit stress, dissipate energy, and couple to exterior regions usually described as vacuum. The paper formulates this surface–vacuum coupling in terms of effective interface parameters, including impedance, leakage rate, and relaxation time. This framework does not require a specific ontology for the vacuum, but clarifies that successful modelling already assigns operational response properties to exterior boundary conditions. The analysis motivates observational tests in helioseismology, magnetar quasi-periodic oscillations, and compact-object ringdown.