Geometry of our model stellar system. The hypothetical central star is a radially pulsating O or B star (see text). As the star expands and contracts, the surface temperature changes the emitted blackbody spectrum, and thus varies the availability of UV photons of different wavelengths. Hot, X-ray emitting gas with temperature in the range of 10⁶ K and containing He-like ions is located on-order 1 r _⋆  above the surface.