Upper panel: opacity broadening β_τ (=ΔV/ ΔV_int) as a function of the central opacity of the line τ₀ (i.e., curve of growth): (black) analytic prediction (Phillips et al. 1979); (orange) values derived from the observed ΔV directly measured from the line emission; (blue) values derived ΔV from Gaussian fits. Lower panel: contribution of both opacity (red solid line) and intrinsic, Doppler broadening (thermal + non-thermal motions; black dashed line) to the total observed linewidth for different line opacities.