Variation in ratio R / ⟨R⟩ for each of the six helium-like ions, C v (red); N vi (yellow); O vii (green); Ne ix (teal); Mg xi (blue); and Si xiii (purple), for our model for a star with average effective temperatures 10 000, 20 000, 30 000 and 40 000 K (top to bottom). Note that the absolute scale of variation changes dramatically – see the values of R₀ and ⟨R⟩ for each ion in Table 3. This figure shows the differences in the width and height of the peaks and troughs in the periodic variation of the ratio.