Comparison of different stars’ spectra (black) with the spectra of inactive stars (gray). For the lowest-activity stars (top), no excess flux can be seen, whereas for higher activity, the observed fill-in increases with activity. The determined excess flux in the first Ca ii line in these three cases are 0.1 × 10⁵ erg s^-1 cm^-2, 2.0 × 10⁵ erg s^-1 cm^-2 and 4.4 × 10⁵ erg s^-1 cm^-2.