Example spectra of a hot star (top) and a cool star (bottom). Both spectra are normalised by their median flux, and a small offset is added to improve visibility. The vertical lines indicate several spectral features used for the analysis, labelled on the top. Both spectra have Br γ emission due to surrounding gas (the hot star has even He I emission), but only the cool star has strong Na I, Ca I and CO 2–0 absorption features, and plenty of other metal lines. The regions used to measure the spectral indices are marked by different colours in the cool star spectrum, solid lines for the feature, and dotted lines for the pseudo-continuum regions. We annotate the K_S and S/N (computed using the PPXF fit residual) for each star.