Observed and modeled center-to-limb behavior for line depths and widths. Predictions from CO ⁵BOLD models with parameters bracketing those of HD 209458 are shown. Left panels show theoretical center-to-limb changes for differently strong lines at full spectral resolution; the right-hand panels compare observations (bold curves) to synthetic profiles degraded with the spectrometer instrumental profile. Line widths and depths were obtained by fitting modified Gaussian functions to both observed and modeled line profiles.