  Bisectors of the HARPS cross-correlation function (CCF) can discern between
planetary radial-velocity (RV) signals and spurious RV signals from stellar
magnetic activity variations. However, little is known about the effects of the
stellar atmosphere on CCF bisectors or how these effects vary with spectral
type and luminosity class. Here we investigate the variations in the shapes of
HARPS CCF bisectors across the HR diagram in order to relate these to the basic
stellar parameters, surface gravity and temperature. We use archive spectra of
67 well studied stars observed with HARPS and extract mean CCF bisectors. We
derive previously defined bisector measures (BIS, v_bot, c_b) and we define and
derive a new measure called the CCF Bisector Span (CBS) from the minimum radius
of curvature on direct fits to the CCF bisector. We show that the bisector
measures correlate differently, and non-linearly with log g and T_eff. The
resulting correlations allow for the estimation of log g and T_eff from the
bisector measures. We compare our results with 3D stellar atmosphere models and
show that we can reproduce the shape of the CCF bisector for the Sun.
