  Extreme precision radial-velocity spectrometers enable extreme precision
stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars
are limited by the physical variability in stellar spectra, such as the
short-term jittering of apparent radial velocities. To understand the physical
origins of such jittering, the solar spectrum is assembled, as far as possible,
from basic principles. Surface convection is modeled with time-dependent 3D
hydrodynamics, followed by the computation of hyper-high resolution spectra
during numerous instances of the simulation sequences. The behavior of
different classes of photospheric absorption lines is monitored to identify
commonalities or differences between different classes of lines: weak or
strong, neutral or ionized, high- or low-excitation, atomic or molecular. For
Fe I and Fe II lines, the radial-velocity jittering over the small simulation
area typically amounts to +-150 m/s, scaling to about 2 m/s for the full solar
disk. Most photospheric lines vary in phase but with different amplitudes among
different classes of lines. Radial-velocity excursions are greater for stronger
and for ionized lines, decreasing at longer wavelengths. The differences
between various line-groups are about one order of magnitude less than the full
jittering amplitudes. By matching very precisely measured radial velocities to
the characteristic jittering patterns between different line-groups should
enable to identify and to remove a significant component of the stellar noise
originating in granulation. To verify the modeling toward such a filter,
predictions of solar center-to-limb dependences of jittering amplitudes are
presented for different classes of lines, testable with spatially resolving
solar telescopes connected to existing radial-velocity instruments.
