  We show that microscopic calculations based on chiral effective field theory
interactions constrain the properties of neutron-rich matter below nuclear
densities to a much higher degree than is reflected in commonly used equations
of state. Combined with observed neutron star masses, our results lead to a
radius R = 9.7 - 13.9 km for a 1.4 M_{solar} star, where the theoretical range
is due, in about equal amounts, to uncertainties in many-body forces and to the
extrapolation to high densities.
