  Neutron stars, produced at the death of massive stars, are often regarded as
giant neutron-rich nuclei. This picture is especially relevant for low-mass
(below about solar mass) neutron stars, where non-nucleonic components are not
expected to occur. Due to the saturation property of nucleonic matter, leading
to the celebrated liquid-drop picture of atomic nuclei, empirical nuclear
masses and radii can be approximately expressed as function of atomic mass
number. It is, however, not straightforward to express masses and radii of
neutron stars even in the low-mass range where the structure is determined by a
balance between the pressure of neutron-rich nucleonic matter and the gravity.
Such expressions would be of great use given possible simultaneous mass and
radius measurements. Here we successfully construct theoretical formulas for
the masses and radii of low-mass neutron stars from various models that are
consistent with empirical masses and radii of stable nuclei. In this process,
we discover a new equation-of-state parameter that characterizes the structure
of low-mass neutron stars. This parameter, which plays a key role in connecting
the mass-radius relation of the laboratory nuclei to that of the celestial
objects, could be constrained from future observations of low-mass neutron
stars.
