  In dense stars such as collapsing cores of supernovae and neutron stars,
nuclear "pasta" such as rod-like and slab-like nuclei are speculated to exist.
However, whether or not they are actually formed in supernova cores is still
unclear. Here we solve this problem by demonstrating that a lattice of rod-like
nuclei is formed from a bcc lattice by compression. We also find that the
formation process is triggered by an attractive force between nearest neighbor
nuclei, which starts to act when their density profile overlaps, rather than
the fission instability. We also discuss the connection between pasta phases in
neutron star crusts and ultracold Fermi gases.
