Compressed Beyond Belief

When a massive star's core collapses in a supernova, protons and electrons are crushed together to form neutrons. The result is a neutron star — roughly 20 kilometers across but containing 1.4 to 2 solar masses. A sugar cube of neutron star material would weigh about 1 billion tons — a mass so large it is better compared with mountains than with ordinary materials.

Neutron stars spin incredibly fast, some rotating hundreds of times per second. This rapid spin is a consequence of conservation of angular momentum — the original star's slow rotation is amplified enormously as the core shrinks.