> A neutron star is what is left after a massive star goes
> supernova. After the explosion throws off the star's outer layers,
> the remaining material continues to burn until the iron in the star's
> core ignites. Then it collapses under its own gravity, fusing
> protons and electrons to form neutrons.
The latter stages of a giant star involve the fusion of light elements forming elements up to iron, which is the last element that can be produced exothermically. From this point, an onion skin layering of lighter elements is formed that is held up by electron degeneracy pressure within an iron-nickel core. Once this core exceeds 1.44 solar masses, electron degeneracy pressure can't hold it and the whole thing collapses rather quickly to form a degenerate neutron core. At this point, the rest of the infalling star "bounces" off forming a shock wave. The material then stalls, but reabsorbs energy from a massive burst of neutrinos from the exceedingly hot core, and thus forms the subsequent explosion we see.
All the "heavy" elements above iron are formed as a result of the collapse and subsequent energy release (through neutron capture, and other proposed mechanisms) - these are all endothermic reactions and cannot be formed during the life of the star .