To understand how neutron stars are formed, we first must understand how a star dies.
How a Star Dies
Thare are many final forms of a star, and the one a star picks depends on its mass. The mass of a star is often measured in solar mass (M☉) which is about the mass of the sun (2×10^30 kg).
stars that are less then 7M☉ in mass are classified as medium stars and will swell before eventually fissling out to make a planitary/star-forming nebulae with a white dwarf star formed from the core.
stars of 7 or more M☉ in weight are classifed as massive stars. these stars do swell but unlike medium stars they explode when they die, in an event known as a supernova. from there they usualy choose either a black hole, a neutron star, or a star-forming nebula.
R.N. Bailey, CC BY 4.0,
via Wikimedia Commons
How we get from Here to Neutron Stars
Whan a massive star is nearing the end of it's life, it starts to run out of hydrogen to fuse into heleum. This is a problem, because stars would collapse without the energy provided by the fusion of elements. As the star runs out of hydrogen, pressure forces it to fuse heaverer and heaver elements, heleum to carbon, carbon to neon, and so on untill we hit iron. Iron produces a net loss when fused, and thus the star has no energy to prop itself up. The outer layers race twards the core at 25% the speed of light, rebounding off of the core. The presure is so greate that it forces protons and electrons colse enough to bond int neutrons. a neurton star is born.