For a sufficiently massive star, an iron core is formed. If the gravitational collapse has enough energy to heat it up to a high enough temperature and fuse or fission Iron. the aftermath of a supernova or in just a collapsing massive star, the energy gets high enough to break down the iron into alpha particles and other smaller units, and still the pressure continues to build. When it reaches the starting point energy necessary to force the combining of electrons and protons to form neutrons, the declining stage has passed and the continue of collapse goes on until it is stopped by neutron degeneracy. At this point it stars with less than two or three solar masses will come to a finish. Resulting in the collection of neutrons forming what is called a neutron star. The periodic emitters called pulsars are also known to be neutron stars.

If the mass exceeds about three solar masses, then even neutron degeneracy will not stop the collapse, and the core shrinks toward the black hole condition. This neutron degeneracy radius is about 20 km for a solar mass, compared to about earth size for a solar mass white dwarf. The density is quoted as about a billion tons per teaspoonful compared to 5 tons per teaspoonful for the white dwarf. A neutron star can have an atmosphere a few centimeters thick and mountain ranges poking up a few centimeters through the atmosphere .It is thought to be about 1/100,000 the diameter of the Sun, and a nucleus is on the order of 100,000 times smaller than an atom.

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Neutron Star