A star with mass equivalent to the Sun but whose diameter is only about 10 kilometers in comparison to the diameter of the Sun of 1 392 000 kilometers. Although theory had predicted the existence of neutron stars, this concept was not taken seriously by most astronomers until the 1960s discovery of pulsars. This discovery was important to quantum mechanics because it confirmed the accuracy of those early predictions, which had been made on the basis of quantum properties of stellar evolution after the nuclear fusion reactions stopped.

The stellar quantum theory asserts that the loss of radiation pressure to counter the force of gravity, a star at the end of its life would shrink into a ball of matter composed mostly of carbon nuclei produced by thermonuclear (hot) fusion embedded in a sea of electrons. On average each carbon nucleus contains 6 protons and 6 neutrons. The star is formed into a white dwarf held up by quantum degeneracy pressure balancing the force of gravity. White dwarfs are earth-size stars with 1 solar mass. But if the initial mass of the star exceeds the Chandrasekhar limit of 1.4 solar masses, and by gravity alone all the remaining protons fuse with the sea of electrons and the star completely transformed into a sea of neutrons by inverse beta decay becoming a neutron star held up again by quantum degeneracy pressure from the neutrons. If the initial mass exceeds three solar masses then its gravity will overcome the quantum degeneracy pressure and the neutron star collapsed into a black hole.