A region, which is labeled the Big Bang, is in the upper left corner of Figs. In the first microsecond after the Big Bang, the entire universe should have been in this state. At that time, the universe would have fit inside the present orbit of the planet Jupiter. These accelerators will produce conditions similar to that found in our early universe. When a massive star undergoes a supernova explosion, a core of iron nuclei remains.
Gravity brings the nuclei together.
The short-range nuclear repulsive force is not strong enough to keep the nuclei apart. As the core collapses, the individual nucleons separate from the nucleus. The protons become neutrons by inverse beta decay. Therefore, the neutron star is very large collection of neutrons, typically a few kilometers in diameter, which is held together by gravity. Some theorists predict that a neutron star of a large enough mass could be of high enough density to produce a quark-gluon plasma.