All objects in the Universe are made of atoms and atoms consist of atomic particles in rapid motion. The forces holding an atom in shape are very powerful, but even they can be broken. When a star runs out of nuclear fuel and collapses, the pressure at the center can be so enormous that the atoms themselves are compressed into a tiny ball of solid neutrons, which are protons and electrons force together. This tiny ball is millions of times smaller the atoms in the original star.
Aggressive explosions called supernovas, triggered by stars that run out of fuel, create Neutron stars.
In order to collapse into neutron star, a star must have a mass about 1.4 times the size of the Sun’s mass, which is called, the mass limit. Such a star would be over one hundred times the larger than the Earth, but it would shrink into a neutron star 12 miles across.
Walter Baade and Fritz Zwicky, to explain the triggering of a supernova, first hypothesized neutron stars right after the discovery of neutron, in 1933.
If the Earth were compressed into a solid neutron, it would a ball the diameter of a large sports field.