Neutronium is the informal term for an extremely dense phase of matter which occurs only in the intense pressure found in the core of neutron stars. Neutronium is formed when a massive star at least 8 times as massive as the Sun exhausts its fuel and collapses in a type II supernova. The star's core must be more massive than the Chandrasekar limit (1.4 solar masses); if it is less, neutronium will not form and a white dwarf star is produced instead.
The core of the collapsing star is initially composed of iron supported by electron degeneracy pressure, since the fusion of iron doesn't release energy. When the core collapses, the densities and pressures in the core overcome even the electron degeneracy pressure and the iron atoms' electrons are compressed into their nuclei where they combine with protons to form neutrons.
The neutrino is emitted from the core, leaving the neutron behind. The core rapidly becomes almost pure neutronium, with only a crust of degenerate matter and a thin layer of normal matter remaining on the outside.
The density of neutronium is similar to that of the nuclei of atoms, approximately 1014-1015 grams per cubic centimeter. The entire core of the star is squeezed down into a sphere about 10 km in diameter; A teaspoonfull of neutronium has a mass of about 100 million tons. Neutronium is only stable under this extreme gravitational pressure, and is never found in a mass less than approximately that of the Sun; it explosively decays back into protons and electrons if the pressure is reduced.
The mass of any neutronium body can be no more than 2-3 times the mass of the Sun; above that the nuclear degeneracy pressure is no longer enough to prevent further collapse. Neutronium may condense into quark matter (also known as strange matter) at this point, or collapse all the way into a black hole.