Neutronium (sometimes shortened to neutrium,[1] also referred to as neutrite[2]) is a hypothetical substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the hypothetical "element of atomic number zero" (with zero protons in its nucleus) that he placed at the head of the periodic table (denoted by dash, no element symbol).[3][4] However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; hereinafter "degenerate neutronium" will refer to this..