This is a neutron star. It's core is hypothesized to be made of neutron degenerate matter, also known as neutronium.
Neutronium (n) (also called Element Zero or Neutron Degenerate Matter) is the name for the element with an atomic number of zero. It's average atomic mass is 1, because it is only ever found naturally, outside of neutron stars, as a lone neutron.
Neutronium and The Periodic Table
Neutronium would be the head of the periodic table, as it would be placed in group zero and in period zero. It would be placed in period zero, because it has no electron shells (as it has no electrons). And since it has no electrons or shells , it would have no electrons in its outermost shell.
Isotopes
Neutronium has 2 confirmed isotopes and three unconfirmed isotopes that all have very short half-lives.
Confirmed Isotopes
Mononeutronium, or 1n is simply a neutron. It has a lifetime of 15 minutes, with a half life of 10 minutes. It undergoes beta decay, and turns into a proton, emitting an electron and an electron neutrino.
Dineutronium, or 2n is two unbound neutrons. It was observed during beta decay of beryllium-16. They may not be bound, bur there attraction nearly makes them so.
Unconfirmed Isotopes
Unconfirmed isotopes include trineutronium, tetraneutronium, and pentaneutronium. The existence of trineutronium for even a short period of time is doubted.