Neutron star

A neutron star is the collapsed remnant of a Type II or Ib supernova.

It has a structure that begins with an iron layer on the outside, going to atoms with increasingly more neutrons to the inside. The core is believed to be a superdense matter, whose exact nature is still not well understood. Some researchers refer to this theoretical substance as neutronium. It could be a superfluid mixture of neutrons with a few protons and electrons, other high energy particles like pions and kaons may be present, and even sub-atomic quark matter is possible. However so far observations have not indicated nor ruled out such exotic states of matter.

Some neutron stars that can be observed are:

Burster - a neutron star with a low mass binary companion from which matter is accreted resulting in irregular bursts of energy from the surface of the neutron star.
Pulsar - general term for neutron stars that emit directed pulses of radiation towards us at regular intervals due to their strong magnetic fields.
Magnetar - a type of Soft gamma repeater that has a very, very strong magnetic field

Neutron stars rotate extremely rapidly after their creation due to the conservation of angular momentum; like an ice skater pulling in his arms, the slow rotation of the original star's core speeds up as it shrinks. A newborn neutron star can rotate several times a second; sometimes, when they orbit a companion star and are able to accrete matter from it, they can increase this to several thousand times per second, distorting into an oblate spheroid shape despite its own immense gravity. Over time, however, neutron stars slow down due to drag on their magnetic fields; older neutron stars may take several seconds or minutes for each revolution.

When pulsars were first discovered, they were believed by some to be evidence of extra-terrestrial intelligences. Because of their highly regular pattern of emmisions, they were initially thought to be beacons of some type.