Neutron star
A neutron star is a compact star in which the weight of the star is carried by the pressure of free neutrons. It is a so called degenerate star. The neutron is an elementary particle and one of the building blocks of atomic nuclei. Neutrons are electrically neutral (hence the name) and in contrast to protons, can be packed to form extremely large "nuclei", up to several times the mass of the Sun. Neutron stars are the first major astronomical object whose existence was first predicted from theory (1933) and later (1968) found to exist, at first as radio pulsars.
Neutron stars have a mass of the same order as the mass of the Sun. Their size (radius) is of order 10 km, about 70,000 times smaller than the Sun. So a neutron star's mass is packed in a volume 70,000³ or approximately 1014 times smaller than the Sun and the average mass density can be 1014 times higher than the density in the Sun. Such dense matter cannot be produced in the laboratory. Neutron stars are the densest objects known. It is about the density 'inside' an atomic nucleus. Indeed, one could see a neutron star as a giant atomic nucleus, bound by the gravitational force.
Due to its small size and high density, a neutron star possesses a surface gravitational field about 2×1011 times that of Earth. One of the measures for the gravity is the escape velocity, the velocity one would need to give an object, such that it can escape from the gravitational field into infinity. For a neutron star such velocities are typically 100,000 km/s, about 1/3 of the velocity of light. Conversely: an object falling onto the surface of a neutron star would impact the star also at 100,000 km/s. To put this in perspective, if an average human were to encounter a neutron star, they would impact with roughly the energy yield of a 100 megaton nuclear explosion.
Neutron stars are one of the few possible endpoints of stellar evolution, therefore sometimes called a dead star. They are formed in a supernova as the collapsed remnant of a massive star (a Type II or Ib supernova) or as the remnant of a collapsing white dwarf in a Type Ia supernova.
Neutron stars are typically about 20 km in diameter, have greater than 1.4 times the mass of our Sun (the Chandrasekhar limit, below which they'd be white dwarfs instead) and less than about 3 times the mass of our Sun (otherwise they'd be black holes), and spin very rapidly (one revolution can take anything from thirty seconds to a hundredth of a second).
The matter at the surface of a neutron star is composed of ordinary nuclei as well as ionized electrons. The "atmosphere" of the star is roughly one metre thick, below which one encounters a solid "crust". Proceeding inward, one encounters nuclei with ever increasing numbers of neutrons; such nuclei would quickly decay on Earth, but are kept stable by tremendous pressures. Proceeding deeper, one comes to a point called neutron drip where free neutrons leak out of nuclei. In this region we have nuclei, free electrons, and free neutrons. The nuclei become smaller and smaller until the core is reached, by definition the point where they disappear altogether. The exact nature of the superdense matter in the core is still not well understood. Some researchers refer to this theoretical substance as neutronium, though this term can be misleading and is more frequently used in science fiction. 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.