The density of nuclear matter within the nucleus of differing elements is exceedingly high but constant. However, the density of nuclear matter in a neutron star is 3 times that of the nucleus (4x1011 gm/cm3). A neutron star or pulsar has a diameter of 10km, an enormous density 1015 times that of water, spins at up to 1000 times a second (any faster and it would fly apart), has an intense magnetic field (>1012 gauss), and accelerates charged particles to immense energies in two narrow beams that are swept around lighthouse fashion by the spin (the rotation axis does not correspond with the magnetic axis). Rapidly spinning pulsars with huge magnetic fields are also known as magnetars, and are thought to only form from stars that were originally greater than 25 solar masses. The magnetic field from magnetars is strong enough to wipe the magnetic stripe on a credit card from 160,000 kilometres. It has just been observed that a magnetar only 30,000 to 50,000 light years away and in our own galaxy is responsible for at least one known source of gamma ray bursts. It is thought that, like the sun, the gamma ray bursts are released when the highly-spin-twisted magnetic fields suddenly re-arrange themselves which, like a released rubber band, accelerates particles to extremely high energies similar to a sling-shot. The sun releases coronal mass ejections (CMEs) (giant solar flares that, unlike ordinary solar flares that loop back towards the sun, are shot out from the sun) in a similar way. The magnetic fields from these CMEs have been known to induce such high voltages in long aerial power cables carried on pylons on Earth that Black-outs are the result.

Neutron stars are formed from the gravitational collapse of the inner core of a large star following a supernova explosion. The inner core must be between 1.4 and 3 solar masses for a neutron star to form after the explosion, any greater and a black hole will form instead. Thus neutron stars weigh between 1.4 and 3 solar masses, but the majority weigh between 1.31 and 1.39 solar masses. Newly created black holes are between 3 and 5 solar masses. Those larger that 5 solar masses have grown from smaller ones by accretion. The extremely strong gravitational force squeezes the electrons of the atoms inside the protons in the nucleus, turning them into neutrons to join the other neutrons already there. This is similar to inverse beta decay, and anti-neutrinos are emitted, 1057 of them within 10 seconds. This blasts the outer layers of the star apart. See neutrinos

The 1057 neutrons in a neutron star are bound together mainly by gravitational attraction unlike those in a nucleus which are bound by the strong nuclear force and which is insufficient to bind even 2 neutrons together. (The attractive force between two different nucleons is only weak; the binding energy of the proton to the neutron in deuterium nuclei is only 2MeV per nucleon, it is thus only weakly bound and is much larger in size than an alpha particle, the most strongly bound nuclear cluster. But when protons and neutrons gather in greater numbers than three, the binding energy per nucleon reaches a saturation value of about 8MeV). However, in the depths of a neutron star, the long-range interaction between neutrons is strong enough to bind them in pairs, forming a superfluid boson state.

Neutron stars have a high peculiar velocity, enough to escape the galaxy in which they were born. When a neutron star is born, it emits huge numbers of neutrinos in all directions, but if just 1% more were emitted in one favoured direction, this would jet-propel it like a rocket. The asymmetry may arise when, under the intense magnetic field, some electron neutrinos travelling with the field in one direction were preferentially changed into - and-neutrinos by the magnetic field. Because, under the extreme density, the - and-neutrinos can escape easier than electron neutrinos, this would propel the neutron star.

The characteristic measure of the size of relativistic effects in a celestial body is GM/Rc2, where G is the Universal constant of gravitation, c the speed of light, M the mass of the body, and R the radius of the body. For the sun this factor is 10-5; for a white dwarf is 10-3; but for a neutron star is as high as 0.3, so a neutron star experiences relativistic effects placing a maximum size and rotation rate on a neutron star. For a black hole it is 1.