Collapse of stars,type II supernovae and type Ib and type Ic SN caused to have neutron stars.This stars whic are completely/dominantly/ composed of neutrons have masses between 1.35~2.1 solar mass and radius of 12 km which means that neutron stars have high densities of 3.7~5.9 * 10^17 kg/m^3 .At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy/Pauli exclusion principle/ managing to oppose gravity.
The neutron stars are formed by high rotation speed because of conservation of angular momentum.Also because of the high speed of stars they can escape from disk of galaxies and can be found in perpendicular direction of disk galaxies.
The high density of neutron star cause to have a escape velocity of about 33% of light speed as a comparison a teaspoon of neutron star has a mass of 5*10^12 kg.
Neutron stars are believed to form in supernovae such as the one that formed the Crab Nebula (or check out this cool X-ray image of the nebula, from the Chandra X-ray Observatory).
up to today there are 2000 known neutron stars are found in milky way and Magellanic clouds.
it is also worth that they appear in binary type with 5%.
Some good links for neutron stars.
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