Neutron stars are ancient remnants of stars that have reached the end of their evolutionary journey. They began as stars between four and eight times the mass of the sun before exploding in catastrophic supernovae. After such an explosion blows a star’s outer layers into space, the core remains, no longer producing nuclear fusion. With no outward pressure from fusion to counterbalance gravity’s inward pull, the star condenses and collapses in upon itself.
Despite their small diameters – 20 km, neutron stars are usually about 1.5 times more massive than the sun, and are thus incredibly dense. Think of a sugar cube 2cm in length having a weight of 10TN on Earth. The energy of electrons will increase upon compression, so pressure must be exerted on the electron gas to compress it, producing electron degeneracy pressure. With sufficient compression, electrons are forced into nuclei in the process of electron capture, relieving the pressure. In brief, protons capture electrons forming neutrons—the process that gives such stars their name. The composition of their cores is unknown, but they may consist of a neutron superfluid or some unknown state of matter.
When they are formed, neutron stars rotate in space. As they compress and shrink, this spinning speeds up because of the conservation of angular momentum—the same principle that causes a spinning skater to speed up when she pulls in her arms. These stars gradually slow down but those bodies that are still spinning rapidly may emit radiation that from Earth appears to blink on and off as the star spins, like the beam of light from a turning lighthouse. This “pulsing” appearance gives some neutron stars the name pulsars.
After spinning for several million years pulsars are drained of their energy by gravitational drag from the outer layers and become normal neutron stars. Few of the known existing neutron stars are pulsars. Only about 1,000 pulsars are known to exist, though there may be hundreds of millions of old neutron stars in the galaxy.
The huge pressures that exist at the core of neutron stars may be like those that existed at the time of the Big Bang.