Neutron Star

Dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of a star with mass 8-18 (?) Msun. A neutron star has mass ~2-3 Msun, density ~1014 g/cm3, and is supported by neutron degeneracy pressure. Typical neutron stars are 10-20 km across and have escape velocities ~70% speed of light. Neutron stars rotate extremely rapidly as a consequence of the conservation of angular momentum, and have incredibly strong magnetic fields due to conservation of magnetic flux. The relatively slowing rotating core of the massive precursor star increases its rotation rate enormously as it collapses to form the much smaller neutron star. At the same time, the magnetic field lines of the massive star are pulled closer together as the core collapses. This intensifies the magnetic field of the star to around 1012 times that of the Earth. The result is that neutron stars can rotate up to at least 60 times per second when born. If they are part of a binary system, they can increase this rotation rate through the accretion of material, to over 600 times per second!

Neutron stars that have lost energy through radiative processes have been observed to rotate as slowly as once every 8 seconds while still maintaining radio pulses, and neutron stars that have been braked by winds in X-ray systems can have rotation rates as slow as once every 20 minutes. Observations also reveal that the rotation rate of isolated neutron stars slowly changes over time, generally decreasing as the star ages and rotational energy is lost to the surroundings through the magnetic field (though occasionally glitches are seen). An example is the Crab pulsar, which is slowing its spin at a rate of 38 nanoseconds per day, releasing enough energy to power the Crab nebula.

The interior of a neutron star contains exotic matter such as superfluid neutrons, superconducting protons, and stranger subatomic particles (e.g., pion condensates, λ hyperons, Δ isobars, and quark-gluon plasmas). Rapidly rotating neutron stars are called pulsars.

Some or all content above used with permission from J. H. Wittke.