A neutron star is the collapsed core of a large star that has undergone a supernova explosion. Neutron stars are incredibly dense, with a typical diameter of just 10 kilometers and containing around 2×10^30 kg of matter—about as much mass as the Sun but compressed into an object no larger than Earth. Because they are so compact, their gravitational field is extremely strong—around 200,000 times stronger than Earth’s.
Neutron stars rotate rapidly, often completing a rotation every few seconds. This rapid rotation combined with their intense gravitational fields generates powerful magnetic fields around the star. These magnetic fields channel particles from the star’s surface out along its poles at high speeds, creating beams of high-energy radiation that sweep across the Universe like lighthouse beams. When these beams intersect our line of sight, we observe them as pulsars.
The interior of a neutron star consists mostly of neutrons packed tightly together by the immense gravity of the star. However, in addition to neutrons, there is also a small fraction (around 1%) of protons and electrons present. The pressure inside a neutron star is so extreme that even this tiny fraction of protons and electrons is forced to combine to form new particles known as hyperons.
The extremely high temperature and pressure inside a neutron star also cause strange effects such as “neutron superfluidity” and “neutron degeneracy pressure”. Neutron degeneracy pressure is what prevents further collapse once a critical point has been reached—beyond which even the repulsive force between neutrons cannot overcome gravity anymore. As such, neutron stars can be considered “dead” remnants of massive stars whose cores have undergone gravitational collapse.