Background image: The Crab Nebula, located at a distance of about 6,500 light-years from the Earth, can be seen in favourable conditions with binoculars. At its centre is the Crab Pulsar, a neutron star about 30 km across. Emissions from this pulsar, primarily in the X-ray and gamm-ray spectrum, make it the strongest persistent source of radiation in the night sky.
Peer into the heart of the Crab Nebula >
Neutron stars are typically remnants of Type II supernovae (SNII). They weigh about 1.4 solar masses for a size of few kilometres (i.e. imagine the sun packed within a ring of 5 miles diameter around Oxford city).
During the core collapse leading to SNII the gravitational push is so strong that the density of the core increases to sufficiently high values to reduce matter to a plasma of electrons and protons. At higher densities the Fermi energy of the electrons becomes so high (tens of MeV) that they interact with the protons reducing the whole core to a neutron gas. At densities of about 1015gcm-3 a degenerate neutron gas forms which is the regime at which neutron stars form.
The contraction of the core during core collapse causes it to spin very fast, once or as man as several hundred times per second. Pulsars are fast rotating neutron stars with a strong magnetic field. When the axis of the magnetic field is not aligned with the axis of rotation of the neutron star this generates a lighthouse effect. Some neutron stars have a weak magnetic field that is not strong enough to create a pulse. The Crab Nebula hosts a pulsar that pulsates more than 30 times per second.
Pulsars were discovered in 1967 by Jocelyn Bell-Burnell as a graduate student in Jordell Bank