Pulsar and companion star, illustration. In this binary system, material from the companion star (left) is being pulled across to form an accretion disc around the pulsar (right). Pulsars are a form of neutron star, stellar cores compressed to nuclear densities. Pulsars rotate rapidly and cast out narrow beams of energy (shown here) as they rotate. The beams are confined by extremely strong magnetic fields. The rotation periods range from a few seconds to hundredths of a second. Here, the influx of material from the companion star can eventually spin up the pulsar to the millisecond range, rotating hundreds of times per second. Gravitational waves are thought to limit the rotation speed of these millisecond pulsars, preventing them from disintegrating.