Pulsar, artwork. This stellar object is a rapidly- rotating neutron star. Neutron stars have powerful magnetic fields (shown by the purple lines, though they would not be visible) and rotate very fast (in less than a second). This causes acceleration of charged particles by the magnetic field and the release of radiation at the magnetic poles (beams, upper left and lower right). When the axis of rotation does not coincide with the magnetic axis, then the rotation sends the two beams sweeping across the sky. Correctly aligned beams are seen as pulses on Earth. Neutron stars are remnant stellar cores, having a mass greater than the Sun in a volume less than 20 kilometres wide.