- Star emitting radio, optical or other radiation in regular pulses, usually many per second. Pulsars (the best-known of which is the one hidden in the Crab Nebula) are rapidly rotating neutron stars formed in supernova explosions, and produce energy in pulses because energy is emitted from them in a tight beam, often compared to the beam from a lighthouse, along the axis of rotation of the neutron star left over from the explosion. A pulse is observed each time the beam sweeps over the Earth. The supernova remnants seem in some cases to be slowing in their rotation, so that the extraordinarily unvarying interval between pulses becomes slightly longer over a period of years.
- A neutron star which emits sharp and brief pulses of energy, as opposed to the usual steady stream of radiation. Pulsar pulse rates have been observed ranging from around one millisecond, to approximately 5 seconds.