Pulsar. Computer illustration of a pulsar in its nebulous supernova remnant. Pulsars are rapidly rotating neutron stars which cast out narrow beams of energy as they rotate. Any pulsar whose beam chances to cross Earth will appear to be flashing like a lighthouse, with regular pulses of energy sweeping across Earth. Pulsars rotate extremely fast, with periods ranging from hundredths of seconds to a few seconds. The pulse is visible from radio to X-ray wavelengths. Neutron stars form in supernova explosions when the core of a giant star collapses to the density of an atomic nucleus. The star's outer layers are thrown off to form expanding gas shells around the pulsar.