Supernova Remnants and PulsarsSupernova remnants are the clouds of gas that have been expelled in the violent nuclear explosion
of a star known as a supernova. These clouds, which often appear as hollow spherical shells, contain large quantities of
electrons of
relativistic energy, that is, electrons moving at speeds approaching the speed of light. Radiating through the synchrotron process, the electrons were evidently created in the supernova explosion; in some cases, however, this explosion has left behind a spinning neutron star, which is both a pulsar and a continuous producer of new relativistic electrons and cosmic rays.The pulsars are radio sources in the Milky Way that are very unusual because, rather than producing a continuous radio intensity at the Earth, they produce short, regularly spaced bursts of radio emission. These bursts typically last about 1/20th of a second and occur at intervals of about one second. The time between pulses is extremely regular, except that a slow increase in the period between pulses is detected if a pulsar is studied for many months. Astronomers have deduced from such data that the objects are spinning neutron stars, the products of supernova explosions. They are made of bulk nuclear matter, primarily neutrons, and their density is about 10xz times the density of water. They have very strong magnetic fields, about 10x6 times stronger than the Earth's field. This field, spinning with the neutron star, acts as a super powerful electric generator capable of accelerating electrically charged particles to relativistic energies that exceed the highest energies produced in terrestrial nuclear accelerators. The particles accelerated in this way radiate synchrotron radiation in well-defined beams that spin with the pulsar. As these beams sweep across the Earth, the pulse of the pulsar radiation can be observed.The most important example of a pulsar is in the Crab nebula, the remains of a star that exploded on July 4, 1054, and was so bright that it was visible for three weeks in the daytime and for almost two years at night. This remnant is about 6,000 light-years from the Earth, and is still expanding outward at a speed of about 1,000 km/sec (6 mi/sec). At most frequencies it is the third brightest cosmic radio source and is a shell filled with relativistic electrons that radiate synchrotron radiation at X-ray and optical frequencies as well as radio. The total power radiated is equal to about 100,000 times the total power radiated by the Sun. The source of this power is particles accelerated and expelled from the most rapidly spinning pulsar discovered so far, a pulsar that is in the center of the object and that spins about 30 times per second.