Pulsars
Sometimes when a star goes supernova, all that is left after this most violent of processes is a cloud of expanding gas and the tiny remnant of extremely dense material only a few tens of kilometers in diameter. The supernova implosion is so intense that the protons and electrons in the atoms of the star are jammed together, thus canceling out their electrical charges and forming neutrons. This neutron star may be 1014 times as dense as water! It will have extremely powerful magnetic fields and may rotate very rapidly. Because the magnetic axis may not correspond to the spin axis, a beam of radiation emitted from the magnetic poles may seem to an observer to pulse like a rotating searchlight. Thus we call these rotating neutron stars pulsars. Although some pulsars are seen at visible and x-ray frequencies, many more are seen at radio frequencies.
Since 1967, when the first pulsar was detected by Jocelyn Bell, hundreds of pulsars have been discovered. The Crab pulsar spins at 30 times per second. The pulsar 1937+21 in Cygnus pulses 642 times per second. We receive this emission on Earth as if it were a signal produced by a cosmic clock. Over the brief period we have been observing them, however, they all them seem to be gradually slowing down. Their energy is dissipating with age. After correction for this effect, some millisecond pulsars are at least as accurate at timekeeping as the best atomic clocks. The rate at which pulsars slow down has been helpful in confirming aspects of Einstein’s theory of general relativity. Also, the timing of pulsars can be useful in determining properties of the interstellar medium.