Neutron stars contain approximately 1.5 times the mass of the sun in a sphere measuring only 10 miles or so in diameter. Scientists predicted the existence of neutron stars in the 1930s. After a star exploded in a supernova, scientists speculated, an ultracompressed core of matter made up of all neutrons would be left over. But scientists figured that by the time the supernova cloud of debris had cleared, the neutron star itself would be too small and too cold to detect from Earth. Then pulsars were discovered in 1967.
Pulsars send radio flashes sweeping across the sky like lighthouse beacons.
Scientists realized that a beacon rotating as rapidly and as steadily as a pulsar had to come from a neutron star. Pulsars steadily flash their radio beacons for years, but occasionally they undergo a sudden increase in rotation speed that astronomers refer to as glitches. One of the best-known pulsars, in the Crab Nebula about 6,000 light years from Earth, has experienced six glitches since its discovery in 1968. We know about more than 700 radio pulsars. And of these 20, at least three behave like the Crab pulsar, where after a glitch you see them spin down faster than they were spinning down before.