When a star becomes a supernova, its brightness increases many millions of times. For a while it may even shine more brightly a whole galaxy. In our Galaxy three supernovae have been seen during the last 1,000 years. Chinese astronomers spotted one in AD 1054, Tycho Brache studied one in 1572, and Johannes Kepler saw one in 1604. Supernovae are so brilliant that we can see them in other galaxies.
What happens to a star after it has exploded as a supernova depends on how massive it is. One with a mass of more than seven times the Sun's mass becomes a neutron star. Stars more massive can become black holes.
A neutron star is formed when the central core of an exploding star collapses under gravity. As it gets smaller and smaller, the protons and electrons in its atoms are crushed together and form neutrons. The star turns into a "sea" of neutrons that is incredibly dense: a teaspoon would weigh 100 million tonnes! Astronomers think that the bodies known as pulsars are neutron stars that rotate rapidly. As they do so, they give out a beam of radio pulses when the beam flashes in our direction. One of the first pulsars to be discovered was the Crab pulsar, in the Crab nebula.
Very heavy stars do not stop collapsing even when they shrink to the neutron-star stage. Their gravity is so great that the collapse continues. The matter which they contain is eventually crushed into a point, know as a singularity. In the region around this point gravity is so intense that nothing, not even light, is able to escape from it. That is why astronomers call such a region a black hole.
