If further fusion takes place the star becomes a red giant. Large red giants fuse until the formation of iron, but thereafter no fusion can take place without energy addition. So the star contracts, and heats up because of the large KE in the particles. When it can't be compressed further it explodes in a supernova. Such a great explosion may leave behind a very dense star made up of mostly neutrons a neutron star.
Pulsars are neutron stars that radiate energy at regular periods See.
F.1.1 Quasars suggest the existence of black holes since the accelerated matter that black holes draw in could release its energy in the form large amounts of light. X-ray is possibly produced as the accelerated matter due to Black holes is compressed and heated to millions of degrees. Since black holes are really massive the 'stretched rubber sheet' of space-time becomes very deformed. For Earth this radius is 1cm. The event horizon is the boundary at which light can not escape from the black hole.