Step by step, heavy element fusion continues in the core of the supergiant as the by-products of one reaction become the fuel for the next reaction. Eventually, iron accumulates at the core of the supergiant. As the core of this supergiant contracts, density reaches a billion g/cm3. When the iron core exceeds 1.4 solar masses, electrons are forced onto protons by gravity to form neutrons and, because there are no longer electrons to prevent the star from collapsing, the core collapses violently. 
 
Even as this core collapses, the outer part of the star is also collapsing inward. As the collapsing materials hit the neutron core, they rebound and move outward as a shock wave. These outer layers, compressed by gravity and the shock wave, fuse and release enormous amounts of energy explosively. It is the violent fusion of these layers that generates enough energy to create iron and heavier elements. In turn, the energy from the shock wave and from these expanding elements causes the outer layers of this supergiant to fuse at virtually one time, and they blow outward at nearly the speed of light. Much of the energy liberated takes the form of EM radiation expanding at the speed of light, and for a few weeks this star may outshine an entire galaxy. 

This cataclysmic death of a supergiant is called a supernova. Their expanding materials mix with interstellar gases and compress them, and in many cases begin the formation of new stars enriched in heavier elements. 

If the collapsing core's mass lies between 1.4 and 3 solar masses, the core is reduced to a ball of neutrons some 10-20 miles across. This is a neutron star. As neutron stars rotate extremely rapidly, strong magnetic fields direct the EM waves they emit into beams of radio and light waves, very much like the rotating beams of light from lighthouses. These pulsating neutron stars that we detect from earth have been called pulsars. 

If the collapsing core's mass is greater than 3 solar masses, the collapse will not stop as a ball of neutrons. In fact, nothing can stop gravity from crushing this star remnant to a point sized or ring object, a singularity with a gravitational pull so great that nothing, not even light can leave it at a certain distance from the singularity. This is a black hole. How can we detect such singularities and the black holes that surround them if they do not give off any EM radiation? While the black hole itself will not give off any signals directly, any matter that is being captured by the gravitational field of this singularity will give of X-rays as it swirls around the black hole in an accretion disk before vanishing into the event horizon, the distance from the singularity within which nothing will/can escape. This is especially true if the black hole has a companion star that can supply large amounts of matter which it can transfer into the black hole.