The Crab Nebula is one of the most spectacular supernova remnants. The fate of the stellar core left behind by a Type II supernova depends on the mass of the original star. Normal atoms are made up of positively charged particles called protons, particles with no electric charge called neutrons, and much smaller, negatively charged particles called electrons. If the original star had a mass about ten times that of the Sun, the core collapses with such force that its protons and electrons combine to form neutrons. The resulting body is composed entirely of neutrons, so astronomers call it a neutron star .
Most neutron stars created by supernovas are pulsars .
Pulsars are neutron stars that spin rapidly as they emit powerful beacons of radio waves. From Earth, these spinning beacons appear as pulses of radiation. If the mass of the original star is greater than about ten solar masses, the nuclear forces that hold up a neutron star are too weak to resist the core's gravitational pull. The core continues to collapse past the neutron star stage. It crushes itself until its mass is concentrated into a volume of space smaller than a typical city on Earth.