Throughout their existence, stars fight a dramatic battle against the forces of gravity. Gravity tries to collapse the star by pulling its outer layers towards its center. But the star fights back by releasing nuclear energy, which is fueled by a rich supply of hydrogen. Eventually, usually after billions of years, stars deplete their fuel supply and must give up the fight. Some aging stars die quietly; others suffer violent deaths. The method depends on a star's mass. Stars about the same size as our Sun become white dwarfs, which shine for a very long time from leftover heat. Stars that have about 10 times the Sun's mass blow apart and often form neutron stars. Scientists believe that the Crab Nebula came from such a star.
Once a large star exhausts its fuel supply, gravity takes over and the star is collapsed without opposition. Usually a star will find other sources of fuel like helium, carbon, oxygen and nitrogen, but these offer a short reprieve. Eventually the densities at the center of the star get so high that the star cannot collapse much more at all. Instead all the pressure from the collapse is "stored," ready for release. Finally the conditions become so extreme at the center of the star that all the "stored" pressure from the years of collapse are released in a single brilliant burst: a nova or a supernova, depending on the mass of the star. This explosion throws off the outer layers of the star and compresses its core even more. It was a supernova that created the Crab Nebula. During the explosion, the star gives off more energy than a galaxy of 100 billion stars. The outer layers being ejected create an expanding shell of dust and gas that become a supernova remnant.
Besides the interstellar debris, supernova explosions often leave behind a cinder, the star's dense, collapsed neutron core, which was created by the compression of electrons and protons. Called a neutron star, the object is about 10 miles wide, has a mass greater than our Sun, and a density of about a billion tons per teaspoonful. Because of its small size and high density, the neutron star possesses a gravitational field 300,000 times stronger than the Earth's. Its rotation also increases dramatically during the collapse. Most celestial objects rotate, but neutron stars rotate very rapidly. The neutron star in the Crab Nebula rotates 30 times per second or 3.4 million miles per hour. The neutron star PSR B1937+21 rotates 642 times per second, or 100 million miles per hour. A neutron star is the only kind of star that can rotate so rapidly without breaking apart.
Some neutron stars -- such as the Crab -- emit radio waves, light, and other forms of radiation that appear to pulse on and off like a lighthouse beacon. Called pulsars, they don't really turn radio waves on and off -- it just appears that way to observers on Earth because the star is spinning. Astronomers pick up the radio waves only when the pulsar's beam sweeps across the Earth.
Pulsars possess a powerful magnetic field that traps and accelerates charged particles, and shoots them through space as radio waves. Their rapid rotation makes them powerful electric generators, capable of accelerating charged particles to energies of millions of volts. The Crab, the youngest and most energetic pulsar, produces enough energy to power the nebula and make it expand. The real difference between a neutron star and a pulsar is that a pulsar has a magnetic field that is misaligned with the rotation axis -- being tilted at an angle of about 30 degrees to the rotation poles.
A pulsar's energy output lights up and expands the nebula around it. This action robs energy from the pulsar's rotation, so that it spins slower over time. This "spin-down" rate is a tiny percentage per year, so that it will take about 10,000 years for the pulsar to slow to half its current rotation speed. As time progresses, the Crab's pulses will become less intense, and its X-ray emissions eventually will end. The nebula itself will disappear after only a few thousands years. Eventually only the radio pulsar, beaming every few seconds, will remain.
First discovered in 1967, scientists jokingly dubbed pulsars LGM for "Little Green Men," because the radio signals were so regular it seemed to be a sign of intelligent life. Scientists can predict the arrival times of pulses a year ahead with an accuracy of better than a millisecond. They have cataloged more than 300 of them. But only two, the Crab and Vela, emit detectable visible pulses. The Crab emits radiation throughout the entire spectrum, including gamma and X-rays.