Explanation Eventually, however, a star runs out of fuel in its center. When this happens, there is no longer a generation of heat and energy in the interior, and nothing is present to counteract the self-gravitation of the star. For very massive stars (more than 10 times bigger than our sun) the sudden, catastrophic, gravitational collapse of the star results in the supernova explosion. After a supernova explosion, all that is left of the original star is the core - called a neutron star. Neutron stars are very small by astronomical standards. Our own Sun's radius is 100 times bigger than the radius of the Earth. However, the typical radius of a neutron star is thought to be only about 10 kilometers. At the same time, a neutron star contains up to 1.5 times as much matter as the Sun, making the density of these objects tremendous. A teaspoon of neutron star material weighs about a billion tons. This much matter in such a small space creates an enormous gravitational field, so powerful, in fact, that it can bend light! Neutron stars also have very large magnetic fields. The magnetic field on Earth, which makes compasses point north, is a trillion times weaker than the typical neutron star’s magnetic field. The magnetic field is so strong that it causes most of the light and radiation that the neutron star emits to be concentrated into cones of emission, like beams from a lighthouse. In fact, the key to a pulsar is the combination of the extraordinary magnetic field and the rotation of a neutron star. If the neutron star is spinning, like the Earth rotates on its axis, and if the Earth happens to lie in the path of the beams, we see a pulse of light each time a beam sweeps across the earth. The center of the Crab Nebula contains a pulsar, which rotates an amazing 33 times per second! Other pulsars rotate at different rates, but they are as precise as atomic clocks.