An ordinary star is a gigantic ball of gas, about a million times more massive than the earth—our sun is a medium-sized star. It is potentially stable for a long time, because the energy produced by the core produces an enormous outward pressure, which balances the inward force of gravity on its huge mass.
However, when the nuclear fuel runs out, there is no longer any force to balance its gravity. If the star is very massive, most of it collapses very fast — in about two seconds. This releases a huge amount of energy—one supernova will out-shine all the billions of stars in its galaxy. The collapse is so violent that the electrons and nuclei are crushed together and produce a core of neutrons. This core is so dense that a teaspoonful would weigh 50 thousand million tons on earth. It cannot be compressed any further, so the incoming material from the rest of the star meets a solid wall. This material bounces off the core, rushes outward and shines very brightly. The remaining core, only about 20 km [about 12 miles] in diameter, is called a neutron star. Because it is spinning very fast, and has a strong magnetic field, we observe regular radio pulses, so the object is called a pulsar.
The energy produced by a supernova is mind-boggling: 10E44 joules [or 1 followed by 44 zeros]. It is the same as if each and every gram of the earth’s mass was converted to a nuclear bomb 200 times more powerful than the one dropped on Hiroshima. That amount of energy would fuel 80 million sun-like stars for 100 years!
However, when the nuclear fuel runs out, there is no longer any force to balance its gravity. If the star is very massive, most of it collapses very fast — in about two seconds. This releases a huge amount of energy—one supernova will out-shine all the billions of stars in its galaxy. The collapse is so violent that the electrons and nuclei are crushed together and produce a core of neutrons. This core is so dense that a teaspoonful would weigh 50 thousand million tons on earth. It cannot be compressed any further, so the incoming material from the rest of the star meets a solid wall. This material bounces off the core, rushes outward and shines very brightly. The remaining core, only about 20 km [about 12 miles] in diameter, is called a neutron star. Because it is spinning very fast, and has a strong magnetic field, we observe regular radio pulses, so the object is called a pulsar.