As a large star between 10 and 29 solar masses comes to the end of its life and uses up its fuel, it will collapse down to the size of a small sphere about 12 miles in diameter and form what is called a neutron star. The collapse of the star sets off a colossal explosion known as a supernova and gravity compresses the left over core of the star into a large mass of neutrons.
At first the neutron star is very hot and can spin rapidly. The fastest spinning neutron star observed so far is over 700 rotations per second. Over time the stars spin slows down. Also as the neutron star ages it gradually cools because it no longer has fuel to burn. The cooling dims the star and it becomes darker over time until it is no longer visible.
As you may have guessed this type of star is remarkably dense and heavy. A sugar cube volume of the neutron star would weigh an astounding one billion tons or about the mass of a mountain.
One type of neutron star is called a pulsar. These stars have extremely strong magnetic fields. These magnetic fields send out particles along each pole of the star. As the star spins, these particles like a sky beam search light becomes visible and the pulsar appears to turn on and off.
