A Neutron Star forms when a medium-sized star (about 4-8 times the size of our Sun) comes to the end of its life (runs out of fuel) and explodes in a Super Nova. It is basically the end product of an intermediate size star, too large to collapse into a White Dwarf, but too small to create a Black Hole.
The Super Nova explosion strips away all protons and electrons from the nucleus of each atom of mass, leaving only neutrons (which have a slightly higher mass than protons). The end result is plasma composed of neutrons only. This plasma is so dense, that a tea spoon of mass from a Neutron Star would weigh 1 Billion tons! In addition, because of its small size and enormous density their gravitional pull is 2 x 1011 times that of Earth!
More than half the stars systems in the Universe are binary systems (2 stars) and many of these are Neutron Stars. In many cases, these binary systems containing Neutron Stars rotate around each other and draw closer to each other. They finally come close enough to merge and WHAM!... a Black Hole is created.
The creation of a Black Hole from 2 merging Neutron Stars is shown in this animated NASA video.....VERY COOL.
As shown in the Nasa video above, colliding Neutron Stars are detected by very bright but temporary burst of gamma rays. When these occur, another Black Hole is created!
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