After a star has expelled its outer materials in the supernova explosion the remaining core compresses so much that it either becomes a neutron star or a black hole.
A neutron star is an object which packs the mass equivalent to the Sun into an area smaller than New York City.
It is so dense that a pinhead of neutron star material weighs as much as double the world's biggest supertanker.
Neutron stars are not made of gas. They are a combination of liquid and solid. The outer shell is made up of solid iron under which is liquid that is made up entirely of neutrons. These form when the star collapsed and the protons and electrons were forced to merge.
The gravitational field and magnetic field in neutron stars are so intense that bursts of energy can be released. Neutron stars that do this are known as pulsars. These are either radio pulsars (because they release radio waves) or X-Ray pulsars(because they release X-Rays). The fastest pulsar sends out a pulse 642 times a second and the slowest sends out a pulse once every 5.1 seconds.