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A neutron star is a very small and dense star made almost completely of neutrons. It is a very large nucleus held together by gravity.Neutron stars have a radius of about 10 kilometers (6 mi) and a mass from about 1.4 to 5 times the mass of the Sun.They are usually what is left of very big stars that have exploded (these are called supernovas). Some are what happen to a white dwarf (small star) that has a lot of energy.Neutron stars usually turn very fast, taking from 0.001 second up to 30 seconds to turn.

They come in different types. One kind is the pulsar which spins and sends a beam of radio waves (like a lighthouse in space). When this beam moves past Earth, it shows up as a pulse (like the light from a lighthouse is seen as a flash on the horizon). This 'pulse' is why they are called pulsars.