A neutron star is a superdense remnant of a massive sun. A star that holds at least 8 times the Sun’s mass will burn through its material much faster than the Sun will. This massive star will then die dramatically as a supernova. This explosive event tears layers of gas away from the star, leaving behind its dense core.
The leftover neutron star is about 12 miles (20 kilometers) in diameter and holds between 1 and 3 times the Sun’s mass, approximately. It’s so dense that a teaspoon of this matter weighs about a billion tons. A neutron star’s density increases toward its center, where it holds a fluid of electrons, protons, and neutrons.