neutron star: The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugarcube of neutron-star stuff on Earth would weigh as much as all of humanity! This illustrates again how much of humanity is empty space." Neutron stars can be observed as pulsars.