A neutron star is an ultra-dense remnant of a massive star formed after a supernova: when the star runs out of fuel, its core collapses, and protons and electrons merge into neutrons, creating an object only about 10–20 km in diameter but with a mass of roughly 1.4–2.1 times that of the Sun; its density is so extreme that a teaspoon of its matter would weigh billions of tons, and its gravity and magnetic fields are among the strongest in the universe; these stars can spin rapidly and appear as p