A neutron star is the stellar endpoint of a star once several times larger than the Sun. After the star exhausts its nuclear fuel, it blows off its outer shell of gas in a fiery display known as a supernova. The remaining dense core, still as massive as the Sun, collapses further into a compact, city-sized sphere. This sphere, called a neutron star because the core's tightly squeezed protons and electrons essentially form a single neutron, is characterized by a strong gravitational field and rapid spin. A teaspoon of neutron star matter would weigh three billion tons on Earth.
Scientists find some neutron stars within the glow their supernova remnant.
Other neutron stars are part of a binary star system, with the gas of one star falling onto the nearby neutron star due to the neutron star's gravitational field, thus lighting the path to the tiny sphere. Isolated neutron stars, however, are in the middle of nowhere, without a supernova remnant or companion star to reveal their presence. The isolated neutron star was either blown away from its companion star in the supernova explosion or perhaps never had a neighbor. Its supernova remnant has since faded or dispersed. Thus, the theory is that these neutron stars are millions of years old embers from stellar explosions.