The image is illustrative of both stellar collapse and the big bang. Big bang theory is the prevailing cosmological model for the early development of the universe. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, including protons, neutrons, and electrons. Gravitational collapse, in for instance a red giant, is the inward fall of an astronomical object due to the influence of its own gravity which tends to draw the object toward its center of mass. A neutron star is a type of compact star that can result from the gravitational collapse of a massive star after a supernova. Neutron stars are the densest and smallest stars known to exist in the universe.