Define a neutron star.

A neutron star is defined as the incredibly dense remnant of a supernova explosion. When a massive star exhausts its nuclear fuel, it undergoes a catastrophic collapse. During this process, the outer layers are expelled in a supernova, while the core collapses under gravity to form the neutron star. This core becomes so compressed that protons and electrons are forced to combine, forming neutrons, which results in a star primarily composed of neutrons. The density of a neutron star is astounding, with a mass greater than that of the Sun compressed into a sphere about the size of a city, typically around 20 kilometers in diameter. This extreme density leads to very strong gravitational fields and is accompanied by the emission of radiation, particularly in the form of X-rays.

In contrast, the other options describe different types of stellar objects or processes. A large star still undergoing fusion refers to a star like a main-sequence or giant star. A newly formed star pertains to a protostar that is still in the process of formation within a nebula, while a white dwarf is the remnant of a smaller star that has shed its outer layers at the end of its life cycle, representing a different stellar evolution path than that of a neutron star.