What is the main characteristic of a neutron star?

A neutron star is primarily characterized by its extreme density and its composition, which consists largely of neutrons. This occurs as a result of the gravitational collapse of a massive star after it has exploded in a supernova. During this collapse, the electrons and protons combine to form neutrons, leading to a star that is incredibly compact—often only a few kilometers in diameter but with a mass greater than that of the Sun. This intense density means that a single teaspoon of material from a neutron star can weigh as much as several tons on Earth.

The other options do not accurately describe neutron stars. While they may possess high surface temperatures and specific structures, these traits do not define what a neutron star is; it is primarily recognized by its dense, neutron-rich composition.