neutron star, extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933. The central core of a neutron star is composed of neutrons or possibly a quark-gluon plasma (see elementary particles); there are no stable atoms or nuclei because these cannot survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutrons squeezed in close contact. The fluid is encased in a rigid crystalline crust a mile or two thick. The outer gaseous atmosphere is probably only a few feet thick. The neutron star resembles a single giant nucleus because the density everywhere except in the outer shell is as high as the density in the nuclei of ordinary matter. There is observational evidence of the existence of several classes of neutron stars: pulsars are periodic sources of radio frequency, X ray, or gamma ray radiation that fluctuate in intensity and are considered to be rotating neutron stars. A neutron star may also be the smaller of the two components in an X-ray binary star.