during most of its lifetime, a star maintains an equilibrium size in which the inward force of gravity on each atom is balanced by an outward pressure force due to the heat of the nuclear reactions in the core. but after all the hydrogen fuel is consumed by nuclear fusion, the pressure force drops and the star undergoes a gravitational collapse until it becomes a neutron star. in a neutron star, the electrons and protons of the atoms are squeezed together by gravity until they fuse into neutrons. neutron stars spin very rapidly and emit intense pulses of radio and light waves, one pulse per rotation. these pulsing stars were discovered in the 1960s and are called pulsars.