Pulsars are magnetized neutron stars that spin incredibly fast and blast out a beam of radiation, kind of like a lighthouse beacon. The fastest known pulsar is PSR J1748-2446ad, located around 18,000 light-years away in the constellation Sagittarius. Though it is of average size for a neutron star, it spins an astonishing 716 times per second. This is nearly a quarter of the speed of light, and exceeds what theories say is possible.