A pulsar is a star that provides a “pulsating radio source” due to the highly magnetized rotating neutron star at its core. Much like a lighthouse is only visible when its beam passes directly over an observer, the radiated energy from a pulsar can only be seen when its beam of emission is pointing directly toward an observer (like an observatory on Earth). Astronomers can use pulsars to detect the gravitational waves produced by colliding supermassive black holes.