A pulsar is a vastly powerful, unstable object, probably a neutron star, very small and very dense, which emits enormous bursts of light and radio waves in very regular pulses—often up to ten per second. To an observer the whole star appears to flash on and off, although the mechanism is probably more like that of a very fast lighthouse, where the light is always present but the star’s magnetic field allows only a narrow beam to escape once every revolution.

Black Widow Pulsar (B1957+20), Chandra, courtesy NASA/CSC