Pulsars vary in radio intensity with periods from milliseconds to seconds, and at the time of discovery some even suspected signals from aliens. Hewish built a large phasing array at Cambridge that could scan large parts of the sky quickly in search of "twinkling" radio sources. Graduate student Jocelyn Bell detected an unusual signal, and the team determined that it was strictly periodic. Analysis showed the signal came from a neutron star rotating hundreds of times per second, whose beam grazes Earth. Pulsars have since become tools for studying stellar evolution, gravity, and cosmic distance scales.

Hewish’s antenna, designed for measuring interplanetary scintillation, used a phased-array of roughly 2,000 wire elements, providing high time resolution. While inspecting kilometers of paper-chart data, a 1.337-second pulse train was spotted and announced as CP1919. Subsequent observations revealed additional periodic sources; measured spin-down implied energy loss consistent with highly magnetized neutron stars. Theoretical work proposed the “lighthouse model,” in which a misalignment between magnetic and rotational axes sweeps a beam across Earth. Pulsar timing is now used for probing interstellar dispersion, searching for a gravitational-wave background, and testing general relativity in double-neutron-star systems.

Hewish upgraded the interplanetary-scintillation phased array to 64 simultaneous beams, enabling detection of sub-second pulses. CP1919, discovered by Bell, exhibited a dispersion measure of ≈19 pc cm⁻³, immediately ruling out atmospheric interference. Spin-down–derived dP/dt implied B≈10¹² G, bolstering Gold’s rotating neutron-star hypothesis. Observations of PSR B0531+21 in the Crab Nebula soon confirmed an association with a historical supernova remnant. Pulsar spectroscopy and polarization studies constrain magnetospheric geometry, and pulsar-timing arrays now achieve ≈10⁻¹⁵ arrival-time precision, probing nano-hertz gravitational waves.