pulsar

Imagine a lighthouse whose beam sweeps the dark sea, so that a ship sees a flash each time the beam swings past. Now shrink the lighthouse to the size of a city, spin it dozens of times a second, and place it thousands of light-years away. That is a pulsar: a rapidly spinning neutron star that beams radiation from its magnetic poles, and as the beam sweeps across Earth, we receive a metronomic 'tick' of radio waves — astonishingly regular, sometimes hundreds of times a second.

A pulsar is not pulsing on and off; it is a steady beacon that appears to blink only because it is rotating and its beam points at us only briefly each turn. The neutron star inherits a huge amount of spin and an intense magnetic field from the collapse that formed it, and charged particles racing along the magnetic field lines emit tightly focused beams from the poles. Because the magnetic axis is tilted from the spin axis (just as Earth's magnetic pole is offset from its geographic pole), the beams sweep around like a cosmic lighthouse. The pulses are so precise that some rival atomic clocks.

The story of their discovery is famous: in 1967 a young graduate student, Jocelyn Bell Burnell, found a signal so regular her team half-jokingly labelled it 'LGM-1' for 'Little Green Men', wondering if it might be aliens. It was the first pulsar. Today thousands are known, and they serve as precision tools — clocking the slow death of stars, testing Einstein's gravity, hunting for planets, and even being proposed as galaxy-wide 'GPS' beacons for future spacecraft.