Pulsar

A pulsar is an extremely rapidly rotating neutron star that emits two beams of electromagnetic radiation along its magnetic axis. When one of these beams sweeps across Earth, we detect a pulsating signal of extraordinary regularity—like a cosmic lighthouse. The word “pulsar” is a contraction of pulsating star.

The Lighthouse Mechanism

When a massive star explodes as a supernova, its core can collapse into a neutron star— an object about ten kilometers in diameter but with a mass greater than that of the Sun. The conservation of angular momentum causes this remnant to spin at dizzying speeds, sometimes several hundred times per second. Its magnetic field, which is also extraordinarily intense, channels the radiation along the magnetic poles. If these poles are not aligned with the axis of rotation, the beams sweep across space like a lighthouse.

A Cosmic Clock

The pulsation period of pulsars is remarkably regular, comparable to the best atomic clocks. Some millisecond pulsars have such precise frequency stability that they are used as time standards in fundamental physics. International Pulsar Timing Arrays seek to detect low-frequency gravitational waves by measuring minute variations in the timing of several pulsars simultaneously.

A Historic Discovery

The first pulsar was discovered in 1967 by Jocelyn Bell Burnell, then a doctoral student at Cambridge. The regular 1.33-second signal was so unexpected that it was initially nicknamed LGM-1 (Little Green Men) before its stellar nature was understood. This discovery earned the 1974 Nobel Prize in Physics, but it was awarded to her thesis advisor, Antony Hewish— not to Bell Burnell—one of the most famous injustices in the history of science.

A concrete example: the Crab Pulsar

The Crab Pulsar (PSR B0531+21), located at thethe Crab Nebula, about 6,500 light-years away, is one of the most extensively studied. It rotates 30 times per second and is the remnant of the supernova observed by Chinese astronomers in 1054. It emits radiation across nearly all wavelengths, from radio waves to gamma rays.

Did you know?

The fastest known pulsar, PSR J1748-2446ad, rotates 716 times per second—its equatorial surface moves at about 24% of the speed of light. Beyond a certain frequency, the neutron star is expected to disintegrate due to centrifugal force.

Frequently Asked Questions

Are all pulsars neutron stars?

Yes, but the reverse is not true: not all neutron stars are pulsars. For a neutron star to be detected as a pulsar from Earth, one of its beams must be pointing in our direction. Many neutron stars exist without being observable as pulsars.

Can pulsars slow down over time?

Yes. Pulsars gradually lose energy through radiation and slow down their rotation. A typical pulsar’s rotation period increases from a few milliseconds to several seconds over millions of years. When it becomes too slow, the emission mechanism stops—the pulsar goes silent.

What are millisecond pulsars?

These are pulsars with a period of less than 30 milliseconds. They are generallyslow but have been re-accelerated by accretion of matter from a companion. They are often found in globular clusters.