Objects · Milky Way scale
Pulsar
A fast-spinning neutron star that sweeps beams of radio (or X-ray) radiation past Earth with extraordinary regularity — like a cosmic lighthouse. Millisecond pulsars spin hundreds of times per second and are among the most precise clocks in the universe.
Record holder
The fastest known pulsar, PSR J1748−2446ad, spins at 716 times per second — its equator moves at 24% of the speed of light.
Source: ATNF Pulsar Catalogue
How pulsars work
When a massive star (roughly 8–20 solar masses) exhausts its nuclear fuel, its core collapses in a fraction of a second in a supernova explosion. The core collapse compresses protons and electrons together into neutrons, forming a neutron star — 1–2 solar masses squeezed into a sphere ~20 km across. The density is so extreme that a teaspoon of neutron star material would weigh about a billion tonnes on Earth.
The collapsing core conserves angular momentum (like a spinning ice-skater pulling in their arms), dramatically increasing the rotation rate — from a stellar rotation period of weeks to milliseconds. The intense magnetic field (trillions of times Earth's) funnels charged particles along the magnetic poles, emitting beams of radio waves, X-rays, and sometimes gamma rays. When the magnetic axis is misaligned with the spin axis — and when that beam sweeps past Earth — we detect a regular pulse.
For the full context on neutron stars and their extreme properties, see our neutron stars and pulsars guide.
Frequently asked questions
- Pulsars were discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish at the Mullard Radio Astronomy Observatory. Bell noticed a regular radio pulse repeating every 1.337 seconds — so regular it was initially labelled "LGM-1" (Little Green Men). Once confirmed to be natural, the object — now known as PSR B1919+21 — was identified as a rapidly rotating neutron star. The 1974 Nobel Prize in Physics was awarded for the discovery (controversially to Hewish alone, not Bell). Over 3,000 pulsars have since been catalogued. The ATNF Pulsar Catalogue lists all known pulsars.
- Pulsars are among the most precise clocks in the known universe. The spin rate of a pulsar — defined by the interval between pulses — is extremely stable. Millisecond pulsars (MSPs) spin hundreds of times per second and are so stable that their long-term timing rivals atomic clocks. This extraordinary precision allows pulsars to be used as physics laboratories: deviations in pulse timing can reveal the presence of unseen companions (the first exoplanet was detected via pulsar timing in 1992), test general relativity in strong-field conditions, and collectively — via pulsar timing arrays (PTAs) — detect gravitational waves with wavelengths of light-years. In 2023, PTAs detected the gravitational wave background for the first time.
- All three are neutron stars — the ultra-dense remnant cores of massive stars after supernova explosions, roughly 20 km in diameter but 1–2 times the Sun's mass. They are differentiated by activity: a pulsar is a neutron star whose radio (or X-ray/gamma-ray) emission beam sweeps across our line of sight, producing pulses. A magnetar is a neutron star with an extraordinarily powerful magnetic field (10¹⁵ Gauss — roughly a trillion times Earth's), which powers occasional, violent X-ray and gamma-ray outbursts called soft gamma repeaters and anomalous X-ray pulsars. Every magnetar is a neutron star; some pulsars are magnetars; not all neutron stars are visible as pulsars (the beam may not sweep past Earth). Our guide to neutron stars and pulsars covers all three in depth.
- In theory, yes — there is a maximum spin rate set by the breakup limit, where centrifugal force overwhelms the strong force binding the neutron star. For a typical neutron star this is roughly 1,000–1,500 rotations per second (Hz). The fastest known pulsar, PSR J1748−2446ad, spins at 716 Hz — about 716 times per second — just under half the theoretical breakup limit. It completes a full rotation in 1.4 milliseconds. At its equator the surface is moving at roughly 24% of the speed of light. Millisecond pulsars are thought to be "recycled" pulsars — old, slow pulsars spun up by accreting matter from a binary companion star, which transfers angular momentum and dramatically increases the spin rate.
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