Pulsars
Rapidly spinning neutron stars that beam radiation.
Overview
A pulsar is a highly magnetised, rapidly rotating neutron star whose beamed radiation sweeps past Earth as regular pulses. Their timing stability is so extreme that they function as natural precision clocks and have delivered some of the strongest tests of general relativity.
- The first pulsar was detected in 1967 by Jocelyn Bell Burnell, then a graduate student.
- A neutron star packs more than the Sun's mass into a sphere roughly 20 kilometres across.
- Millisecond pulsars spin hundreds of times per second, spun up by accretion from a companion.
- Timing of the Hulse–Taylor binary pulsar provided the first evidence for gravitational waves — indirectly, decades before LIGO.
Discovery
In 1967 Jocelyn Bell Burnell noticed an unusually regular repeating radio signal in data from a Cambridge array. Its precision — pulses arriving every 1.34 seconds — was initially so unlike any known astronomical source that the possibility of an artificial origin was briefly entertained, and the source was informally labelled LGM-1.
The discovery of further sources with different periods ruled that out, and the signals were identified as rotating neutron stars. The 1974 Nobel Prize in Physics went to Bell Burnell's supervisor Antony Hewish, cited for his decisive role in the discovery of pulsars, and was shared with Martin Ryle, cited separately for aperture synthesis. Bell Burnell's own omission has been widely discussed since.
What a neutron star is
A neutron star is the collapsed core left when a massive star's iron core can no longer support itself. Electron degeneracy pressure fails, protons and electrons combine, and the core collapses until it is supported by neutron degeneracy pressure and the strong nuclear force — packing more than the Sun's mass into a sphere of order 20 kilometres across.
Conservation of angular momentum during that collapse spins the remnant up enormously, and conservation of magnetic flux amplifies its magnetic field to extraordinary strengths. Charged particles accelerated along the magnetic axis produce beamed emission, and because the magnetic axis is generally not aligned with the rotation axis, that beam sweeps around like a lighthouse.
Timing precision and what it buys
Pulsar periods are stable enough that arrival times can be predicted over years, and departures from prediction reveal physics. Gradual spin-down is measurable and gives the energy loss rate. Sudden small speed-ups — glitches — probe the interior superfluid.
Millisecond pulsars, spun up to hundreds of rotations per second by accreting matter from a binary companion, are the most stable of all, rivalling atomic clocks over long intervals. Pulsar timing arrays exploit this by monitoring many millisecond pulsars simultaneously, searching for the correlated timing deviations that a background of nanohertz gravitational waves would produce.
Testing gravity
The binary pulsar PSR B1913+16, discovered by Russell Hulse and Joseph Taylor in 1974, provided the first evidence that gravitational waves exist. Its orbital period decays at precisely the rate general relativity predicts for energy lost to gravitational radiation — a match sustained over decades of timing, recognised with the 1993 Nobel Prize in Physics.
Later systems, including double-pulsar binaries where both components are detectable, have tightened these tests further and continue to constrain alternatives to general relativity in the strong-field regime more sharply than any laboratory experiment can.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why do pulsars pulse?
- Because they emit beamed radiation along their magnetic axis, which is usually tilted relative to their rotation axis. As the star spins, the beam sweeps across space, and an observer whose line of sight the beam crosses sees a pulse once per rotation. The star's emission is continuous; the pulsing is a geometric effect.
- How fast can a pulsar spin?
- The fastest known millisecond pulsars rotate several hundred times per second. They reach those rates by accreting matter from a binary companion, which transfers angular momentum and spins the neutron star up — which is why they are described as recycled pulsars.
- Were pulsars mistaken for alien signals?
- Briefly and informally. The first detected source was so regular that an artificial origin was considered and the signal was labelled LGM-1 as a working joke. Finding additional sources with different periods in different parts of the sky ruled that out quickly, and the rotating-neutron-star explanation followed.
- How dense is a neutron star?
- Extremely — comparable to atomic nuclei. More than the Sun's mass is contained in a sphere roughly 20 kilometres across, so a volume the size of a sugar cube would have a mass of order a hundred million tonnes. This is matter compressed beyond anything reproducible in a laboratory, and its exact equation of state is still being constrained.