Neutron stars and pulsars
More than the Sun's mass in a city-sized sphere, spinning hundreds of times a second.
4 min read · Published 26 August 2026
The short answer
A neutron star is the collapsed core left by a massive star's supernova, packing more than the Sun's mass into a sphere roughly 20 kilometres across. Its material is so dense that a teaspoon would weigh hundreds of millions of tonnes. A pulsar is a neutron star whose beamed radiation sweeps past Earth as it rotates, producing pulses so regular they rival atomic clocks.
The density
Gravitational collapse crushes electrons into protons, producing neutrons packed at roughly the density of an atomic nucleus. The result is essentially one enormous nucleus, about 20 kilometres across.
A teaspoon of neutron star material would weigh on the order of hundreds of millions of tonnes. Surface gravity is around a hundred billion times Earth's, and the escape velocity is a substantial fraction of the speed of light.
Their structure is layered: a solid crust of nuclei, then a region where neutrons form a superfluid — flowing with zero viscosity — and a core whose composition remains genuinely unknown. Matter at those pressures may take forms with no laboratory analogue, and this is one of the open problems in physics.
Why they spin so fast
Conservation of angular momentum. The collapsing core shrinks from roughly Earth-sized to 20 kilometres, and its rotation speeds up enormously — the skater's-arms effect taken to an extreme.
Typical young pulsars rotate several times per second. Millisecond pulsars rotate hundreds of times per second, with the fastest known exceeding 700 rotations per second. At that rate the surface moves at a substantial fraction of light speed.
Millisecond pulsars are old neutron stars that have been spun up by accreting material from a companion, which is why they are found in binary systems and in globular clusters where interactions are common.
The magnetic field is similarly extreme — compressed along with everything else to strengths trillions of times Earth's.
The LGM-1 story
In 1967 Jocelyn Bell Burnell, then a graduate student, noticed a regular pulsing signal in radio data — a pulse every 1.337 seconds, far too regular for any known astrophysical source.
The signal was labelled LGM-1, for Little Green Men, as a half-joking acknowledgement that an artificial origin could not immediately be excluded. The possibility was taken seriously enough to be discussed before being ruled out when a second source was found in a different part of the sky — two independent civilisations transmitting on the same principle was implausible.
The 1974 Nobel Prize for the discovery went to her supervisor and a colleague, not to Bell Burnell. The omission has been widely criticised since and she has spoken about it with notable generosity. It sits alongside the story of the Harvard computers in the history of credit in astronomy.
What they are used for
- Timekeeping. Millisecond pulsars rival atomic clocks in long-term stability.
- Gravitational wave detection. Pulsar timing arrays watch for correlated timing deviations across many pulsars, sensitive to very low frequency gravitational waves from supermassive black hole binaries.
- Testing general relativity. Binary pulsars provide the strongest tests of relativity in strong fields. The orbital decay of one such system matched relativity's prediction for energy loss to gravitational waves, providing the first indirect evidence they exist.
- Heavy element production. The 2017 detection of a neutron star merger, seen in both gravitational waves and light, showed heavy elements being produced directly.
- Navigation. Their positions were engraved on spacecraft plaques as a map to locate the Sun.
Common questions
- What is a neutron star?
- The collapsed core left after a massive star's supernova, packing more than the Sun's mass into a sphere about 20 kilometres across. Its material is at roughly nuclear density — a teaspoon would weigh hundreds of millions of tonnes.
- What is the difference between a neutron star and a pulsar?
- A pulsar is a neutron star whose beamed radiation happens to sweep across Earth as it rotates, producing regular pulses. All pulsars are neutron stars, but a neutron star whose beam misses us is not observed as a pulsar.
- Why were pulsars nicknamed Little Green Men?
- The first pulsar, discovered by Jocelyn Bell Burnell in 1967, produced a signal so regular that an artificial origin could not immediately be excluded. It was labelled LGM-1 until a second source was found elsewhere in the sky, making two independent civilisations implausible.
- How fast do neutron stars spin?
- Typical young pulsars rotate several times per second. Millisecond pulsars, spun up by accreting material from a companion, rotate hundreds of times per second, with the fastest known exceeding 700 rotations per second.