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Answer: A neutron star is what remains when a massive star runs out of fuel and its core collapses, crushing every proton and electron into a neutron. It is the densest object astronomers can observe directly: an object with the mass of a sun squished down to the size of a city, about 20 kilometers (12.5 miles) across.

How they form

If the core of the collapsing star is between about 1 and 3 solar masses, the newly-created neutrons can stop the collapse, leaving behind a neutron star. Stars with higher masses will continue to collapse into stellar-mass black holes.

How dense

One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth, about as much as a mountain.

Pulsars

A pulsar is a neutron star with a strong magnetic field and a beam of light along the magnetic axis. As the neutron star spins, the magnetic field spins with it, sweeping that beam through space. If that beam sweeps over Earth, we see it as a regular pulse of light.

Discovery

Astronomers first hypothesized neutron stars in the 1930s, shortly after the discovery of the neutron itself, but the first evidence came in the 1960s. In 1967, Jocelyn Bell, a PhD student under Antony Hewish, detected a radio signal with very regular pulses 1.3 seconds apart. It turned out to be radio emissions from a pulsar now called PSR B1919+12.

Finding them today

Most pulsars are discovered by their radio signals. Accreting neutron stars in binary systems are observed principally in X-rays, and magnetars in both X-rays and gamma-rays.

Claims

• id: C1 claim: Neutron stars form when a massive star runs out of fuel and collapses. source: S1 evidence: "Neutron stars are formed when a massive star runs out of fuel and collapses."
• id: C2 claim: Neutron stars measure about 20 kilometers across. source: S1 evidence: "These stellar remnants measure about 20 kilometers (12.5 miles) across."
• id: C3 claim: Most pulsars are discovered by their radio signals. source: S2 evidence: "Most pulsars are discovered by their radio signals."

Sources

• [S1] Imagine the Universe!. NASA. https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Imagine the Universe!. NASA. https://imagine.gsfc.nasa.gov/science/objects/neutron_stars2.html . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_73dz6f1j,
  author = {{iinventedlight}},
  title = {{Neutron stars: how they form and how we find them}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/neutron-stars/},
  note = {Permanent ID iil:note:73dz6f1j. License: CC-BY-4.0.},
}

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