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SESK: Stuff Everyone Should Know · sesk.info/astro-neutron-star-size

When a massive star's iron core collapses into a neutron star, about how big does it end up?

Less than about 20 km across, crushed from roughly Earth size in under a second.

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  1. Less than about 20 km across ✔
  2. About the size of Earth
  3. About the size of the Sun

Why: When the core collapses, electrons are forced into nuclei, turning protons into neutrons. A core of about one solar mass shrinks from Earth size to a ball of neutrons under 20 km across.

Huh, didn't know that: Neutron star material has a density of about 10^14 grams per cubic centimeter.

The dad joke
A neutron walks into a bar and asks how much for a drink. The bartender says, for you, no charge.
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References

Answer: Less than about 20 km across. When the core collapses, electrons are forced into nuclei, turning protons into neutrons. A core of about one solar mass shrinks from Earth size to a ball of neutrons under 20 km across.

  1. Astronomy 2e, 23.2 Evolution of Massive Stars: An Explosive Finish openstax.org/books/astronomy-2e/pages/23-2-evolution-of-massive-stars-an-explosive-finish
    which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers.
    Checked 2026-10-10.

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