Phenomena · Space Codex
Neutron stars
The collapsed cores of massive stars: more mass than the Sun crushed into a ball about 20 km wide, so dense a sugar cube of it would weigh about a trillion kilograms.
City-sized stellar cores
At a glance
Key points
- About 20 km across
- A sugar cube weighs a trillion kg
- You could see three-quarters of its surface at once
From prediction to discovery
James Chadwick discovered the neutron in 1932, and by 1934 Walter Baade and Fritz Zwicky had proposed that a supernova marks the collapse of an ordinary star into a ball of neutrons. The idea stayed untested for three decades, until the pulsing radio source Jocelyn Bell Burnell found in 1967 was identified as a spinning neutron star. They are now seen as radio pulsars, X-ray sources feeding on companion stars, magnetars and partners in gravitational-wave mergers.
What holds it up
When fusion stops in a massive star's core, gravity crushes it until electrons are forced into protons, turning them into neutrons. The collapse halts when neutrons packed shoulder to shoulder resist further squeezing, through quantum pressure and the strong nuclear force. Above a limit that the heaviest known neutron stars place somewhere beyond about 2.2 solar masses, nothing can resist and a black hole forms instead.
Weighing and measuring
Masses come from pulsars in binary systems: when a companion passes between us and the pulsar, its gravity delays the pulses slightly. NASA's NICER instrument on the International Space Station measures sizes by timing X-rays from hot spots whose light is bent by the star's gravity. Together, mass and radius constrain the equation of state, the rule linking pressure and density in the core.
Bending light
Gravity at the surface is strong enough to bend light around the star, so you would see more than half of its surface at once, about three-quarters for a typical neutron star.
Mergers and gold
The collision seen as GW170817 was followed by a kilonova, a glow powered by the radioactive decay of freshly forged heavy elements. It showed that neutron-star mergers make at least part of the universe's gold, platinum and other heavy elements; how large a part, compared with rare kinds of supernova, is still debated.
What we still don't know
What matter is like in the core, and the exact maximum mass before a neutron star collapses into a black hole.
Related entries
- PulsarsCosmic lighthouses
- MagnetarsThe strongest magnets in the universe
- Gravitational wavesRipples in spacetime
Sources
- NASA Imagine the Universe: neutron stars (opens in a new tab)
- Romani et al. 2022 (arXiv) (opens in a new tab)
- ESA: ESA/Webb weic2404 (opens in a new tab)
- Burrows 2015, PNAS: Baade and Zwicky's 1934 papers (opens in a new tab)
- Fonseca et al. 2021, ApJL (arXiv) (opens in a new tab)
- Dittmann et al. 2024, ApJ (arXiv) (opens in a new tab)
- LIGO/Virgo 2017, PRL 119:161101 (arXiv) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.