Stellar Remnants
Neutron Star
City-sized stellar corpse — supported by neutron degeneracy, surface gravity 10¹¹× Earth
A neutron star is the ultra-dense remnant of a massive star (8-25 M_sun) that exploded as a supernova. Diameter ~20 km but mass 1.4-3 M_sun. Density ~10¹⁷ kg/m³ — packing the mass of the Sun into a city. Supported by neutron degeneracy pressure. Surface gravity 10¹¹× Earth. Most spin rapidly (milliseconds to seconds) and have strong magnetic fields. Pulsars are rotating neutron stars beaming radio waves. Maximum mass: ~3 M_sun (Tolman-Oppenheimer-Volkoff limit) — above this, neutron stars collapse to black holes.
- Diameter~20 km (Manhattan-sized)
- Mass1.4-3 M_sun (typical 1.4 M_sun)
- Density~10¹⁷ kg/m³ (Sun mass in city)
- Surface gravity~10¹¹ × Earth's
- Magnetic field10⁸-10¹⁵ G (vs 1 G for Earth)
- Maximum mass~3 M_sun (Tolman-Oppenheimer-Volkoff)
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Why neutron stars matter
- Stellar evolution endpoint. Major outcome for high-mass stars.
- Dense matter physics. Lab for QCD at extreme density.
- Pulsar timing. Tests of general relativity.
- Gravitational waves. Mergers detected (GW170817).
- Heavy element production. R-process nucleosynthesis.
- Magnetic fields. Strongest in universe (magnetars).
- FRBs. Fast radio bursts often from magnetars.
Common misconceptions
- Neutron stars are all pulsars. Most are; some misaligned beams undetectable.
- Neutron stars don't have magnetic fields. Strongest fields known.
- Neutron stars are inside-out atoms. Different physics; nuclear matter.
- Neutron stars are huge. ~20 km diameter — extremely small.
- Neutron stars stop spinning. They slow but rarely stop.
- Neutron stars are stable forever. Most stable stellar end-state, but mergers do occur.
Frequently asked questions
How does a neutron star form?
Massive star (8-25 M_sun) exhausts fuel sequentially: H → He → C → O → Si → Fe. Iron core can't fuse to release energy. Core collapses; electrons captured by protons → neutrons. Neutron core resists further collapse via degeneracy pressure. Outer layers explode as supernova; collapse continues to ~20 km radius. Neutron star is the leftover compact core.
Why is it so dense?
Atomic nuclei pushed together. Normal matter has electron clouds making atoms 10⁵× larger than nuclei. In neutron star, electrons combined with protons → neutrons; nuclei pack at nuclear density. Density: ~10¹⁷ kg/m³ — same as inside an atomic nucleus. A teaspoon weighs ~6 billion tons.
What's a pulsar?
Rotating neutron star with strong magnetic field. Radiation beam aligned with magnetic axis sweeps through space — observed as pulses. Periods: milliseconds (millisecond pulsars) to seconds (slower, older). Discovery: 1967 by Jocelyn Bell Burnell. First found pulsar (LGM-1) was nicknamed "Little Green Men" because pulses seemed artificially regular.
What's a magnetar?
Neutron star with extreme magnetic field — 10¹⁴-10¹⁵ G (a quadrillion times Earth). Cause: turbulent dynamo during early life. Decay via flares (giant magnetar flares). Some magnetars are sources of fast radio bursts (FRBs). About 30 known.
How fast can a neutron star spin?
Up to ~700 Hz. Period in milliseconds. PSR J1748-2446ad rotates at 716 Hz (~25% speed of light at equator). Newly-formed neutron stars spin even faster (50-1000 Hz). Slow over time as they radiate energy. Spin-up by mass transfer in binaries → millisecond pulsars.
Could a neutron star turn into a black hole?
Yes, if mass exceeds ~3 M_sun (TOV limit). Through accretion or merger. Two neutron stars merging often produce a black hole. GW170817 (gravitational wave + electromagnetic detection 2017) was a NS merger event — produced kilonova.
What are kilonovae?
Mergers of two neutron stars. Release ~10⁴² J, including gravitational waves, gamma-ray burst, optical/UV/IR emission, kilonova ejecta. Heavy elements (gold, platinum) produced via r-process. Kilonova GW170817 confirmed neutron star mergers as origin of much of universe's heavy elements.