Neutron Stars
Ultra-dense stellar remnants formed in supernovae: their structure, the pulsar mechanism, and what their mergers teach about matter and elements.
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A neutron star is the compact remnant left behind when the core of a massive star collapses in a supernova. It packs roughly 1.4 to 2 solar masses into a sphere about 20 kilometers across, giving it a density comparable to an atomic nucleus — a teaspoon of neutron-star material would weigh about a billion tonnes on Earth. Neutron stars are supported against gravity by neutron degeneracy pressure, with possible additional contributions from exotic states of matter whose nature is still uncertain.
Formation begins when a star of roughly eight solar masses or more exhausts its fusion fuel and its iron core (built by stellar nucleosynthesis) can no longer produce energy. The core collapses in fractions of a second; protons and electrons combine into neutrons, and the collapse is halted by degeneracy pressure, while the infalling outer layers rebound and are ejected as a supernova. If the remnant's mass exceeds the maximum stable mass — the exact value depends on the poorly constrained equation of state of dense matter, with limits near 2 to 3 solar masses — it collapses further into a black hole.
The surface gravity of a neutron star is extreme: the escape velocity approaches half the speed of light, and general-relativistic effects such as gravitational redshift and frame dragging are measurable. Magnetic fields reach 108 to 1011 tesla in the strongest objects, the magnetars, whose decaying fields power intense X-ray and gamma-ray bursts.
Many neutron stars are observed as pulsars: rapidly rotating objects whose magnetic axis is tilted relative to the rotation axis, so that beams of radio (and sometimes optical, X-ray, or gamma-ray) emission sweep past Earth like a lighthouse. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell, who jokingly labeled the periodic signal LGM-1 ("little green men") before it was identified as a rotating neutron star. Millisecond pulsars spin hundreds of times per second, and their clock-like regularity lets astronomers test general relativity: the Hulse–Taylor binary pulsar showed orbital decay matching the emission of gravitational waves, work recognized with the 1993 Nobel Prize in Physics.
Mergers of neutron stars are now directly observable. The 2017 event GW170817 was detected by the LIGO and Virgo gravitational-wave observatories and followed across the electromagnetic spectrum; it produced a kilonova and confirmed that neutron-star mergers synthesize heavy r-process elements such as gold and platinum. Observing these collisions also probes the equation of state of matter at nuclear densities, one of the most active frontiers in stellar astrophysics.
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gravity neutron stars pulsars stars