Neutron star

Neutron stars are the compact stellar remnants produced when the core of a massive star collapses under gravity following a supernova explosion. They represent one of the densest forms of matter in the universe, exceeded only by black holes. Formed from the gravitationally compressed cores of supergiant stars, neutron stars display extreme physical properties, including intense gravity, rapid rotation and powerful magnetic fields.

Formation and Structure

A neutron star originates when a massive star—typically with an initial mass greater than eight solar masses—exhausts its nuclear fuel and can no longer support its core against gravitational collapse. As the iron-rich core grows and surpasses the Chandrasekhar limit, electron degeneracy pressure fails and the core collapses to nuclear densities. Under these conditions, electrons and protons fuse through electron capture to form neutrons and neutrinos. Once the collapsing core reaches densities comparable to those of atomic nuclei, neutron degeneracy pressure and strong-force repulsion halt further collapse, producing a compact remnant a few tens of kilometres in diameter.
The formation process triggers a Type II, Type Ib or Type Ic supernova, ejecting the star’s outer layers. If the remnant core exceeds the Tolman–Oppenheimer–Volkoff limit, neutron degeneracy and nuclear forces are insufficient to prevent further collapse, and a black hole forms instead.

Physical Properties

A typical neutron star has:

• A radius of about 10–12 kilometres.
• A mass between approximately 1.1 and 2.3 solar masses, depending on its progenitor and subsequent evolution.
• An average density comparable to that of atomic nuclei; a matchbox-sized sample of neutron star material would weigh around three billion tonnes, equivalent to a 0.5-cubic-kilometre block of terrestrial rock (about 800 metres on each side).
• Extreme gravity, with surface accelerations billions of times that of Earth, and an escape velocity exceeding half the speed of light.

Newly formed neutron stars may have surface temperatures above ten million kelvin but cool over thousands to millions of years. Even older neutron stars, with temperatures in the hundreds of thousands of kelvin, remain detectable, especially via X-ray observatories.
As the core collapses, conservation of angular momentum causes the remnant to spin at extraordinary rates. Rotation periods range from milliseconds to seconds. The fastest-spinning neutron star known, PSR J1748−2446ad, rotates approximately 716 times per second.

Types and Observational Signatures

Neutron stars exhibit different behaviours depending on their environment and history:

• Pulsars emit beams of electromagnetic radiation from their magnetic poles. As the star rotates, the beam sweeps across space, producing periodic pulses. Pulsars provided the first observational evidence for neutron stars in 1967.
• X-ray pulsars form in binary systems when accreted matter from a companion star heats the neutron star’s surface, generating hotspots visible in X-rays.
• Millisecond pulsars are old neutron stars spun up through prolonged accretion, achieving rotation periods of only a few milliseconds.
• Thermally emitting isolated neutron stars represent a cooler, less active class, detected largely through their black-body radiation.

Many neutron stars are difficult to detect if they are not accreting material or producing strong beams of radiation. The Milky Way may harbour hundreds of millions to a billion neutron stars, though only a small fraction are observable.

Binary Systems and Stellar Evolution

In binary systems, neutron stars can accrete matter from companion stars, leading to significant observable phenomena. Accretion increases both temperature and luminosity, often leading to X-ray emission. Accreting neutron stars can undergo “recycling,” becoming millisecond pulsars with extreme rotation rates.
Binary evolution can produce a range of compact-object pairings, including neutron star–white dwarf systems and neutron star–neutron star pairs. Some interactions may destroy the companion star through ablation or direct collision.

Neutron Star Mergers and Gravitational Waves

When two neutron stars in a binary system gradually lose orbital energy through gravitational radiation, they spiral closer together and eventually merge. These mergers are among the most powerful events in the cosmos, producing:

• Gravitational waves detectable by observatories such as LIGO and Virgo.
• Kilonovae, transient optical and infrared events powered by the radioactive decay of heavy elements.
• Short gamma-ray bursts, resulting from relativistic jets.

In 2017, the event GW170817 became the first directly observed neutron star merger, linking gravitational-wave astronomy with electromagnetic observations and confirming neutron star collisions as sites of heavy-element nucleosynthesis.

Equation of State and Interior Composition

The internal structure of neutron stars remains one of the major unsolved problems in physics. Conditions deep inside these objects cannot be replicated in terrestrial laboratories, making direct testing impossible. Modelling requires the simultaneous application of:

• General relativity
• Quantum chromodynamics
• Superfluidity and superconductivity theories
• Nuclear physics at extreme densities

A neutron star is believed to consist of multiple layers:

• Outer crust, where nuclei coexist with a sea of electrons.
• Inner crust, containing neutron-rich nuclei embedded in superfluid neutrons.
• Outer core, comprising ultra-dense neutron-rich matter.
• Inner core, whose composition is highly uncertain—possibilities include exotic states such as hyperon-rich matter, deconfined quark matter or meson condensates.

Determining the equation of state that relates pressure, density and temperature in this environment is key to understanding neutron star masses, radii and stability limits.

Significance

Neutron stars represent natural laboratories for nuclear and gravitational physics under extreme conditions. They illuminate stellar evolution pathways, serve as precise cosmic clocks and provide insights into the behaviour of matter at densities unreachable on Earth. Their study continues to inform models of high-energy astrophysics, the synthesis of heavy elements and the nature of compact objects in the universe.