A neutron star is a compact remnant formed when the core of a massive star collapses, usually during a supernova. It contains matter dominated by neutrons, compressed to densities comparable to or exceeding those inside atomic nuclei. A typical neutron star has a mass around 1.4 times that of the Sun, packed into a sphere only about 20–30 kilometres across. Its extreme density, rapid rotation, and intense magnetic environment make it an important natural laboratory for matter under conditions inaccessible to terrestrial experiments. (nasa.gov)
Formation
In a massive star approaching the end of its life, successive stages of nuclear burning produce a core that can no longer remain stable against gravity. As collapse proceeds, electrons are captured by protons, producing neutrons and neutrinos. The collapsing material becomes increasingly neutron-rich. When nuclear densities are reached, the resistance of dense matter can halt the core’s compression, leaving a hot proto-neutron star while the surrounding stellar layers may be expelled. (heasarc.gsfc.nasa.gov)
The outcome is not determined by the original stellar mass alone. The core’s properties, the explosion, and material falling back onto the remnant influence whether a neutron star survives or a black hole forms. Other proposed formation channels include the collapse of an accreting white dwarf and some mergers of compact remnants; these channels do not imply that every such event produces a stable neutron star. (science.nasa.gov)
Structure and dense matter
A neutron star is not a uniform ball of neutrons. Models generally distinguish a thin atmosphere, a solid crust, and a predominantly fluid core. The outer crust contains neutron-rich atomic nuclei arranged in a lattice and immersed in electrons. Deeper in the crust, neutrons begin to exist outside nuclei. Near the crust–core boundary, some models predict elongated or sheet-like nuclear structures, collectively called nuclear pasta. (pmc.ncbi.nlm.nih.gov)
The outer core is commonly modelled as mainly neutrons, with smaller proportions of protons and leptons. At greater depths, the composition remains uncertain: proposed possibilities include additional strongly interacting particles or matter containing deconfined quarks. Such possibilities are theoretical alternatives, not established descriptions of every neutron star. Their consequences are tested through the relationship between pressure, density, and composition known as the equation of state. (science.nasa.gov)
Support against collapse includes degeneracy pressure, associated with the Pauli exclusion principle, together with contributions from the strong interaction. Neutron-star structure therefore cannot be adequately described as an ideal gas of noninteracting neutrons. Gravity must also be treated using general relativity. For a specified equation of state, relativistic equilibrium calculations predict a mass–radius relation and a maximum mass for a cold, nonrotating star. This maximum is not a single precisely established universal number. (heasarc.gsfc.nasa.gov)
Pulsars and magnetars
A pulsar is a rotating neutron star observed through regularly recurring emission. Radiation associated with its magnetic environment or surface passes through the observer’s line of sight as the star rotates, producing a lighthouse-like signal. Pulsars were discovered in 1967 through radio observations by Jocelyn Bell Burnell. Their measured pulses reveal rotation periods and changes in rotation over time. (nasa.gov)
Rotation-powered pulsars gradually lose rotational energy. In a binary system, however, accretion of matter from a companion can transfer angular momentum to a neutron star and increase its rotation rate. This recycling process explains many millisecond pulsars, which complete a rotation in only a few thousandths of a second. Accreting neutron stars can also appear as luminous X-ray pulsars. (fermi.gsfc.nasa.gov)
A magnetar is a neutron star distinguished by an exceptionally strong magnetic field and associated high-energy activity. Magnetars can produce powerful bursts and occasional giant flares. The terms pulsar and magnetar are not mutually exclusive: one describes observable pulsations, while the other identifies a strongly magnetized type of neutron star. (science.nasa.gov)
Cooling and internal motion
Young neutron stars lose much of their thermal energy through neutrino emission from their interiors; radiation from the surface also contributes to cooling. Surface X-ray observations, interpreted using atmosphere and thermal-evolution models, can constrain interior composition and heat transport. Cooling rates depend on which particle reactions are possible and on whether particles form paired states. (arxiv.org)
Theoretical models predict neutron superfluidity and, in appropriate regions, proton superconductivity. Superfluid components may rotate differently from the crust. Sudden increases in a pulsar’s rotation rate, called glitches, are commonly interpreted through angular-momentum transfer between internal components, although the detailed mechanisms remain under investigation. (arxiv.org)
Measurements and mergers
Neutron-star masses can be constrained through binary orbital observations, while radii require more indirect inference. NASA’s NICER instrument measures variations in surface X-ray emission as stars rotate. Models account for relativistic light bending, which allows some radiation from the far side to reach an observer. Combining these measurements with mass estimates constrains dense-matter equations of state. (nasa.gov)
Binary neutron stars can spiral together as their orbit loses energy through gravitational waves. On August 17, 2017, LIGO and Virgo detected GW170817, the first gravitational-wave observation of a binary neutron-star inspiral. Associated electromagnetic observations established a major example of multi-messenger astronomy. (ligo.org)
Material expelled during a merger can undergo rapid neutron capture, a form of nucleosynthesis that produces heavy elements. Radioactive decay of the ejecta powers a transient glow called a kilonova. Observations of GW170817 supported this production mechanism, and subsequent analysis identified strontium in its ejecta. The gravitational-wave signal also carries information about tidal deformation, providing an independent constraint on neutron-star structure. (arxiv.org)