Una estrella de neutrones es un tipo de remanente estelar resultante del colapso gravitacional de una estrella supergigante masiva después de agotar el combustible en su núcleo y explotar como una supernova tipo II, tipo Ib o tipo Ic. Como su nombre lo indica, estas estrellas están compuestas principalmente de neutrones, más otro tipo de partículas tanto en su corteza sólida de hierro, como en su interior, que puede contener tanto protones y electrones, como piones y kaones. Las estrellas de neutrones son muy calientes y se apoyan en contra de un mayor colapso mediante presión de degeneración cuántica, debido al fenómeno descrito por el principio de exclusión de Pauli. Este principio establece que dos neutrones (o cualquier otra partícula fermiónica) no pueden ocupar el mismo espacio y estado cuántico simultáneamente. Una estrella de neutrones típica tiene una masa entre 1,35 y 2,1 masas solares, con un radio correspondiente aproximado de 12 km. En cambio, el radio del Sol es de unas 60 000 veces esa cifra. Las estrellas de neutrones tienen densidades totales de 3,7×1017 a 5,9×1017 kg/m3 (de 2,6×1014 a 4,1×1014 veces la densidad del Sol), lo que se compara con la densidad aproximada de un núcleo atómico de 3×1017 kg/m3. La densidad de una estrella de neutrones varía desde menos de 1×109 kg/m3 en la corteza, aumentando con la profundidad a más de 6×1017 u 8×1017 kg/m3 aún más adentro (más denso que un núcleo atómico). Esta densidad equivale aproximadamente a la masa de un Boeing 747 comprimido en el tamaño de un pequeño grano de arena. En general, estrellas compactas de menos de 1,44 masas solares —el límite de Chandrasekhar— son enanas blancas, y por encima de 2 a 3 masas solares —el límite de Tolman-Oppenheimer-Volkoff— puede crearse una estrella de quarks; no obstante, esto es incierto. El colapso gravitatorio generalmente ocurre en cualquier estrella compacta de entre 10 a 25 masas solares, y producirá un agujero negro. Algunas estrellas de neutrones giran rápidamente y emiten rayos de radiación electromagnética como púlsares.

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A neutron star is the collapsed core of a large star (10–29 solar masses). Neutron stars are the smallest and densest stars known to exist. With a radius on the order of 10 km, they can, however, have a mass of about twice that of the Sun. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past the white dwarf star density to that of atomic nuclei. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons, which are subatomic particles with no net electrical charge and with slightly larger mass than protons. They are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle. If the remnant has too great a density, something which occurs in excess of an upper limit of the size of neutron stars at 2-3 solar masses, it will continue collapsing to form a black hole. Neutron stars that can be observed are very hot and typically have a surface temperature around 6×105 K. They are so dense that a normal-sized matchbox containing neutron-star material would have a mass of approximately 13 million tonnes, or a 2.5 million m3 chunk of the Earth (a cube with edges of about 135 metres). They have strong magnetic fields, between 108 and 1015 times that of Earth's. The gravitational field at the neutron star's surface is about 2×1011 times that of the Earth's. As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. Indeed, the discovery of pulsars in 1967 was the first observational suggestion that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space (the so-called "lighthouse effect"). The fastest rotation rate for a neutron star was a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.165 c. There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, most are old and cold, and neutron stars can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are virtually undetectable; however, since the Hubble Space Telescope detection of RX J185635-3754, a few nearby neutron stars that appear only to emit thermal radiation have been detected. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them. Neutron stars in binary systems can undergo accretion which typically makes the system bright in x-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and potentially cause them to gain mass and spin-up to very fast rotation rates, forming the so-called millisecond pulsars. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. Though as of 2016 no direct detection of the gravitational waves from such an event has been made, gravitational waves have been indirectly detected in a system where two neutron stars orbit each other.

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