Neutron Stars are the most dense category of stars that form when a star collapses in on itself. These stars have 1.4 solar masses (1.4 times larger than our sun) and have a radius of 6.2 miles. The reason they have the name "Neutron Stars" is also because it is believed that they are comprised of mostly neutrons.Neutron Stars also lose the ability to generate heat because of how tightly packed the particles are against each other. Despite this surface cooling, it may still be very hot due through further collisions with other stars or an accretion (a gravitationally bound disk of particles).

What is a Neutron Star?

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A neutron star is the <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_collapse" title="Gravitational collapse">collapsed </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_structure" title="Stellar structure">core </a>of a massive <a dir="ltr" href="https://en.wikipedia.org/wiki/Supergiant_star" title="Supergiant star">supergiant star </a>, which had a total <a dir="ltr" href="https://en.wikipedia.org/wiki/Mass" title="Mass">mass </a>of between 10 and 25 <a dir="ltr" href="https://en.wikipedia.org/wiki/Solar_mass" title="Solar mass">solar masses </a>, possibly more if the star was especially metal-rich.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-1">[1] </a> Except for <a dir="ltr" href="https://en.wikipedia.org/wiki/Black_hole" title="Black hole">black holes </a>, and some <a dir="ltr" href="https://en.wikipedia.org/wiki/Hypothetical" title="Hypothetical">hypothetical </a>objects (e.g. <a dir="ltr" href="https://en.wikipedia.org/wiki/White_hole" title="White hole">white holes </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Quark_star" title="Quark star">quark stars </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Strange_star" title="Strange star">strange stars </a>), neutron stars are the smallest and densest currently known class of stellar objects.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-2">[2] </a> Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass of about 1.4 solar masses.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-3">[3] </a> They result from the <a dir="ltr" href="https://en.wikipedia.org/wiki/Supernova" title="Supernova">supernova </a>explosion of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_evolution#Massive_star" title="Stellar evolution">massive star </a>, combined with <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_collapse" title="Gravitational collapse">gravitational collapse </a>, that compresses the core past <a dir="ltr" href="https://en.wikipedia.org/wiki/White_dwarf" title="White dwarf">white dwarf </a>star density to that of <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">atomic nuclei </a>.

Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through <a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_collision" title="Stellar collision">collision </a>or <a dir="ltr" href="https://en.wikipedia.org/wiki/Accretion_(astrophysics)" title="Accretion (astrophysics)">accretion </a>. Most of the basic models for these objects imply that neutron stars are composed almost entirely of <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron" title="Neutron">neutrons </a>(subatomic particles with no net <a dir="ltr" href="https://en.wikipedia.org/wiki/Electrical_charge" title="Electrical charge">electrical charge </a>and with slightly larger mass than <a dir="ltr" href="https://en.wikipedia.org/wiki/Proton" title="Proton">protons </a>); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are partially supported against further collapse by <a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy" title="Degenerate matter">neutron degeneracy pressure </a>, a phenomenon described by the <a dir="ltr" href="https://en.wikipedia.org/wiki/Pauli_exclusion_principle" title="Pauli exclusion principle">Pauli exclusion principle </a>, just as white dwarfs are supported against collapse by <a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_matter#Electron_degeneracy" title="Degenerate matter">electron degeneracy pressure </a>. However, neutron degeneracy pressure is not by itself sufficient to hold up an object beyond 0.7<a dir="ltr" href="https://en.wikipedia.org/wiki/Solar_mass" title="Solar mass"><var dir="ltr">M </var>☉ </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-4">[4] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-5">[5] </a> and repulsive nuclear forces play a larger role in supporting more massive neutron stars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-6">[6] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-7">[7] </a> If the remnant star has a <a dir="ltr" href="https://en.wikipedia.org/wiki/Mass" title="Mass">mass </a>exceeding the <a dir="ltr" href="https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit" title="Tolman–Oppenheimer–Volkoff limit">Tolman–Oppenheimer–Volkoff limit </a>of around 2 solar masses, the combination of degeneracy pressure and nuclear forces is insufficient to support the neutron star and it continues collapsing to form a <a dir="ltr" href="https://en.wikipedia.org/wiki/Black_hole" title="Black hole">black hole </a>. The most massive neutron star detected so far, <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_J0740%2B6620" title="PSR J0740+6620">PSR J0740+6620 </a>, is estimated to be 2.14 solar masses.

Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000  K .<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Kiziltan-8">[8] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-9">[9] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-10">[10] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Haensel-11">[11] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-12">[a] </a> They are so dense that a normal-sized <a dir="ltr" href="https://en.wikipedia.org/wiki/Phillumeny#Matchbox" title="Phillumeny">matchbox </a>containing neutron-star material would have a weight of approximately 3 <a dir="ltr" href="https://en.wikipedia.org/wiki/Billion" title="Billion">billion </a>tonnes, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres) from Earth's surface.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-13">[12] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-14">[13] </a> Their <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetic_field" title="Magnetic field">magnetic fields </a>are between 108 and 1015 (100 <a dir="ltr" href="https://en.wikipedia.org/wiki/Million" title="Million">million </a>to 1 <a dir="ltr" href="https://en.wikipedia.org/wiki/Quadrillion" title="Quadrillion">quadrillion </a>) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about 2× 1011 (200 billion) times that of Earth's gravitational field.

As the star's core collapses, its rotation rate increases as a result of <a dir="ltr" href="https://en.wikipedia.org/wiki/Conservation_of_angular_momentum" title="Conservation of angular momentum">conservation of angular momentum </a>, and newly formed neutron stars hence rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar" title="Pulsar">pulsars </a>. Indeed, the discovery of pulsars by <a dir="ltr" href="https://en.wikipedia.org/wiki/Jocelyn_Bell_Burnell" title="Jocelyn Bell Burnell">Jocelyn Bell Burnell </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Antony_Hewish" title="Antony Hewish">Antony Hewish </a>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-spinning neutron star known is <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_J1748-2446ad" title="PSR J1748-2446ad">PSR J1748-2446ad </a>, rotating at a rate of 716 times a second<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-15">[14] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-16">[15] </a> or 43,000 <a dir="ltr" href="https://en.wikipedia.org/wiki/Revolutions_per_minute" title="Revolutions per minute">revolutions per minute </a>, giving a linear speed at the surface on the order of 0.24 <a dir="ltr" href="https://en.wikipedia.org/wiki/Speed_of_light" title="Speed of light">c </a> (i.e., nearly a quarter the <a dir="ltr" href="https://en.wikipedia.org/wiki/Speed_of_light" title="Speed of light">speed of light </a>).

There are thought to be around one billion neutron stars in the <a dir="ltr" href="https://en.wikipedia.org/wiki/Milky_Way" title="Milky Way">Milky Way </a>,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-17">[16] </a> and at a minimum several hundred million, a figure obtained by estimating the number of stars that have undergone supernova explosions.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-18">[17] </a> However, most are old and cold and radiate very little; most neutron stars that have been detected occur only in certain situations in which they do radiate, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are almost undetectable; however, since the <a dir="ltr" href="https://en.wikipedia.org/wiki/Hubble_Space_Telescope" title="Hubble Space Telescope">Hubble Space Telescope </a> detection of <a dir="ltr" href="https://en.wikipedia.org/wiki/RX_J185635%E2%88%923754" title="RX J185635−3754">RX J185635−3754 </a>in the 1990s, a few nearby neutron stars that appear to emit only thermal radiation have been detected. <a dir="ltr" href="https://en.wikipedia.org/wiki/Soft_gamma_repeater" title="Soft gamma repeater">Soft gamma repeaters </a>are conjectured to be a type of neutron star with very strong magnetic fields, known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetar" title="Magnetar">magnetars </a>, or alternatively, neutron stars with fossil disks around them.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-19">[18] </a>

Neutron stars in binary systems can undergo <a dir="ltr" href="https://en.wikipedia.org/wiki/Accretion_(astrophysics)" title="Accretion (astrophysics)">accretion </a>which typically makes the system bright in <a dir="ltr" href="https://en.wikipedia.org/wiki/X-ray" title="X-ray">X-rays </a>while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified <a dir="ltr" href="https://en.wikipedia.org/wiki/X-ray_pulsar" title="X-ray pulsar">X-ray pulsar </a>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 <a dir="ltr" href="https://en.wikipedia.org/wiki/Millisecond_pulsar" title="Millisecond pulsar">millisecond pulsars </a>. These binary systems will continue to <a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_evolution" title="Stellar evolution">evolve </a>, and eventually the companions can become <a dir="ltr" href="https://en.wikipedia.org/wiki/Compact_star" title="Compact star">compact objects </a>such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through <a dir="ltr" href="https://en.wikipedia.org/wiki/Ablation" title="Ablation">ablation </a>or merger. The merger of binary neutron stars may be the source of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gamma-ray_burst" title="Gamma-ray burst">short-duration gamma-ray bursts </a>and are likely strong sources of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_wave" title="Gravitational wave">gravitational waves </a>. In 2017, a direct detection (<a dir="ltr" href="https://en.wikipedia.org/wiki/GW170817" title="GW170817">GW170817 </a>) of the gravitational waves from such an event was observed,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-20">[19] </a> and gravitational waves have also been indirectly observed in a <a dir="ltr" href="https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_binary" title="Hulse–Taylor binary">system where two neutron stars orbit each other </a>.

Any <a dir="ltr" href="https://en.wikipedia.org/wiki/Main_sequence" title="Main sequence">main-sequence </a>star with an initial mass of above 8 times the mass of the sun (8 <a dir="ltr" href="https://en.wikipedia.org/wiki/Solar_mass" title="Solar mass"><var dir="ltr">M </var>☉ </a>) has the potential to produce a neutron star. As the star evolves away from the main sequence, subsequent nuclear burning produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of mass from shell burning cause the core to exceed the <a dir="ltr" href="https://en.wikipedia.org/wiki/Chandrasekhar_limit" title="Chandrasekhar limit">Chandrasekhar limit </a>. Electron-degeneracy pressure is overcome and the core collapses further, sending temperatures soaring to over 5× 109  K . At these temperatures, <a dir="ltr" href="https://en.wikipedia.org/wiki/Photodisintegration" title="Photodisintegration">photodisintegration </a>(the breaking up of iron nuclei into <a dir="ltr" href="https://en.wikipedia.org/wiki/Alpha_particle" title="Alpha particle">alpha particles </a>by high-energy gamma rays) occurs. As the temperature climbs even higher, electrons and protons combine to form neutrons via <a dir="ltr" href="https://en.wikipedia.org/wiki/Electron_capture" title="Electron capture">electron capture </a>, releasing a flood of <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutrino" title="Neutrino">neutrinos </a>. When densities reach nuclear density of 4× 1017  kg/m3 , a combination of <a dir="ltr" href="https://en.wikipedia.org/wiki/Strong_force" title="Strong force">strong force </a>repulsion and neutron degeneracy pressure halts the contraction.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-21">[20] </a> The infalling outer envelope of the star is halted and flung outwards by a flux of neutrinos produced in the creation of the neutrons, becoming a supernova. The remnant left is a neutron star. If the remnant has a mass greater than about 3 <var dir="ltr">M </var>☉ , it collapses further to become a black hole.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-22">[21] </a>

As the core of a massive star is compressed during a <a dir="ltr" href="https://en.wikipedia.org/wiki/Type_II_supernova" title="Type II supernova">Type II supernova </a>or a <a dir="ltr" href="https://en.wikipedia.org/wiki/Type_Ib_and_Ic_supernovae" title="Type Ib and Ic supernovae">Type Ib or Type Ic </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Supernova" title="Supernova">supernova </a>, and collapses into a neutron star, it retains most of its <a dir="ltr" href="https://en.wikipedia.org/wiki/Angular_momentum" title="Angular momentum">angular momentum </a>. But, because it has only a tiny fraction of its parent's radius (and therefore its <a dir="ltr" href="https://en.wikipedia.org/wiki/Moment_of_inertia" title="Moment of inertia">moment of inertia </a>is sharply reduced), a neutron star is formed with very high rotation speed, and then over a very long period it slows. Neutron stars are known that have rotation periods from about 1.4 ms to 30 s. The neutron star's density also gives it very high <a dir="ltr" href="https://en.wikipedia.org/wiki/Surface_gravity" title="Surface gravity">surface gravity </a>, with typical values ranging from 1012 to 1013  m/s2 (more than 1011 times that of <a dir="ltr" href="https://en.wikipedia.org/wiki/Earth" title="Earth">Earth </a>).<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Haensel-11">[11] </a> One measure of such immense gravity is the fact that neutron stars have an <a dir="ltr" href="https://en.wikipedia.org/wiki/Escape_velocity" title="Escape velocity">escape velocity </a>ranging from <a dir="ltr" href="https://en.wikipedia.org/wiki/Orders_of_magnitude_(speed)#8" title="Orders of magnitude (speed)">100,000 km/s </a>to <a dir="ltr" href="https://en.wikipedia.org/wiki/Orders_of_magnitude_(speed)#8" title="Orders of magnitude (speed)">150,000 km/s </a>, that is, from a third to half the <a dir="ltr" href="https://en.wikipedia.org/wiki/Speed_of_light" title="Speed of light">speed of light </a>. The neutron star's gravity accelerates infalling matter to tremendous speed. The force of its impact would likely destroy the object's component atoms, rendering all the matter identical, in most respects, to the rest of the neutron star.

A neutron star has a mass of at least 1.1 <a dir="ltr" href="https://en.wikipedia.org/wiki/Solar_mass" title="Solar mass">solar masses </a>(<var dir="ltr">M </var>☉ ). The upper limit of mass for a neutron star is called the <a dir="ltr" href="https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit" title="Tolman–Oppenheimer–Volkoff limit">Tolman–Oppenheimer–Volkoff limit </a>and is generally held to be around 2.1 <var dir="ltr">M </var>☉ ,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-23">[22] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Chamel-24">[23] </a> but a recent estimate puts the upper limit at 2.16 <var dir="ltr">M </var>☉ .<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-25">[24] </a> The maximum observed mass of neutron stars is about 2.14 <var dir="ltr">M </var>☉ for <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_J0740%2B6620" title="PSR J0740+6620">PSR J0740+6620 </a>discovered in September, 2019.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-26">[25] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Compact_star" title="Compact star">Compact stars </a>below the <a dir="ltr" href="https://en.wikipedia.org/wiki/Chandrasekhar_limit" title="Chandrasekhar limit">Chandrasekhar limit </a>of 1.39 <var dir="ltr">M </var>☉ are generally <a dir="ltr" href="https://en.wikipedia.org/wiki/White_dwarf" title="White dwarf">white dwarfs </a>whereas compact stars with a mass between 1.4 <var dir="ltr">M </var>☉ and 2.16 <var dir="ltr">M </var>☉ are expected to be neutron stars, but there is an interval of a few tenths of a solar mass where the masses of low-mass neutron stars and high-mass white dwarfs can overlap. It is thought that beyond 2.16 <var dir="ltr">M </var>☉ the stellar remnant will overcome the strong force repulsion and <a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy" title="Degenerate matter">neutron degeneracy pressure </a>so that <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_collapse" title="Gravitational collapse">gravitational collapse </a>will occur to produce a black hole, but the smallest observed mass of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_black_hole" title="Stellar black hole">stellar black hole </a>is about 5 <var dir="ltr">M </var>☉ .<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Black_Hole-28">[b] </a> Between 2.16 <var dir="ltr">M </var>☉ and 5 <var dir="ltr">M </var>☉ , hypothetical intermediate-mass stars such as <a dir="ltr" href="https://en.wikipedia.org/wiki/Quark_star" title="Quark star">quark stars </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Electroweak_star" title="Electroweak star">electroweak stars </a>have been proposed, but none have been shown to exist.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Black_Hole-28">[b] </a>

The temperature inside a newly formed neutron star is from around <a dir="ltr" href="https://en.wikipedia.org/wiki/Orders_of_magnitude_(numbers)#1011" title="Orders of magnitude (numbers)">1011 </a>to <a dir="ltr" href="https://en.wikipedia.org/wiki/10%5E12" title="10^12">1012 </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Kelvin" title="Kelvin">kelvins </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Miller-29">[27] </a> However, the huge number of <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutrino" title="Neutrino">neutrinos </a>it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around <a dir="ltr" href="https://en.wikipedia.org/wiki/Orders_of_magnitude_(numbers)#106" title="Orders of magnitude (numbers)">106 </a>kelvins.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Miller-29">[27] </a> At this lower temperature, most of the light generated by a neutron star is in X-rays.

Some researchers have proposed a neutron star classification system using <a dir="ltr" href="https://en.wikipedia.org/wiki/Roman_numerals" title="Roman numerals">Roman numerals </a>(not to be confused with the <a dir="ltr" href="https://en.wikipedia.org/wiki/Stellar_classification" title="Stellar classification">Yerkes luminosity classes </a>for non-degenerate stars) to sort neutron stars by their mass and cooling rates: type I for neutron stars with low mass and cooling rates, type II for neutron stars with higher mass and cooling rates, and a proposed type III for neutron stars with even higher mass, approaching 2 <var dir="ltr">M </var>☉ , and with higher cooling rates and possibly candidates for <a dir="ltr" href="https://en.wikipedia.org/wiki/Exotic_star" title="Exotic star">exotic stars </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-30">[28] </a>

Neutron stars have overall densities of 3.7× 1017 to 5.9× 1017  kg/m3 (2.6× 1014 to 4.1× 1014 times the density of the Sun),<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-31">[c] </a> which is comparable to the approximate density of an <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">atomic nucleus </a>of 3× 1017  kg/m3 .<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-32">[29] </a> The neutron star's density varies from about 1× 109  kg/m3 in the crust—increasing with depth—to about 6× 1017 or 8× 1017  kg/m3 (denser than an atomic nucleus) deeper inside.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Miller-29">[27] </a> A neutron star is so dense that one teaspoon (5 <a dir="ltr" href="https://en.wikipedia.org/wiki/Milliliter" title="Milliliter">milliliters </a>) of its material would have a mass over 5.5× 1012  kg , about 900 times the mass of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Great_Pyramid_of_Giza" title="Great Pyramid of Giza">Great Pyramid of Giza </a>. In the enormous gravitational field of a neutron star, that teaspoon of material would <a dir="ltr" href="https://en.wikipedia.org/wiki/Weight" title="Weight">weigh </a>1.1× 1025  N , which is 15 times what the <a dir="ltr" href="https://en.wikipedia.org/wiki/Moon" title="Moon">Moon </a>would weigh if it were placed on the surface of the Earth.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-33">[d] </a> The entire mass of the Earth at neutron star density would fit into a sphere of 305 m in diameter (the size of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Arecibo_Telescope" title="Arecibo Telescope">Arecibo Telescope </a>). The pressure increases from 3.2× 1031 to 1.6× 1034  Pa from the inner crust to the center.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-34">[30] </a>

The <a dir="ltr" href="https://en.wikipedia.org/wiki/Equation_of_state" title="Equation of state">equation of state </a>of matter at such high densities is not precisely known because of the theoretical difficulties associated with extrapolating the likely behavior of <a dir="ltr" href="https://en.wikipedia.org/wiki/Quantum_chromodynamics" title="Quantum chromodynamics">quantum chromodynamics </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Superconductivity" title="Superconductivity">superconductivity </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Superfluidity" title="Superfluidity">superfluidity </a>of matter in such states. The problem is exacerbated by the empirical difficulties of observing the characteristics of any object that is hundreds of <a dir="ltr" href="https://en.wikipedia.org/wiki/Parsec" title="Parsec">parsecs </a>away, or farther.

A neutron star has some of the properties of an <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">atomic nucleus </a>, including density (within an order of magnitude) and being composed of <a dir="ltr" href="https://en.wikipedia.org/wiki/Nucleon" title="Nucleon">nucleons </a>. In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the <a dir="ltr" href="https://en.wikipedia.org/wiki/Strong_interaction" title="Strong interaction">strong interaction </a>, whereas a neutron star is held together by <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravity" title="Gravity">gravity </a>. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities.

The magnetic field strength on the surface of neutron stars ranges from <abbr dir="ltr" title="circa">c. </abbr> 104 to 1011  <a dir="ltr" href="https://en.wikipedia.org/wiki/Tesla_(unit)" title="Tesla (unit)">tesla </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-reisenegger-35">[31] </a> These are orders of magnitude higher than in any other object: For comparison, a continuous 16 T field has been achieved in the laboratory and is sufficient to levitate a living frog due to <a dir="ltr" href="https://en.wikipedia.org/wiki/Diamagnetic_levitation" title="Diamagnetic levitation">diamagnetic levitation </a>. Variations in magnetic field strengths are most likely the main factor that allows different types of neutron stars to be distinguished by their spectra, and explains the periodicity of pulsars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-reisenegger-35">[31] </a>

The neutron stars known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetar" title="Magnetar">magnetars </a>have the strongest magnetic fields, in the range of 108 to 1011  tesla,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-mcgill-36">[32] </a> and have become the widely accepted hypothesis for neutron star types <a dir="ltr" href="https://en.wikipedia.org/wiki/Soft_gamma_repeater" title="Soft gamma repeater">soft gamma repeaters </a>(SGRs)<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-sa-37">[33] </a> and <a dir="ltr" href="https://en.wikipedia.org/wiki/Anomalous_X-ray_pulsar" title="Anomalous X-ray pulsar">anomalous X-ray pulsars </a>(AXPs).<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-38">[34] </a> The magnetic <a dir="ltr" href="https://en.wikipedia.org/wiki/Energy_density" title="Energy density">energy density </a>of a 108  T field is extreme, greatly exceeding the <a dir="ltr" href="https://en.wikipedia.org/wiki/Theoretical_total_mass-energy" title="Theoretical total mass-energy">mass-energy </a>density of ordinary matter.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-40">[e] </a> Fields of this strength are able to <a dir="ltr" href="https://en.wikipedia.org/wiki/Vacuum_polarization" title="Vacuum polarization">polarize the vacuum </a>to the point that the vacuum becomes <a dir="ltr" href="https://en.wikipedia.org/wiki/Birefringent" title="Birefringent">birefringent </a>. Photons can merge or split in two, and virtual particle-antiparticle pairs are produced. The field changes electron energy levels and atoms are forced into thin cylinders. Unlike in an ordinary pulsar, magnetar spin-down can be directly powered by its magnetic field, and the magnetic field is strong enough to stress the crust to the point of fracture. Fractures of the crust cause <a dir="ltr" href="https://en.wikipedia.org/wiki/Starquake_(astrophysics)#Starquake" title="Starquake (astrophysics)">starquakes </a>, observed as extremely luminous millisecond hard gamma ray bursts. The fireball is trapped by the magnetic field, and comes in and out of view when the star rotates, which is observed as a periodic soft gamma repeater (SGR) emission with a period of 5–8 seconds and which lasts for a few minutes.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-41">[36] </a>

The origins of the strong magnetic field are as yet unclear.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-reisenegger-35">[31] </a> One hypothesis is that of "flux freezing", or conservation of the original <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetic_flux" title="Magnetic flux">magnetic flux </a>during the formation of the neutron star.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-reisenegger-35">[31] </a> If an object has a certain magnetic flux over its surface area, and that area shrinks to a smaller area, but the magnetic flux is conserved, then the <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetic_field" title="Magnetic field">magnetic field </a>would correspondingly increase. Likewise, a collapsing star begins with a much larger surface area than the resulting neutron star, and conservation of magnetic flux would result in a far stronger magnetic field. However, this simple explanation does not fully explain magnetic field strengths of neutron stars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-reisenegger-35">[31] </a>

The gravitational field at a neutron star's surface is about 2× 1011 times <a dir="ltr" href="https://en.wikipedia.org/wiki/Standard_gravity" title="Standard gravity">stronger than on Earth </a>, at around 2.0× 1012  m/s2 .<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-43">[38] </a> Such a strong gravitational field acts as a <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_lens" title="Gravitational lens">gravitational lens </a>and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Zahn-42">[37] </a> If the radius of the neutron star is 3GM /c 2 or less, then the photons may be <a dir="ltr" href="https://en.wikipedia.org/wiki/Photon_sphere" title="Photon sphere">trapped in an orbit </a>, thus making the whole surface of that neutron star visible from a single vantage point , along with destabilizing photon orbits at or below the 1 radius distance of the star.

A fraction of the mass of a star that collapses to form a neutron star is released in the supernova explosion from which it forms (from the law of mass–energy equivalence, E = mc 2 ). The energy comes from the <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_binding_energy" title="Gravitational binding energy">gravitational binding energy </a>of a neutron star.

Hence, the gravitational force of a typical neutron star is huge. If an object were to fall from a height of one meter on a neutron star 12 kilometers in radius, it would reach the ground at around 1400 kilometers per second.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-44">[39] </a> However, even before impact, the <a dir="ltr" href="https://en.wikipedia.org/wiki/Tidal_force" title="Tidal force">tidal force </a>would cause <a dir="ltr" href="https://en.wikipedia.org/wiki/Spaghettification" title="Spaghettification">spaghettification </a>, breaking any sort of an ordinary object into a stream of material.

Because of the enormous gravity, <a dir="ltr" href="https://en.wikipedia.org/wiki/Time_dilation" title="Time dilation">time dilation </a>between a neutron star and Earth is significant. For example, eight years could pass on the surface of a neutron star, yet ten years would have passed on Earth, not including the time-dilation effect of the star's very rapid rotation.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-45">[40] </a>

Neutron star relativistic equations of state describe the relation of radius vs. mass for various models.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-46">[41] </a> The most likely radii for a given neutron star mass are bracketed by models AP4 (smallest radius) and MS2 (largest radius). BE is the ratio of gravitational binding energy mass equivalent to the observed neutron star gravitational mass of "M" kilograms with radius "R" meters,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-47">[42] </a>

and star masses "M" commonly reported as multiples of one solar mass,

then the relativistic fractional binding energy of a neutron star is

A 2 <var dir="ltr">M </var>☉ neutron star would not be more compact than 10,970 meters radius (AP4 model). Its mass fraction gravitational binding energy would then be 0.187, −18.7% (exothermic). This is not near 0.6/2 = 0.3, −30%.

The <a dir="ltr" href="https://en.wikipedia.org/wiki/Equation_of_state" title="Equation of state">equation of state </a>for a neutron star is not yet known. It is assumed that it differs significantly from that of a white dwarf, whose equation of state is that of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_gas" title="Degenerate gas">degenerate gas </a>that can be described in close agreement with <a dir="ltr" href="https://en.wikipedia.org/wiki/Special_relativity" title="Special relativity">special relativity </a>. However, with a neutron star the increased effects of general relativity can no longer be ignored. Several equations of state have been proposed (FPS, UU, APR, L, SLy, and others) and current research is still attempting to constrain the theories to make predictions of neutron star matter.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Haensel-11">[11] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-NASAEOS-49">[44] </a> This means that the relation between density and mass is not fully known, and this causes uncertainties in radius estimates. For example, a 1.5 <var dir="ltr">M </var>☉ neutron star could have a radius of 10.7, 11.1, 12.1 or 15.1 kilometers (for EOS FPS, UU, APR or L respectively).<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-NASAEOS-49">[44] </a>

Current understanding of the structure of neutron stars is defined by existing mathematical models, but it might be possible to infer some details through studies of <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron-star_oscillations" title="Neutron-star oscillations">neutron-star oscillations </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/Asteroseismology" title="Asteroseismology">Asteroseismology </a>, a study applied to ordinary stars, can reveal the inner structure of neutron stars by analyzing observed <a dir="ltr" href="https://en.wikipedia.org/wiki/Frequency_spectrum" title="Frequency spectrum">spectra </a>of stellar oscillations.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Haensel-11">[11] </a>

Current models indicate that matter at the surface of a neutron star is composed of ordinary <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">atomic nuclei </a>crushed into a solid lattice with a sea of <a dir="ltr" href="https://en.wikipedia.org/wiki/Electron" title="Electron">electrons </a>flowing through the gaps between them. It is possible that the nuclei at the surface are <a dir="ltr" href="https://en.wikipedia.org/wiki/Iron" title="Iron">iron </a>, due to iron's high <a dir="ltr" href="https://en.wikipedia.org/wiki/Binding_energy" title="Binding energy">binding energy </a>per nucleon.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Surface-50">[45] </a> It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like <a dir="ltr" href="https://en.wikipedia.org/wiki/Helium" title="Helium">helium </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Hydrogen" title="Hydrogen">hydrogen </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Surface-50">[45] </a> If the surface temperature exceeds 106 kelvins (as in the case of a young pulsar), the surface should be fluid instead of the solid phase that might exist in cooler neutron stars (temperature &lt;106 kelvins).<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Surface-50">[45] </a>

The "atmosphere" of a neutron star is hypothesized to be at most several micrometres thick, and its dynamics are fully controlled by the neutron star's magnetic field. Below the atmosphere one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities on the order of millimetres or less), due to the extreme gravitational field.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-51">[46] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-mt-ls-52">[47] </a>

Proceeding inward, one encounters nuclei with ever-increasing numbers of neutrons; such nuclei would decay quickly on Earth, but are kept stable by tremendous pressures. As this process continues at increasing depths, the <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_drip_line" title="Neutron drip line">neutron drip </a>becomes overwhelming, and the concentration of free neutrons increases rapidly. In that region, there are nuclei, free electrons, and free neutrons. The nuclei become increasingly small (gravity and pressure overwhelming the <a dir="ltr" href="https://en.wikipedia.org/wiki/Strong_force" title="Strong force">strong force </a>) until the core is reached, by definition the point where mostly neutrons exist. The expected hierarchy of phases of nuclear matter in the inner crust has been characterized as "<a dir="ltr" href="https://en.wikipedia.org/wiki/Nuclear_pasta" title="Nuclear pasta">nuclear pasta </a>", with fewer voids and larger structures towards higher pressures.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-53">[48] </a> The composition of the superdense matter in the core remains uncertain. One model describes the core as <a dir="ltr" href="https://en.wikipedia.org/wiki/Superfluid" title="Superfluid">superfluid </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy" title="Degenerate matter">neutron-degenerate matter </a>(mostly neutrons, with some protons and electrons). More exotic forms of matter are possible, including degenerate <a dir="ltr" href="https://en.wikipedia.org/wiki/Strange_matter" title="Strange matter">strange matter </a>(containing <a dir="ltr" href="https://en.wikipedia.org/wiki/Strange_quark" title="Strange quark">strange quarks </a>in addition to <a dir="ltr" href="https://en.wikipedia.org/wiki/Up_quark" title="Up quark">up </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Down_quark" title="Down quark">down quarks </a>), matter containing high-energy <a dir="ltr" href="https://en.wikipedia.org/wiki/Pion" title="Pion">pions </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Kaon" title="Kaon">kaons </a>in addition to neutrons,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-Haensel-11">[11] </a> or ultra-dense <a dir="ltr" href="https://en.wikipedia.org/wiki/Degenerate_matter#Quark_degeneracy" title="Degenerate matter">quark-degenerate matter </a>.

Neutron stars are detected from their <a dir="ltr" href="https://en.wikipedia.org/wiki/Electromagnetic_radiation" title="Electromagnetic radiation">electromagnetic radiation </a>. Neutron stars are usually observed to <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulse_(physics)" title="Pulse (physics)">pulse </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Radio_wave" title="Radio wave">radio waves </a>and other electromagnetic radiation, and neutron stars observed with pulses are called <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar" title="Pulsar">pulsars </a>.

Pulsars' radiation is thought to be caused by particle acceleration near their <a dir="ltr" href="https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies#Magnetic_poles" title="Poles of astronomical bodies">magnetic poles </a>, which need not be aligned with the <a dir="ltr" href="https://en.wikipedia.org/wiki/Axis_of_rotation" title="Axis of rotation">rotational axis </a>of the neutron star. It is thought that a large <a dir="ltr" href="https://en.wikipedia.org/wiki/Electrostatic_field" title="Electrostatic field">electrostatic field </a>builds up near the magnetic poles, leading to <a dir="ltr" href="https://en.wikipedia.org/wiki/Electron" title="Electron">electron </a>emission.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a> These electrons are magnetically accelerated along the field lines, leading to <a dir="ltr" href="https://en.wikipedia.org/wiki/Curvature_radiation" title="Curvature radiation">curvature radiation </a>, with the radiation being strongly <a dir="ltr" href="https://en.wikipedia.org/wiki/Polarization_(waves)" title="Polarization (waves)">polarized </a>towards the plane of curvature.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a> In addition, high energy <a dir="ltr" href="https://en.wikipedia.org/wiki/Photons" title="Photons">photons </a>can interact with lower energy photons and the magnetic field for <a dir="ltr" href="https://en.wikipedia.org/wiki/Electron%E2%88%92positron_pair_production" title="Electron−positron pair production">electron−positron pair production </a>, which through <a dir="ltr" href="https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation" title="Electron–positron annihilation">electron–positron annihilation </a>leads to further high energy photons.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

The radiation emanating from the magnetic poles of neutron stars can be described as magnetospheric radiation , in reference to the <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetosphere" title="Magnetosphere">magnetosphere </a>of the neutron star.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-pavlov-55">[50] </a> It is not to be confused with <a dir="ltr" href="https://en.wikipedia.org/wiki/Multipole_radiation" title="Multipole radiation">magnetic dipole radiation </a> , which is emitted because the <a dir="ltr" href="https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies#Magnetic_poles" title="Poles of astronomical bodies">magnetic </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis" title="Rotation around a fixed axis">axis </a>is not aligned with the rotational axis, with a radiation frequency the same as the neutron star's rotational frequency.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

If the axis of rotation of the neutron star is different to the magnetic axis, external viewers will only see these beams of radiation whenever the magnetic axis point towards them during the neutron star rotation. Therefore, <a dir="ltr" href="https://en.wikipedia.org/wiki/Periodic_function" title="Periodic function">periodic </a>pulses are observed, at the same rate as the rotation of the neutron star.

In addition to pulsars, non-pulsating neutron stars have also been identified, although they may have minor periodic variation in luminosity.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-cco-56">[51] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-non-pulsating-57">[52] </a> This seems to be a characteristic of the X-ray sources known as Central Compact Objects in <a dir="ltr" href="https://en.wikipedia.org/wiki/Supernova_remnant" title="Supernova remnant">Supernova remnants </a>(CCOs in SNRs), which are thought to be young, radio-quiet isolated neutron stars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-cco-56">[51] </a>

In addition to <a dir="ltr" href="https://en.wikipedia.org/wiki/Radio_wave" title="Radio wave">radio </a>emissions, neutron stars have also been identified in other parts of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Electromagnetic_spectrum" title="Electromagnetic spectrum">electromagnetic spectrum </a>. This includes <a dir="ltr" href="https://en.wikipedia.org/wiki/Light" title="Light">visible light </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Near_infrared" title="Near infrared">near infrared </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Ultraviolet" title="Ultraviolet">ultraviolet </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/X-ray" title="X-ray">X-rays </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Gamma_ray" title="Gamma ray">gamma rays </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-pavlov-55">[50] </a> Pulsars observed in X-rays are known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Accretion-powered_pulsars" title="Accretion-powered pulsars">X-ray pulsars if accretion-powered </a>, while those identified in visible light are known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Optical_pulsar" title="Optical pulsar">optical pulsars </a>. The majority of neutron stars detected, including those identified in optical, X-ray, and gamma rays, also emit radio waves;<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-jb-58">[53] </a> the <a dir="ltr" href="https://en.wikipedia.org/wiki/Crab_Pulsar" title="Crab Pulsar">Crab Pulsar </a>produces electromagnetic emissions across the spectrum.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-jb-58">[53] </a> However, there exist neutron stars called <a dir="ltr" href="https://en.wikipedia.org/wiki/Radio-quiet_neutron_star" title="Radio-quiet neutron star">radio-quiet neutron stars </a>, with no radio emissions detected.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-ras-59">[54] </a>

Neutron stars rotate extremely rapidly after their formation due to the conservation of angular momentum; in analogy to spinning ice skaters pulling in their arms, the slow rotation of the original star's core speeds up as it shrinks. A newborn neutron star can rotate many times a second.

Over time, neutron stars slow, as their rotating magnetic fields in effect radiate energy associated with the rotation; older neutron stars may take several seconds for each revolution. This is called spin down . The rate at which a neutron star slows its rotation is usually constant and very small.

The <a dir="ltr" href="https://en.wikipedia.org/wiki/Periodic_time" title="Periodic time">periodic time </a>(P ) is the <a dir="ltr" href="https://en.wikipedia.org/wiki/Rotation_period" title="Rotation period">rotational period </a>, the time for one rotation of a neutron star. The spin-down rate, the rate of slowing of rotation, is then given the symbol

(P -dot), the <a dir="ltr" href="https://en.wikipedia.org/wiki/Derivative" title="Derivative">derivative </a>of P with respect to time. It is defined as periodic time increase per unit time; it is a <a dir="ltr" href="https://en.wikipedia.org/wiki/Dimensionless_quantity" title="Dimensionless quantity">dimensionless quantity </a>, but can be given the units of s⋅s−1 (seconds per second).<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

The spin-down rate (P -dot) of neutron stars usually falls within the range of 10−22 to 10−9  s⋅s−1 , with the shorter period (or faster rotating) observable neutron stars usually having smaller P -dot. As a neutron star ages, its rotation slows (as P increases); eventually, the rate of rotation will become too slow to power the radio-emission mechanism, and the neutron star can no longer be detected.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

P and P -dot allow minimum magnetic fields of neutron stars to be estimated.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a> P and P -dot can be also used to calculate the characteristic age of a pulsar, but gives an estimate which is somewhat larger than the true age when it is applied to young pulsars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

P and P -dot can also be combined with neutron star's <a dir="ltr" href="https://en.wikipedia.org/wiki/Moment_of_inertia" title="Moment of inertia">moment of inertia </a>to estimate a quantity called spin-down <a dir="ltr" href="https://en.wikipedia.org/wiki/Luminosity" title="Luminosity">luminosity </a> , which is given the symbol

(E -dot). It is not the measured luminosity, but rather the calculated loss rate of rotational energy that would manifest itself as radiation. For neutron stars where the spin-down luminosity is comparable to the actual <a dir="ltr" href="https://en.wikipedia.org/wiki/Luminosity" title="Luminosity">luminosity </a>, the neutron stars are said to be "<a dir="ltr" href="https://en.wikipedia.org/wiki/Rotation_powered_pulsar" title="Rotation powered pulsar">rotation powered </a>".<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-pavlov-55">[50] </a> The observed luminosity of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Crab_Pulsar" title="Crab Pulsar">Crab Pulsar </a>is comparable to the spin-down luminosity, supporting the model that rotational kinetic energy powers the radiation from it.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a> With neutron stars such as <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetar" title="Magnetar">magnetars </a>, where the actual luminosity exceeds the spin-down luminosity by about a factor of one hundred, it is assumed that the luminosity is powered by magnetic dissipation, rather than being rotation powered.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-ufrgs-60">[55] </a>

P and P -dot can also be plotted for neutron stars to create a P –P -dot diagram. It encodes a tremendous amount of information about the pulsar population and its properties, and has been likened to the <a dir="ltr" href="https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram" title="Hertzsprung–Russell diagram">Hertzsprung–Russell diagram </a>in its importance for neutron stars.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nrao-54">[49] </a>

Neutron star rotational speeds can increase, a process known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star_spin-up" title="Neutron star spin-up">spin up </a>. Sometimes neutron stars absorb orbiting matter from companion stars, increasing the rotation rate and reshaping the neutron star into an <a dir="ltr" href="https://en.wikipedia.org/wiki/Oblate_spheroid" title="Oblate spheroid">oblate spheroid </a>. This causes an increase in the rate of rotation of the neutron star of over a hundred times per second in the case of <a dir="ltr" href="https://en.wikipedia.org/wiki/Millisecond_pulsar" title="Millisecond pulsar">millisecond pulsars </a>.

The most rapidly rotating neutron star currently known, <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_J1748-2446ad" title="PSR J1748-2446ad">PSR J1748-2446ad </a>, rotates at 716 revolutions per second.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-61">[56] </a> A 2007 paper reported the detection of an X-ray burst oscillation, which provides an indirect measure of spin, of 1122 <a dir="ltr" href="https://en.wikipedia.org/wiki/Hertz" title="Hertz">Hz </a>from the neutron star <a dir="ltr" href="https://en.wikipedia.org/wiki/XTE_J1739-285" title="XTE J1739-285">XTE J1739-285 </a>,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-KaaretPrieskorn2007-62">[57] </a> suggesting 1122 rotations a second. However, at present, this signal has only been seen once, and should be regarded as tentative until confirmed in another burst from that star.

Sometimes a neutron star will undergo a <a dir="ltr" href="https://en.wikipedia.org/wiki/Glitch_(astronomy)" title="Glitch (astronomy)">glitch </a>, a sudden small increase of its rotational speed or spin up. Glitches are thought to be the effect of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Starquake_(astrophysics)" title="Starquake (astrophysics)">starquake </a>—as the rotation of the neutron star slows, its shape becomes more spherical. Due to the stiffness of the "neutron" crust, this happens as discrete events when the crust ruptures, creating a starquake similar to earthquakes. After the starquake, the star will have a smaller equatorial radius, and because angular momentum is conserved, its rotational speed has increased.

Starquakes occurring in <a dir="ltr" href="https://en.wikipedia.org/wiki/Magnetar" title="Magnetar">magnetars </a>, with a resulting glitch, is the leading hypothesis for the gamma-ray sources known as <a dir="ltr" href="https://en.wikipedia.org/wiki/Soft_gamma_repeater" title="Soft gamma repeater">soft gamma repeaters </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-journal-63">[58] </a>

Recent work, however, suggests that a starquake would not release sufficient energy for a neutron star glitch; it has been suggested that glitches may instead be caused by transitions of vortices in the theoretical superfluid core of the neutron star from one metastable energy state to a lower one, thereby releasing energy that appears as an increase in the rotation rate.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-64">[59] </a>

An "anti-glitch", a sudden small decrease in rotational speed, or spin down, of a neutron star has also been reported.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nature-65">[60] </a> It occurred in the magnetar <a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=1E_2259%2B586&amp;action=edit&amp;redlink=1" title="1E 2259+586 (page does not exist)">1E 2259+586 </a>, that in one case produced an X-ray luminosity increase of a factor of 20, and a significant spin-down rate change. Current neutron star models do not predict this behavior. If the cause was internal, it suggests differential rotation of solid outer crust and the superfluid component of the magnetar's inner structure.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-nature-65">[60] </a>

At present, there are about 2,000 known neutron stars in the <a dir="ltr" href="https://en.wikipedia.org/wiki/Milky_Way" title="Milky Way">Milky Way </a>and the <a dir="ltr" href="https://en.wikipedia.org/wiki/Magellanic_Clouds" title="Magellanic Clouds">Magellanic Clouds </a>, the majority of which have been detected as radio <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar" title="Pulsar">pulsars </a>. Neutron stars are mostly concentrated along the disk of the Milky Way, although the spread perpendicular to the disk is large because the supernova explosion process can impart high translational speeds (400 km/s) to the newly formed neutron star.

Some of the closest known neutron stars are <a dir="ltr" href="https://en.wikipedia.org/wiki/RX_J1856.5%E2%88%923754" title="RX J1856.5−3754">RX J1856.5−3754 </a>, which is about 400 <a dir="ltr" href="https://en.wikipedia.org/wiki/Light-year" title="Light-year">light-years </a>from Earth, and <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_J0108%E2%88%921431" title="PSR J0108−1431">PSR J0108−1431 </a>about 424 light years.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-aaa496-67">[62] </a> RX J1856.5-3754 is a member of a close group of neutron stars called <a dir="ltr" href="https://en.wikipedia.org/wiki/The_Magnificent_Seven_(neutron_stars)" title="The Magnificent Seven (neutron stars)">The Magnificent Seven </a>. Another nearby neutron star that was detected transiting the backdrop of the constellation Ursa Minor has been nicknamed <a dir="ltr" href="https://en.wikipedia.org/wiki/Calvera_(X-ray_source)" title="Calvera (X-ray source)">Calvera </a>by its Canadian and American discoverers, after the villain in the 1960 film <a dir="ltr" href="https://en.wikipedia.org/wiki/The_Magnificent_Seven" title="The Magnificent Seven">The Magnificent Seven </a> . This rapidly moving object was discovered using the <a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=ROSAT/Bright_Source_Catalog&amp;action=edit&amp;redlink=1" title="ROSAT/Bright Source Catalog (page does not exist)">ROSAT/Bright Source Catalog </a>.

Neutron stars are only detectable with modern technology during the earliest stages of their lives (almost always less than 1 million years) and are vastly outnumbered by older neutron stars that would only be detectable through their blackbody radiation and gravitational effects on other stars.

About 5% of all known neutron stars are members of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Binary_system_(astronomy)" title="Binary system (astronomy)">binary system </a>. The formation and evolution of binary neutron stars can be a complex process.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-68">[63] </a> Neutron stars have been observed in binaries with ordinary <a dir="ltr" href="https://en.wikipedia.org/wiki/Main-sequence_star" title="Main-sequence star">main-sequence stars </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Red_giant" title="Red giant">red giants </a>, white dwarfs, or other neutron stars. According to modern theories of binary evolution, it is expected that neutron stars also exist in binary systems with black hole companions. The merger of binaries containing two neutron stars, or a neutron star and a black hole, has been observed through the emission of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_waves" title="Gravitational waves">gravitational waves </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-69">[64] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-70">[65] </a>

Binary systems containing neutron stars often emit X-rays, which are emitted by hot gas as it falls towards the surface of the neutron star. The source of the gas is the companion star, the outer layers of which can be stripped off by the gravitational force of the neutron star if the two stars are sufficiently close. As the neutron star accretes this gas, its mass can increase; if enough mass is accreted, the neutron star may collapse into a black hole.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-71">[66] </a>

Neutron star binary mergers and nucleosynthesis

The distance between two neutron stars in a close binary system is observed to shrink as <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_waves" title="Gravitational waves">gravitational waves </a>are emitted.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-72">[67] </a> Ultimately, the neutron stars will come into contact and coalesce. The coalescence of binary neutron stars is one of the leading models for the origin of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gamma-ray_burst#Short_gamma-ray_bursts" title="Gamma-ray burst">short gamma-ray bursts </a>. Strong evidence for this model came from the observation of a <a dir="ltr" href="https://en.wikipedia.org/wiki/Kilonova" title="Kilonova">kilonova </a>associated with the short-duration gamma-ray burst GRB 130603B,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-73">[68] </a> and finally confirmed by detection of gravitational wave <a dir="ltr" href="https://en.wikipedia.org/wiki/GW170817" title="GW170817">GW170817 </a>and short <a dir="ltr" href="https://en.wikipedia.org/wiki/GRB_170817A" title="GRB 170817A">GRB 170817A </a>by <a dir="ltr" href="https://en.wikipedia.org/wiki/LIGO" title="LIGO">LIGO </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Virgo_interferometer" title="Virgo interferometer">Virgo </a>, and 70 observatories covering the electromagnetic spectrum observing the event.<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-SM-20171016-74">[69] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-NYT-20171016-75">[70] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-NAT-20170825-76">[71] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-PhysRev-77">[72] </a> The light emitted in the kilonova is believed to come from the radioactive decay of material ejected in the merger of the two neutron stars. This material may be responsible for the production of many of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Chemical_element" title="Chemical element">chemical elements </a>beyond <a dir="ltr" href="https://en.wikipedia.org/wiki/Iron" title="Iron">iron </a>,<a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_star#cite_note-78">[73] </a> as opposed to the <a dir="ltr" href="https://en.wikipedia.org/wiki/Supernova_nucleosynthesis" title="Supernova nucleosynthesis">supernova nucleosynthesis </a>theory.

Neutron stars can host <a dir="ltr" href="https://en.wikipedia.org/wiki/Exoplanet" title="Exoplanet">exoplanets </a>. These can be original, <a dir="ltr" href="https://en.wikipedia.org/wiki/Circumbinary_planet" title="Circumbinary planet">circumbinary </a>, captured, or the result of a second round of planet formation. Pulsars can also strip the atmosphere off from a star, leaving a planetary-mass remnant, which may be understood as a <a dir="ltr" href="https://en.wikipedia.org/wiki/Chthonian_planet" title="Chthonian planet">chthonian planet </a>or a stellar object depending on interpretation. For pulsars, such <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar_planet" title="Pulsar planet">pulsar planets </a>can be detected with the <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar_timing_method" title="Pulsar timing method">pulsar timing method </a>, which allows for high precision and detection of much smaller planets than with other methods. Two systems have been definitively confirmed. The first exoplanets ever to be detected were the three planets Draugr, Poltergeist and Phobetor around <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_B1257%2B12" title="PSR B1257+12">PSR B1257+12 </a>, discovered in 1992–1994. Of these, <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_B1257%2B12_A" title="PSR B1257+12 A">Draugr </a>is the smallest exoplanet ever detected, at a mass of twice that of the Moon. Another system is <a dir="ltr" href="https://en.wikipedia.org/wiki/PSR_B1620%E2%88%9226" title="PSR B1620−26">PSR B1620−26 </a>, where a <a dir="ltr" href="https://en.wikipedia.org/wiki/Circumbinary_planet" title="Circumbinary planet">circumbinary planet </a>orbits a neutron star-white dwarf binary system. Also, there are several unconfirmed candidates. Pulsar planets receive little visible light, but massive amounts of ionizing radiation and high-energy stellar wind, which makes them rather hostile environments.

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