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{{for|the story by Larry Niven|Neutron Star (short story)}}File:Neutron_Star_bending_light_simulation.png|thumb|right|300px|Simulated view of a neutron star. Due to its strong gravity, the background is gravitationally lensed, making it appear distorted.]]File:PIA18848-PSRB1509-58-ChandraXRay-WiseIR-20141023.jpg|thumb|right|Radiation from the pulsar PSR B1509-58, a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, from Chandra) and illuminates the rest of the nebula, here seen in infrared (blue and red, from WISE).]]A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses{{Citation needed|date=August 2019}}. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars.BOOK, Compact Stars: Nuclear Physics, Particle Physics and General Relativity, illustrated, Norman K., Glendenning, Springer Science & Business Media, 2012, 978-1-4684-0491-3, 1,weblink Neutron stars have a radius on the order of {{convert|10|km|mi}} and a mass of about 1.4 solar masses.BOOK, Astronomy: The Solar System and Beyond, 6th, Michael, Seeds, Dana, Backman, Cengage Learning, 2009, 978-0-495-56203-0, 339,weblink They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); 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 neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure. However neutron degeneracy pressure is not sufficient to hold up an object beyond 0.7{{Solar mass|link=y}}JOURNAL, R. C., Tolman, 1939, Static Solutions of Einstein's Field Equations for Spheres of Fluid, Physical Review, 55, 4, 364–373, 10.1103/PhysRev.55.364, 1939PhRv...55..364T,weblink JOURNAL, J. R., Oppenheimer, G. M., Volkoff, 1939, On Massive Neutron Cores, Physical Review, 55, 4, 374–381, 10.1103/PhysRev.55.374, 1939PhRv...55..374O, and repulsive nuclear forces play a larger role in supporting more massive neutron stars.WEB, Neutron Stars,weblink www.astro.princeton.edu, 14 December 2018, JOURNAL, Douchin, F., Haensel, P., December 2001, A unified equation of state of dense matter and neutron star structure, Astronomy & Astrophysics, 380, 1, 151–167, 10.1051/0004-6361:20011402, 0004-6361, astro-ph/0111092, 2001A&A...380..151D, If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit 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 black hole.Neutron stars that can be observed are very hot and typically have a surface temperature of around {{val|600000|u=K}}.BOOK, Reassessing the Fundamentals: On the Evolution, Ages and Masses of Neutron Stars, Bulent, Kiziltan, 978-1-61233-765-4, Universal-Publishers, 2011, Neutron star mass measurementsWEB,weblink Ask an Astrophysicist, imagine.gsfc.nasa.gov, BOOK, Neutron Stars, Paweł, Haensel, Alexander Y., Potekhin, Dmitry G., Yakovlev, 978-0-387-33543-8, Springer, 2007, A neutron star's density increases as its mass increases, and its radius decreases non-linearly. (archived image: weblink" title="web.archive.org/web/20111017230141weblink">NASA mass radius graph) A newer page is here: WEB,weblink RXTE Discovers Kilohertz Quasiperiodic Oscillations, NASA, 17 February 2016, (specifically the image weblink) They are so dense that a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion metric tons, 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.WEB,weblink heasarc.gsfc.nasa.gov, Tour the ASM Sky, WEB,weblink Density of the Earth, 2009-03-10, Their magnetic fields are between 108 and 1015 (100 million to 1 quadrillion) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about {{val|2|e=11}} (200 billion) times that of Earth's gravitational field.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 by Jocelyn Bell Burnell and Antony Hewish 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 PSR J1748-2446ad, rotating at a rate of 716 times a secondJOURNAL, Hessels, Jason, 4, Ransom, Scott M., Stairs, Ingrid H., Freire, Paulo C. C., Victoria Kaspi, Kaspi, Victoria M., Camilo, Fernando, A Radio Pulsar Spinning at 716 Hz, Science (journal), Science, 311, 5769, 1901–1904, 2006, 10.1126/science.1123430, 16410486, 2006Sci...311.1901H, astro-ph/0601337, 10.1.1.257.5174, NEWS, Naeye, Robert, 2006-01-13, Spinning Pulsar Smashes Record, Sky & Telescope,weblink 2008-01-18, or 43,000 revolutions per minute, giving a linear speed at the surface on the order of {{val|0.24|ul=c}} (i.e. nearly a quarter the speed of light).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.BOOK, Camenzind, Max, Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes, 24 February 2007, Springer Science & Business Media, 978-3-540-49912-1, 269,weblink 2007coaw.book.....C, 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 almost undetectable; however, since the Hubble Space Telescope detection of RX J185635−3754, a few nearby neutron stars that appear to emit only 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.JOURNAL, Xu, R. X., Qiao, G. J., 2000, Nature and Nurture: a Model for Soft Gamma-Ray Repeaters, The Astrophysical Journal, 545, 2, 127–129, Zhang, Bing, 2000ApJ...545L.127Z, astro-ph/0010225, 10.1086/317889, 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. In 2017, a direct detection (GW170817) of the gravitational waves from such an event was made,JOURNAL, 2017, Multi-messenger Observations of a Binary Neutron Star Merger, The Astrophysical Journal Letters, 848, 2, L12, 1710.05833, 2017ApJ...848L..12A, 10.3847/2041-8213/aa91c9, Abbott, B. P., Abbott, R., Abbott, T. D., Acernese, F., Ackley, K., Adams, C., Adams, T., Addesso, P., Richard, Howard, Adhikari, R. X., Huang-Wei, and gravitational waves have also been indirectly detected in a system where two neutron stars orbit each other.

Formation

(File:Neutronstarsimple.png|thumb|300px|Simplistic representation of the formation of neutron stars.)Any main-sequence star with an initial mass of above 8 times the mass of the sun ({{Solar mass|8|link=y}}) 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 Chandrasekhar limit. Electron-degeneracy pressure is overcome and the core collapses further, sending temperatures soaring to over {{val|5|e=9|u=K}}. At these temperatures, photodisintegration (the breaking up of iron nuclei into alpha particles by high-energy gamma rays) occurs. As the temperature climbs even higher, electrons and protons combine to form neutrons via electron capture, releasing a flood of neutrinos. When densities reach nuclear density of {{val|4|e=17|u=kg/m3}}, neutron degeneracy pressure halts the contraction. 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 {{Solar mass|3}}, it collapses further to become a black hole.BOOK, The Birth of Stars and Planets, illustrated, John, Bally, Bo, Reipurth, Cambridge University Press, 2006, 978-0-521-80105-8, 207,weblink As the core of a massive star is compressed during a Type II supernova, Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum. But, because it has only a tiny fraction of its parent's radius (and therefore its moment of inertia 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 surface gravity, with typical values ranging from 1012 to 1013 m/s2 (more than 1011 times that of Earth). One measure of such immense gravity is the fact that neutron stars have an escape velocity ranging from 100,000 km/s to 150,000 km/s, that is, from a third to half the speed of light. 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.

Properties

Mass and temperature

A neutron star has a mass of at least 1.1 and perhaps up to more than 2.1 solar masses ({{Solar mass|link=y}}) (the Tolman–Oppenheimer–Volkoff limit),JOURNAL, Özel, Feryal, Psaltis, Dimitrios, Narayan, Ramesh, Santos Villarreal, Antonio, On the Mass Distribution and Birth Masses of Neutron Stars, The Astrophysical Journal, September 2012, 757, 1, 13, 10.1088/0004-637X/757/1/55, 1201.1006, 2012ApJ...757...55O, JOURNAL, Chamel, N., Haensel, PaweÅ‚, Zdunik, J. L., Fantina, A. F., On the Maximum Mass of Neutron Stars, International Journal of Modern Physics, 19 November 2013, 1, 28, 10.1142/S021830131330018X, 1307.3995, 2013IJMPE..2230018C, 1330018, although the recent estimate puts the upper limit at {{Solar mass|2.16}}.JOURNAL, 10.3847/2041-8213/aaa401, Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars, The Astrophysical Journal, 852, 2, L25, 2018, Rezzolla, Luciano, Most, Elias R., Weih, Lukas R., 2018ApJ...852L..25R, 1711.00314, The maximum observed mass of neutron stars is about {{solar mass|2.01}}. However, in general, compact stars of less than {{Solar mass|1.39|link=y}} (the Chandrasekhar limit) are white dwarfs, whereas compact stars with a mass between {{Solar mass|1.4}} and {{Solar mass|2.16}} should be neutron stars (though 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 {{Solar mass|2.16}} the stellar remnant will overcome the neutron degeneracy pressure and gravitational collapse will usually occur to produce a black hole, though the smallest observed mass of a stellar black hole is about {{Solar mass|5}}.weblink, a {{Solar mass|10}} star will collapse into a black hole. Between {{Solar mass|2.16}} and {{Solar mass|5}}, hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist.The temperature inside a newly formed neutron star is from around 1011 to 1012 kelvin. However, the huge number of neutrinos it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around 106 kelvin. At this lower temperature, most of the light generated by a neutron star is in X-rays.

Density and pressure

Neutron stars have overall densities of {{val|3.7|e=17}} to {{val|5.9|e=17|u=kg/m3}} ({{val|2.6|e=14}} to {{val|4.1|e=14}} times the density of the Sun),{{val|3.7|e=17|u=kg/m3}} derives from mass {{val|2.68|e=30|u=kg}} / volume of star of radius 12 km; {{val|5.9|e=17|u=kg/m3}} derives from mass {{val|4.2|e=30|u=kg}} per volume of star radius 11.9 km which is comparable to the approximate density of an atomic nucleus of {{val|3|e=17|u=kg/m3}}.WEB,weblink Calculating a Neutron Star's Density, 2006-03-11, NB 3{{E-sp|17}} kg/m3 is {{val|3|e=14|u=g/cm3}} The neutron star's density varies from about {{val|1|e=9|u=kg/m3}} in the crust—increasing with depth—to about {{val|6|e=17}} or {{val|8|e=17|u=kg/m3}} (denser than an atomic nucleus) deeper inside.JOURNAL,weblink Introduction to neutron stars, American Institute of Physics Conference Series, 1645, 1, 61–78, 2007-11-11, 2015AIPC.1645...61L, Lattimer, James M., 2015, 10.1063/1.4909560, AIP Conference Proceedings, A neutron star is so dense that one teaspoon (5 milliliters) of its material would have a mass over {{val|5.5|e=12|u=kg}}, about 900 times the mass of the Great Pyramid of Giza. In the enormous gravitational field of a neutron star, that teaspoon of material would weigh {{val|1.1|e=25|u=N}}, which is 15 times what the Moon would weigh if it were placed on the surface of the Earth.The average density of material in a neutron star of radius 10 km is {{val|1.1|e=12|u=kg/cm3}}. Therefore, 5 ml of such material is {{val|5.5|e=12|u=kg}}, or 5 500 000 000 metric tons. This is about 15 times the total mass of the human world population. Alternatively, 5 ml from a neutron star of radius 20 km radius (average density {{val|8.35|e=10|u=kg/cm3}}) has a mass of about 400 million metric tons, or about the mass of all humans. The gravitational field is ca. {{val|2|e=11}}g or ca. {{val|2|e=12}} N/kg. Moon weight is calculated at 1g. The entire mass of the Earth at neutron star density would fit into a sphere of 305m in diameter (the size of the Arecibo Observatory). The pressure increases from {{val|3.2|e=31}} to {{val|1.6|e=34|u=Pa}} from the inner crust to the center.JOURNAL, Ozel, Feryal, Freire, Paulo, Masses, Radii, and the Equation of State of Neutron Stars, Annu. Rev. Astron. Astrophys., 54, 1, 401–440, 2016, 10.1146/annurev-astro-081915-023322, 2016ARA&A..54..401O, 1603.02698, The equation of state of matter at such high densities is not precisely known because of the theoretical difficulties associated with extrapolating the likely behavior of quantum chromodynamics, superconductivity, and superfluidity of matter in such states. The problem is exacerbated by the empirical difficulties of observing the characteristics of any object that is hundreds of parsecs away, or farther.A neutron star has some of the properties of an atomic nucleus, including density (within an order of magnitude) and being composed of nucleons. 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 strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities.

Magnetic field

The magnetic field strength on the surface of neutron stars ranges from c. 104 to 1011 tesla.WEB,weblink Origin and Evolution of Neutron Star Magnetic Fields, Universidade Federal do Rio Grande do Sul, 21 March 2016, A., Reisenegger, 2003astro.ph..7133R, 2003, astro-ph/0307133, 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 diamagnetic levitation. 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.The neutron stars known as magnetars have the strongest magnetic fields, in the range of 108 to 1011 tesla,WEB,weblink McGill SGR/AXP Online Catalog, 2 Jan 2014, and have become the widely accepted hypothesis for neutron star types soft gamma repeaters (SGRs)JOURNAL,weblink Magnetars, Scientific American, February 2003, 21 March 2016, Chryssa, Kouveliotou, Robert C., Duncan, Christopher, Thompson, and anomalous X-ray pulsars (AXPs).JOURNAL, (Anomalous) X-ray Pulsars, Nuclear Physics B: Proceedings Supplements, 132, 456–465, V. M., Kaspi, F. P., Gavriil, 10.1016/j.nuclphysbps.2004.04.080, 2004, astro-ph/0402176, 2004NuPhS.132..456K, The magnetic energy density of a 108 T field is extreme, exceeding the mass−energy density of ordinary matter.Magnetic energy density for a field B is U = B2/2μ0 per Eric Weisstein's World of Physics. Substituting B = 108 T, U = {{val|4|e=21|u=J|up=m3}}. Dividing by c2 one obtains the equivalent mass density of {{val|44500|u=kg|up=m3}}, which exceeds the standard temperature and pressure density of all known materials, cf. {{val|22590|u=kg|up=m3}} for osmium, the densest stable element. Fields of this strength are able to polarize the vacuum to the point that the vacuum becomes birefringent. 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 starquakes, 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.WEB,weblink 'Magnetars', soft gamma repeaters & very strong magnetic fields, Robert C., Duncan, March 2003, 2018-04-17, The origins of the strong magnetic field are as yet unclear. One hypothesis is that of "flux freezing", or conservation of the original magnetic flux during the formation of the neutron star. 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 magnetic field 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.

Gravity and equation of state

File:Neutronstar 2Rs.svg|thumb|right|Gravitational light deflection at a neutron star. Due to relativistic light deflection more than half of the surface is visible (each grid patch here represents 30 degrees by 30 degrees). In natural units, the mass of the depicted star is 1 and its radius 4, or twice its Schwarzschild radiusSchwarzschild radiusThe gravitational field at a neutron star's surface is about {{val|2|e=11}} times stronger than on Earth, at around {{val|2.0|e=12|u=m/s2}}.BOOK, An Introduction to the Sun and Stars, illustrated, Simon F., Green, Mark H., Jones, S. Jocelyn, Burnell, Cambridge University Press, 2004, 978-0-521-54622-5, 322,weblink Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible.WEB, Corvin, Zahn, Tempolimit Lichtgeschwindigkeit, 1990-10-09,weblink German, Durch die gravitative Lichtablenkung ist mehr als die Hälfte der Oberfläche sichtbar. Masse des Neutronensterns: 1, Radius des Neutronensterns: 4, ... dimensionslosen Einheiten (c, G = 1), 2009-10-09, If the radius of the neutron star is 3GM/c2 or less, then the photons may be trapped in an orbit, 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, {{nowrap|E {{=}} mc2}}). The energy comes from the gravitational binding energy 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.WEB, Peligroso lugar para jugar tenis,weblink Datos Freak, 3 June 2016, Spanish, However, even before impact, the tidal force would cause spaghettification, breaking any sort of an ordinary object into a stream of material.Because of the enormous gravity, time dilation 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 its very rapid rotation.BOOK, Marcia Bartusiak, Black Hole: How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved,weblink 2015, Yale University Press, 978-0-300-21363-8, 130, Neutron star relativistic equations of state describe the relation of radius vs. mass for various models.Neutron Star Masses and Radii, p. 9/20, bottom 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,JOURNAL, astro-ph/0002232, Hessels, Jason W. T, Neutron Star Structure and the Equation of State, The Astrophysical Journal, 550, 426, 426–442, Ransom, Scott M, Stairs, Ingrid H, Freire, Paulo C. C, Kaspi, Victoria M, Camilo, Fernando, 2001, 10.1086/319702, 2001ApJ...550..426L,
BE = frac{0.60,beta}{1 - frac{beta}{2}}      beta = G,M/R,{c}^{2}
Given current values
G = 6.67408times10^{-11}, text{m}^3text{kg}^{-1}text{s}^{-2}CODATA 2014
c = 2.99792458 times10^{8}, text{m}/text{s}
M_odot = 1.98855times10^{30}, text{kg}
and star masses "M" commonly reported as multiples of one solar mass,
M_x = frac{M}{M_odot}
then the relativistic fractional binding energy of a neutron star is
BE = frac{886.0 ,M_x}{R_{left[text{in meters}right]} - 738.3,M_x}
A {{Solar mass|2}} 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 equation of state 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 degenerate gas that can be described in close agreement with special relativity. 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.NASA. Neutron Star Equation of State Science Retrieved 2011-09-26 {{webarchive |url=https://web.archive.org/web/20130220103830weblink |date=February 20, 2013 }} This means that the relation between density and mass is not fully known, and this causes uncertainties in radius estimates. For example, a {{Solar mass|1.5}} 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).

Structure

thumb|right|Cross-section of neutron star. Densities are in terms of ρ0 the saturation nuclear matter density, where nucleons begin to touch.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 neutron-star oscillations. Asteroseismology, a study applied to ordinary stars, can reveal the inner structure of neutron stars by analyzing observed spectra of stellar oscillations.Current models indicate that matter at the surface of a neutron star is composed of ordinary atomic nuclei crushed into a solid lattice with a sea of electrons flowing through the gaps between them. It is possible that the nuclei at the surface are iron, due to iron's high binding energy per nucleon.Beskin, V. S.; (1999); Radiopulsars, УФН. T. 169, №11, p. 1173-1174 It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like helium and hydrogen. If the surface temperature exceeds 106 kelvin (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 }}}} less than two years after the discovery of the neutron by Sir James Chadwick.JOURNAL
, Nature, 129
, 3252, 312
, On the possible existence of a neutron
, James, Chadwick
, 10.1038/129312a0, 1932, 1932Natur.129Q.312C, In seeking an explanation for the origin of a supernova, they tentatively proposed that in supernova explosions ordinary stars are turned into stars that consist of extremely closely packed neutrons that they called neutron stars. Baade and Zwicky correctly proposed at that time that the release of the gravitational binding energy of the neutron stars powers the supernova: "In the supernova process, mass in bulk is annihilated". Neutron stars were thought to be too faint to be detectable and little work was done on them until November 1967, when Franco Pacini pointed out that if the neutron stars were spinning and had large magnetic fields, then electromagnetic waves would be emitted. Unbeknown to him, radio astronomer Antony Hewish and his research assistant Jocelyn Bell at Cambridge were shortly to detect radio pulses from stars that are now believed to be highly magnetized, rapidly spinning neutron stars, known as pulsars.
In 1965, Antony Hewish and Samuel Okoye discovered "an unusual source of high radio brightness temperature in the Crab Nebula".JOURNAL, Nature, 207, 4992, 59–60, Evidence of an unusual source of high radio brightness temperature in the Crab Nebula, Hewish, A., Okoye, S. E., yes, 10.1038/207059a0, 1965, 1965Natur.207...59H, This source turned out to be the Crab Pulsar that resulted from the great supernova of 1054.In 1967, Iosif Shklovsky examined the X-ray and optical observations of Scorpius X-1 and correctly concluded that the radiation comes from a neutron star at the stage of accretion.JOURNAL, Shklovsky, I. S., On the Nature of the Source of X-Ray Emission of SCO XR-1, Astrophysical Journal, 148, 1, L1–L4, April 1967, 10.1086/180001, 1967ApJ...148L...1S, In 1967, Jocelyn Bell Burnell and Antony Hewish discovered regular radio pulses from PSR B1919+21. This pulsar was later interpreted as an isolated, rotating neutron star. The energy source of the pulsar is the rotational energy of the neutron star. The majority of known neutron stars (about 2000, as of 2010) have been discovered as pulsars, emitting regular radio pulses.In 1971, Riccardo Giacconi, Herbert Gursky, Ed Kellogg, R. Levinson, E. Schreier, and H. Tananbaum discovered 4.8 second pulsations in an X-ray source in the constellation Centaurus, Cen X-3.BOOK, Rotation and Accretion Powered Pulsars, illustrated, Pranab, Ghosh, World Scientific, 2007, 978-981-02-4744-7, 8,weblink They interpreted this as resulting from a rotating hot neutron star. The energy source is gravitational and results from a rain of gas falling onto the surface of the neutron star from a companion star or the interstellar medium.In 1974, Antony Hewish was awarded the Nobel Prize in Physics "for his decisive role in the discovery of pulsars" without Jocelyn Bell who shared in the discovery.BOOK, A Companion to Astronomy and Astrophysics: Chronology and Glossary with Data Tables, illustrated, Kenneth, Lang, Springer Science & Business Media, 2007, 978-0-387-33367-0, 82,weblink In 1974, Joseph Taylor and Russell Hulse discovered the first binary pulsar, PSR B1913+16, which consists of two neutron stars (one seen as a pulsar) orbiting around their center of mass. Einstein's general theory of relativity predicts that massive objects in short binary orbits should emit gravitational waves, and thus that their orbit should decay with time. This was indeed observed, precisely as general relativity predicts, and in 1993, Taylor and Hulse were awarded the Nobel Prize in Physics for this discovery.BOOK, Neutron Stars 1: Equation of State and Structure, illustrated, Paweł, Haensel, Alexander Y., Potekhin, Dmitry G., Yakovlev, Springer Science & Business Media, 2007, 978-0-387-47301-7, 474,weblink In 1982, Don Backer and colleagues discovered the first millisecond pulsar, PSR B1937+21.BOOK, Pulsar Astronomy, illustrated, Francis, Graham-Smith, Cambridge University Press, 2006, 978-0-521-83954-9, 11,weblink This object spins 642 times per second, a value that placed fundamental constraints on the mass and radius of neutron stars. Many millisecond pulsars were later discovered, but PSR B1937+21 remained the fastest-spinning known pulsar for 24 years, until PSR J1748-2446ad (which spins more than 700 times a second) was discovered.In 2003, Marta Burgay and colleagues discovered the first double neutron star system where both components are detectable as pulsars, PSR J0737−3039.BOOK, Rotation and Accretion Powered Pulsars, illustrated, Pranab, Ghosh, World Scientific, 2007, 978-981-02-4744-7, 281,weblink The discovery of this system allows a total of 5 different tests of general relativity, some of these with unprecedented precision.In 2010, Paul Demorest and colleagues measured the mass of the millisecond pulsar PSR J1614−2230 to be {{Solar mass|1.97±0.04}}, using Shapiro delay.JOURNAL, 10.1038/nature09466, Demorest, Paul B., Pennucci, T., Ransom, S. M., Roberts, M. S., Hessels, J. W., A two-solar-mass neutron star measured using Shapiro delay, Nature, 467, 7319, 1081–1083, 2010Natur.467.1081D, 2010, 20981094, 1010.5788, This was substantially higher than any previously measured neutron star mass ({{Solar mass|1.67}}, see PSR J1903+0327), and places strong constraints on the interior composition of neutron stars.In 2013, John Antoniadis and colleagues measured the mass of PSR J0348+0432 to be {{Solar mass|2.01±0.04}}, using white dwarf spectroscopy.JOURNAL, 10.1126/science.1233232, Antoniadis, John, A Massive Pulsar in a Compact Relativistic Binary, Science, 340, 6131, 2013Sci...340..448A, 2012, 1304.6875, 1233232, 23620056, 10.1.1.769.4180, This confirmed the existence of such massive stars using a different method. Furthermore, this allowed, for the first time, a test of general relativity using such a massive neutron star.In August 2017, LIGO and Virgo made first detection of gravitational waves produced by colliding neutron stars.WEB,weblink LIGO Detection of Colliding Neutron Stars Spawns Global Effort to Study the Rare Event, Kimberly M., Burtnyk, 16 October 2017, 17 November 2017, In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.NEWS, University of Maryland, All in the family: Kin of gravitational wave source discovered - New observations suggest that kilonovae -- immense cosmic explosions that produce silver, gold and platinum--may be more common than thought,weblink 16 October 2018, EurekAlert!, 17 October 2018, JOURNAL, Troja, E., etal, A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341, 16 October 2018, Nature Communications, 9, 4089, 4089 (2018), 10.1038/s41467-018-06558-7, 30327476, 6191439, 2018NatCo...9.4089T, 1806.10624, NEWS, Mohon, Lee, GRB 150101B: A Distant Cousin to GW170817,weblink 16 October 2018, NASA, 17 October 2018, WEB, Wall, Mike, Powerful Cosmic Flash Is Likely Another Neutron-Star Merger,weblink 17 October 2018, Space.com, 17 October 2018, In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of neutron stars, following the detection of the neutron star merger of GW170817.NEWS, National Radio Astronomy Observatory, New method may resolve difficulty in measuring universe's expansion - Neutron star mergers can provide new 'cosmic ruler',weblink 8 July 2019, EurekAlert!, 8 July 2019, NEWS, Finley, Dave, New Method May Resolve Difficulty in Measuring Universe’s Expansion,weblink 8 July 2019, National Radio Astronomy Observatory, 8 July 2019, Their measurement of the Hubble constant is {{val|70.3|+5.3|-5.0}} (km/s)/Mpc.JOURNAL, Hotokezaka, K., et al., A Hubble constant measurement from superluminal motion of the jet in GW170817,weblink 8 July 2019, Nature Astronomy, 10.1038/s41550-019-0820-1, 8 July 2019, 1806.10596,

Subtypes table

  • Neutron star
    • Isolated neutron star (INS):BOOK, 1008.2891, April 2010, Sandro, Mereghetti, High-Energy Emission from Pulsars and their Systems, 21, 345–363, 10.1007/978-3-642-17251-9_29, X-ray emission from isolated neutron stars, Astrophysics and Space Science Proceedings, 978-3-642-17250-2, 2011ASSP...21..345M, WEB,weblink Thermal Radiation from Isolated Neutron Stars, SLAC National Accelerator Laboratory, 28 April 2016, George, Pavlov, Slava, Zavlin, Divas, Sanwal, Oleg, Kargaltsev, Roger, Romani, not in a binary system.
      • Rotation-powered pulsar (RPP or "radio pulsar"): neutron stars that emit directed pulses of radiation towards us at regular intervals (due to their strong magnetic fields).
Rotating radio transient (RRATs): are thought to be pulsars which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars.
      • Magnetar: a neutron star with an extremely strong magnetic field (1000 times more than a regular neutron star), and long rotation periods (5 to 12 seconds).
Soft gamma repeater (SGR). Anomalous X-ray pulsar (AXP). X-ray dim isolated neutron stars. Central compact objects in supernova remnants (CCOs in SNRs): young, radio-quiet non-pulsating X-ray sources, thought to be Isolated Neutron Stars surrounded by supernova remnants. Millisecond pulsar (MSP) ("recycled pulsar").* Sub-millisecond pulsar.JOURNAL, 1989PThPh..81.1006N, Binary Sub-Millisecond Pulsar and Rotating Core Collapse Model for SN1987A, T., Nakamura, 10.1143/PTP.81.1006, Progress of Theoretical Physics, 81, 5, 1006–1020, 1989, X-ray burster: a neutron star with a low mass binary companion from which matter is accreted resulting in irregular bursts of energy from the surface of the neutron star.

Examples of neutron stars

(File:Artist's impression of disc around a neutron star RX J0806.4-4123.tif|thumb|Artist's impression of disc around a neutron star RX J0806.4-4123.WEB, Artist's impression of disc around a neutron star,weblink www.spacetelescope.org, 18 September 2018, )
  • RX J0806.4-4123 – neutron star source of infrared radiation.WEB, HubbleSite: News - Hubble Uncovers Never Before Seen Features Around a Neutron Star,weblink hubblesite.org, 18 September 2018,
  • PSR J0108−1431 – closest neutron star
  • LGM-1 – the first recognized radio-pulsar
  • PSR B1257+12 – the first neutron star discovered with planets (a millisecond pulsar)
  • SWIFT J1756.9-2508 – a millisecond pulsar with a stellar-type companion with planetary range mass (below brown dwarf)
  • PSR B1509−58 – source of the "Hand of God" photo shot by the Chandra X-ray Observatory.
  • PSR J0348+0432 – the most massive neutron star with a well-constrained mass, 2.01 ± 0.04 {{Solar mass}}.

Gallery

Video – animation

Image:Neutron Star Manhattan.ogv|Neutron stars containing 500,000 Earth-masses in {{convert|25|km|abbr=on}} diameter sphereImage:Crash and Burst.ogv|Neutron stars collidingImage:Neutron star collision.ogv|Neutron star collision

See also

Notes

{{reflist|group=lower-alpha}}

References

{{reflist|30em}}
  • WEB,weblink ASTROPHYSICS: ON OBSERVED PULSARS, scienceweek.com, 6 August 2004,
  • BOOK, Compact Stars, Norman K. Glendenning, 4, R. Kippenhahn, I. Appenzeller, G. Borner, M. Harwit, 2000, 2nd,
  • JOURNAL, Evidence for 1122 Hz X-Ray Burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285, Kaaret, 4, Prieskorn, in 't Zand, Brandt, Lund, Mereghetti, Gotz, Kuulkers, Tomsick, 10.1086/513270, 2006, The Astrophysical Journal, 657, 2, L97, astro-ph/0611716, 2007ApJ...657L..97K,

External links

{{Commons category|Neutron stars}} {{Neutron star}}{{White dwarf}}{{Black holes}}{{Stellar core collapse}}{{Star}}{{Supernovae}}{{Gravitational waves}}{{Authority control}}

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