Neutron star 21869 225976854 2008-07-16T08:19:02Z Tauris 5978943 /* Population and distances */ {{for|the story by Larry Niven|Neutron Star (story)}} A '''neutron star''' is formed from the [[gravitational collapse|collapsed]] remnant of a massive [[star]]; i.e. a [[Type II supernova|Type II]], [[Type Ib and Ic supernovae|Type Ib or Type Ic]] [[supernova]]. Models predict that [[neutron]] stars consist mostly of neutrons, hence the name. Such stars are very hot, as supported by the [[Pauli exclusion principle]] indicating repulsion between neutrons. A neutron star is one of the few possible [[compact star|conclusions]] of [[stellar evolution]]. A typical neutron star has a [[mass]] between [[Orders of magnitude (mass)#30|1.35 and about 2.1]] [[solar mass]]es, with a corresponding [[radius]] between 20 and 10 [[Kilometre|km]],<ref>A neutron star's radius shrinks as its mass increases</ref> respectively &mdash; in contrast, the [[Sun]] is 30,000 to 70,000 times larger. Thus, neutron stars have overall densities of [[Orders of magnitude (density)#16|8.4×10<sup>16</sup> to 1×10<sup>18</sup>]] [[Kilogram|kg]]/[[Cubic metre|m³]],<ref>8.4x10^16kgm-3 derives from mass 2.8x10^30kg / volume of star of radius 20km;1x10^18kgm-3 derives from mass 4.2x10^30kg / volume of star radius 10km</ref> which compares with the approximate density of an [[atomic nucleus]] of [[Orders of magnitude (density)#14|3×10<sup>17</sup>]] kg/m³.<ref>{{cite web| url=http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/ASM/ns.html| title=Calculating a Neutron Star's Density| accessdate=2006-03-11}} NB 3×10<sup>17</sup> kg/m³ is 3×10<sup>14</sup> g/cm³</ref> The neutron star's density varies from below [[Orders of magnitude (density)#9|1×10<sup>9</sup>]] kg/m³ in the crust increasing with depth to above [[Orders of magnitude (density)#17|6 or 8×10<sup>17</sup>]] kg/m³ deeper inside.<ref>{{cite web| url=http://www.astro.umd.edu/~miller/nstar.html| title=Introduction to neutron stars| accessdate=2007-11-11}}</ref> In general, compact stars of less than 1.38 solar masses, the [[Chandrasekhar limit]], are [[white dwarf]]s; above 2 to 3 solar masses (the [[Tolman-Oppenheimer-Volkoff limit]]), a [[quark star]] might be created, however this is uncertain. [[Gravitational collapse]] will always occur on any star over 5 solar masses, inevitably producing a [[black hole]]. ==Formation== As the core of a massive star is compressed during a [[supernova]], and collapses into a neutron star, it retains most of its [[angular momentum]]. Since 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 gradually slows down. Neutron stars are known to have rotation periods between about 1.4ms to thirty seconds. The neutron star's compactness also gives it very high [[surface gravity]], 2×10<sup>11</sup> to 3×10<sup>12</sup> times stronger than that of [[Earth]]. One measure of such immense gravity is the fact that neutron stars have an [[escape velocity]] of around [[Orders of magnitude (speed)#8|150,000 km/s]], about 50% of the [[speed of light]]. Matter falling onto the surface of a neutron star would be super-accelerated by this gravity and the force of impact would likely destroy the object's component atoms, rendering all its matter identical, in most respects, to the rest of the star. ==Structure== [[Image:Neutron star cross section.jpg|thumb|275px|right|A model of a neutron star's internal structure]] Current understanding of the structure of neutron stars is defined by existing mathematical models, but it might be possible to infer through studies of [[Neutron-star oscillations|neutron-star oscillations]]. Similar to [[asteroseismology]] for ordinary stars, the inner structure might be derived by analyzing observed [[Frequency spectrum|frequency spectra]] of stellar oscillations. A neutron star is so dense that one teaspoon ([[Orders of magnitude (volume)#-6|5]] [[millilitre]]) of its material would have a mass over 5×10<sup>12</sup> kg.<ref>5 ml of a 10 km radius neutron star's average density material masses 5 cm3 x 1.1 x 10^12kgcm-3, or 5.5x10^12kg or 5500000000 tonne, about 15 times the total mass of the human world population;<br/>5 ml of a 20 km radius star would mass 5 cm3 x 8.35 x 10^10kgcm-3, or about 400 million tonne or about the mass of all humans</ref> On the basis of current models, the matter at the surface of a neutron star is composed of ordinary [[atomic nucleus|atomic nuclei]] as well as [[electron]]s. The "atmosphere" of the star is roughly one meter thick, below which one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities of ~5 mm <ref>http://www.daviddarling.info/encyclopedia/N/neutronstar.html</ref>), because of the extreme gravitational field. The crust would appear black because all radiation is focused around the X-ray spectrum. 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. Proceeding deeper, one comes to a point called [[neutron drip line|neutron drip]] where free neutrons leak out of nuclei. In this region, there are nuclei, free electrons, and free neutrons. The nuclei become smaller and smaller until the core is reached, by definition the point where they disappear altogether. The exact nature of the superdense matter in the core is still not well understood. While this theoretical substance is referred to as [[neutronium]] in [[science fiction]] and popular literature, the term "neutronium" is rarely used in scientific publications, due to ambiguity over its meaning. The term [[degenerate matter|neutron-degenerate matter]] is sometimes used, though not universally as the term incorporates assumptions about the nature of neutron star core material. Neutron star core material could be a [[superfluid]] mixture of neutrons with a few [[proton]]s and electrons, or it could incorporate high-energy particles like [[pion]]s and [[kaon]]s in addition to neutrons, or it could be composed of [[strange matter]] incorporating [[quark]]s heavier than [[up quark|up]] and [[down quark]]s, or it could be [[QCD matter|quark matter]] not bound into [[hadron]]s. (A compact star composed entirely of strange matter would be called a [[strange star]].) However, so far, observations have neither indicated nor ruled out such exotic states of matter. ==History of discoveries== [[Image:IsolatedNeutronStar.jpg|thumb|right|200px|The first direct observation of a neutron star in visible light. The neutron star is [[RX J185635-3754]].]] In 1932, [[James Chadwick|Sir James Chadwick]] discovered the neutron as an elementary particle,<ref>{{cite journal| journal=Nature| volume=129| pages=312| title=On the possible existence of a neutron|first=James| last= Chadwick| doi=10.1038/129312a0| year=1932}}</ref> for which he was awarded the [[Nobel Prize in Physics]] in 1935. In 1933, [[Walter Baade]] and [[Fritz Zwicky]] proposed the existence of the neutron star,<ref>{{cite journal| journal=Phys. Rev.| volume=46| title=Remarks on Super-Novae and Cosmic Rays|author=Baade, Walter and Zwicky, Fritz|pages=76–77|doi=10.1103/PhysRev.46.76.2}}</ref> only a year after Chadwick's discovery of the neutron. In seeking an explanation for the origin of a [[supernova]], they proposed that the neutron star is formed in a supernova. Supernovae are suddenly appearing dying stars in the sky, whose luminosity in the optical might outshine an entire [[galaxy]] for days to weeks. 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". If the central part of a massive star before its collapse contains (for example) 3 solar masses, then a neutron star of 2 solar masses can be formed. The binding energy ''E'' of such a neutron star, when expressed in mass units via the [[mass-energy equivalence]] formula ''E''&nbsp;=&nbsp;''mc''², is 1 solar mass. It is ultimately this energy that powers the supernova. In 1965, [[Antony Hewish]] and [[Samuel Okoye]] discovered "an unusual source of high radio brightness temperature in the [[Crab Nebula]]".<ref>{{cite journal|journal=Nature| volume=207| pages=59| title=Evidence of an unusual source of high radio brightness temperature in the Crab Nebula| author=Hewish and Okoye| doi=10.1038/207059a0| year=1965}}</ref> This source turned out to be the [[Crab pulsar|Crab Nebula neutron star]] that resulted from the great [[SN 1054|supernova of 1054]] CE. In 1967, [[Jocelyn Bell Burnell|Jocelyn Bell]] and [[Antony Hewish]] discovered regular radio pulses from the location of the Hewish and Okoye radio source. This [[pulsar]] was later interpreted as originating from an isolated, rotating neutron star. The energy source of the pulsar is the rotational energy of the neutron star. The largest number of known neutron stars are of this type (See [[Rotation-powered pulsar]]). 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. They interpreted this as resulting from a rotating hot neutron star. The energy source is gravitational and results from a [[accretion (science)|rain of gas falling]] onto the surface of the neutron star from a [[companion star]] or the [[interstellar medium]] (See [[Accretion-powered pulsar]]). In 1974, [[Antony Hewish]] was awarded the [[Nobel Prize in Physics]] "for his decisive role in the discovery of pulsars" without [[Samuel Okoye]] and [[Jocelyn Bell]] who shared in the discovery. ==Rotation== Neutron stars rotate extremely rapidly after their creation due to the conservation of angular momentum; like a spinning ice skater pulling in his or her arms, the slow rotation of the original star's core speeds up as it shrinks. A newborn neutron star can rotate several times a second; sometimes, when they orbit a companion star and are able to accrete matter from it, they can increase this to [[Millisecond pulsar|several hundred times per second]], distorting into an [[oblate spheroid]] shape despite their own immense gravity (an [[equatorial bulge]]). Over time, neutron stars slow down because their rotating magnetic fields radiate energy; older neutron stars may take several seconds for each revolution. The rate at which a neutron star slows down its rotation is usually constant and ''very'' small: the observed rates are between 10<sup>-10</sup> and 10<sup>-21</sup> second for each rotation. In other words, for a typical slow down rate of 10<sup>-15</sup> seconds per rotation, then a neutron star now rotating in 1 second will rotate in 1.000003 seconds after a century, or 1.03 seconds after 1 million years. Sometimes a neutron star will ''spin up'' or undergo a ''glitch'', a rapid and unexpected increase of its rotation speed (of the same, extremely small scale as the constant slowing down). Glitches are thought to be the effect of a starquake: As the rotation of the star slows down, the shape becomes more spherical. Due to the stiffness of the 'neutron' crust, this happens as discrete events as the crust ruptures, similar to tectonic earthquakes. After the starquake, the star will have a smaller equatorial radius, and since angular momentum is conserved, rotational speed increases. 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 superfluid core of the star from one metastable energy state to a lower one.<ref>{{cite web| url=http://physicsworld.com/cws/article/print/1756| date=Jan 1, 1998| title=Pulsars, glitches and superfluids| publisher=Physicsworld.com| first=M Ali| last= Alpar}}</ref> Neutron stars may "pulse" due to particle acceleration near the [[magnetic pole]]s, which are not aligned with the rotation axis of the star. Through mechanisms not yet entirely understood, these particles produce coherent beams of radio emission. External viewers see these beams as pulses of radiation whenever the magnetic pole sweeps past the line of sight. The pulses come at the same rate as the rotation of the neutron star, and thus, appear periodic. Neutron stars which emit such pulses are called [[pulsar]]s. The most rapidly rotating neutron star currently known, [[PSR J1748-2446ad]], rotates at 716 revolutions per second.<ref>[http://arxiv.org/abs/astro-ph/0601337 [astro-ph/0601337&#93; A Radio Pulsar Spinning at 716 Hz<!-- Bot generated title -->]</ref> A recent paper reported the detection of an X-ray burst oscillation (an indirect measure of spin) at 1122 Hz from the neutron star XTE J1739-285.<ref>[http://www.journals.uchicago.edu/cgi-bin/resolve?doi=10.1086/513270 University of Chicago Press - Millisecond Variability from XTE J1739285 - 10.1086/513270<!-- Bot generated title -->]</ref> However, at present this signal has only been seen once, and should be regarded as tentative until confirmed in another burst from this star. ==Population and distances== At present there are about 2000 known neutron stars in the [[Milky Way]] and the [[Magellanic Clouds]], the majority of which have been detected as radio [[pulsar]]s. The population of neutron stars is concentrated along the disk of the Milky Way although the spread perpendicular to the disk is fairly large. The reason for this spread is that neutron stars are born with high speeds (400 km/s) as a result of an imparted momentum-kick from an asymmetry during the [[supernova]] explosion process. The closest known neutron star is [[PSR J0108-1431]] at a distance of about 85 [[parsec]]s (or 280 [[light year]]s)<ref>Tauris et al. 1994, ApJ.Lett. 428, L53 http://adsabs.harvard.edu/abs/1994ApJ...428L..53T</ref>. Another nearby neutron star is [[RX J185635-3754]] but observations using the [[Chandra X-ray Observatory]] in 2002 appear to show that its distance is greater—about 450 light-years. ==Binary neutron stars== About 5% of all neutron stars are members of a [[binary system (astronomy)|binary system]]. The formation and evolution scenario of binary neutron stars is a rather exotic and complicated process<ref>Tauris & van den Heuvel (2006), in Compact Stellar X-ray Sources. Eds. Lewin and van der Klis, Cambridge University Press http://adsabs.harvard.edu/abs/2006csxs.book..623T</ref>. The companion stars may be either ordinary [[stars]], [[white dwarf]]s or other [[neutron star]]s. According to modern theories of binary evolution it is expected that neutron stars also exist in binary systems with [[black hole]] companions. Such binaries are expected to be prime sources for emitting [[gravitational waves]]. Neutron stars in binary systems often emit [[X-rays]] which is caused by the heating of material (gas) accreted from the companion star. Material from the outer layers of a (bloated) companion star is sucked towards the neutron star as a result of its very strong gravitational field. As a result of this process binary neutron stars may also coalesce into [[black hole]]s if the accretion of mass takes place under extreme conditions. ==Subtypes== * Neutron star ** Protoneutron star (PNS), theorized.<ref>[http://www.astro.princeton.edu/~burrows/eos.wind.thermal/wind.html Neutrino-Driven Protoneutron Star Winds], Todd A. Thompson.</ref> ** [[Radio-quiet neutron star]]s ** Radio loud neutron star *** [[Pulsar|Single pulsar]]s &ndash; general term for neutron stars that emit directed pulses of radiation towards us at regular intervals (due to their strong magnetic fields). **** [[Rotation-powered pulsar]] ''("radio pulsar")'' ***** [[Magnetar]] &ndash; 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) *** [[Binary pulsar]]s **** [[Low-mass X-ray binaries]] (LMXB) **** [[Intermediate-mass X-ray binaries]] (IMXB) **** [[High-mass X-ray binaries]] (HMXB) **** [[Accretion-powered pulsar]] ''("X-ray pulsar")'' ***** [[X-ray burster]] &ndash; 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. ***** [[Millisecond pulsar]] (MSP) ''("recycled pulsar")'' ****** Sub-millisecond pulsar<ref>[http://adsabs.harvard.edu/abs/1989PThPh..81.1006N Binary Sub-Millisecond Pulsar and Rotating Core Collapse Model for SN1987A], Nakamura, T., 1989.</ref> *** [[Exotic star]] **** [[Quark star]] &ndash; currently a hypothetical type of neutron star composed of [[quark matter]], or [[strange matter]]. As of 2008, there are three candidates. **** [[Preon star]] &ndash; currently a hypothetical type of neutron star composed of [[preon matter]]. As of 2008, there is no evidence for the existence of [[preon]]s. **** [[Q star]] &ndash; currently a hypothetical type of heavy neutron star with an exotic state of matter. As of 2008, there is no evidence for their existence. ==Giant nuclei== A neutron star has some of the properties of an [[atomic nucleus]], including density, and being made of [[nucleon]]s. 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. In particular, a nucleus is held together by the [[strong force]], while a neutron star is held together by [[gravity]]. It is generally more useful to consider such objects as [[star]]s. ==See also== <div style="-moz-column-count:2; column-count:2;"> * [[Magnetar]] * [[Millisecond pulsar]] * [[Neutron]] * [[Neutron stars in fiction]] * [[Neutronium]], [[Neutron-degenerate matter]] * [[Preon matter]], [[Preon-degenerate matter]] * [[Pulsar]] * [[Quark matter]], [[Quark-degenerate matter]] * [[Radio quiet neutron stars]] * [[Rotating radio transients]] </div> ==References== {{reflist}} {{refbegin}} * {{cite web | url=http://scienceweek.com/2004/sb040806-1.htm | title=ASTROPHYSICS: ON OBSERVED PULSARS | work=scienceweek.com | accessmonthday=6 August | accessyear=2004 }} * {{cite book | title=Compact Stars | author=Norman K. Glendenning, R. Kippenhahn, I. Appenzeller, G. Borner, M. Harwit | year=2000 | edition=2nd ed }} * {{cite web | url=http://arxiv.org/abs/astro-ph/0611716 | title= Evidence for 1122 Hz X-Ray Burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285 | work=ApJL | accessmonthday=28 February | accessyear=2007 }} {{refend}} ==External links== {{Commonscat|Neutron star}} * [http://www.astro.umd.edu/~miller/nstar.html Introduction to neutron stars] * "[http://spacedaily.com/reports/NASA_Sees_Hidden_Structure_Of_Neutron_Star_In_Starquake.html NASA Sees Hidden Structure Of Neutron Star In Starquake]". SpaceDaily.com. [[April 26]] 2006 * "[http://newscientistspace.com/article.ns?id=dn9397&feedId=online-news_rss20 Mysterious X-ray sources may be lone neutron stars]". ''New Scientist''. * "[http://space.newscientist.com/article/dn9428-massive-neutron-star-rules-out-exotic-matter.html Massive neutron star rules out exotic matter]". ''New Scientist''. According to a new analysis, exotic states of matter such as free quarks or BECs do not arise inside neutron stars. * "[http://space.newscientist.com/article/dn9730-neutron-star-clocked-at-mindboggling-velocity.html Neutron star clocked at mind-boggling velocity]". ''New Scientist''. A neutron star has been clocked traveling at more than 1500 kilometers per second. {{Star}} [[Category:Neutron stars|Neutron stars]] [[Category:Neutron|Star]] [[Category:Star types]] [[Category:Exotic matter]] [[ar:نجم نيتروني]] [[bn:নিউট্রন তারা]] [[bs:Neutronska zvijezda]] [[bg:Неутронна звезда]] [[ca:Estrella de neutrons]] [[cs:Neutronová hvězda]] [[da:Neutronstjerne]] [[de:Neutronenstern]] [[et:Neutrontäht]] [[el:Αστέρας νετρονίων]] [[es:Estrella de neutrones]] [[eo:Neŭtrona stelo]] [[fr:Étoile à neutrons]] [[ga:Neodrónréalta]] [[gl:Estrela de neutróns]] [[ko:중성자별]] [[hr:Neutronska zvijezda]] [[it:Stella di neutroni]] [[he:כוכב נייטרונים]] [[la:Stella neutronica]] [[lv:Neitronu zvaigzne]] [[lt:Neutroninė žvaigždė]] [[hu:Neutroncsillag]] [[ml:ന്യൂട്രോണ്‍ നക്ഷത്രം]] [[ms:Bintang neutron]] [[nl:Neutronenster]] [[ja:中性子星]] [[no:Nøytronstjerne]] [[nn:Nøytronstjerne]] [[pl:Gwiazda neutronowa]] [[pt:Estrela de nêutrons]] [[ru:Нейтронная звезда]] [[simple:Neutron star]] [[sk:Neutrónová hviezda]] [[sl:Nevtronska zvezda]] [[sh:Neutronska zvijezda]] [[fi:Neutronitähti]] [[sv:Neutronstjärna]] [[th:ดาวนิวตรอน]] [[vi:Sao neutron]] [[tr:Nötron yıldızı]] [[uk:Нейтронна зоря]] [[zh:中子星]]