Editing Neutron star (section)

{{short description|Collapsed core of a massive star}}
{{other uses|Neutron Star (disambiguation)}}

[[File:Moving heart of the Crab Nebula.jpg|thumb|upright=1.4|Central neutron star at the heart of the [[Crab Nebula]]]]
[[File:PIA18848-PSRB1509-58-ChandraXRay-WiseIR-20141023.jpg|thumb|upright=1.4|Radiation from the rapidly spinning [[pulsar]] [[PSR B1509-58]] makes nearby gas emit [[X-ray]]s (gold) and illuminates the rest of the [[nebula]], here seen in [[infrared]] (blue and red).]]

A '''neutron star''' is the [[Gravitational collapse|collapsed]] [[Stellar structure|core]] of a massive [[supergiant star]]. It results from the [[supernova]] explosion of a [[stellar evolution#Massive star|massive star]]—combined with [[gravitational collapse]]—that compresses the core past [[white dwarf]] star density to that of [[Atomic nucleus|atomic nuclei]]. Surpassed only by [[black hole]]s, neutron stars are the second smallest and densest known class of stellar objects.<ref>{{cite book |title=Compact Stars: Nuclear Physics, Particle Physics and General Relativity |edition=illustrated |first1=Norman K. |last1=Glendenning |publisher=Springer Science & Business Media |year=2012 |isbn=978-1-4684-0491-3 |page=1 |url=https://books.google.com/books?id=cCDlBwAAQBAJ&pg=PA1 |access-date=2016-03-21 |archive-date=2017-01-31 |archive-url=https://web.archive.org/web/20170131202613/https://books.google.com/books?id=cCDlBwAAQBAJ&pg=PA1 |url-status=live }}</ref> Neutron stars have a radius on the order of {{convert|10|km|sigfig=1|sp=us}} and a mass of about {{solar mass|1.4}}.<ref>{{cite book |title=Astronomy: The Solar System and Beyond |edition=6th |first1=Michael |last1=Seeds |first2=Dana |last2=Backman |publisher=Cengage Learning |year=2009 |isbn=978-0-495-56203-0 |page=339 |url=https://books.google.com/books?id=DajpkyXS-NUC&pg=PT356 |access-date=2018-02-22 |archive-date=2021-02-06 |archive-url=https://web.archive.org/web/20210206222508/https://books.google.com/books?id=DajpkyXS-NUC&pg=PT356 |url-status=live }}</ref> Stars that collapse into neutron stars have a total [[mass]] of between 10 and 25 [[solar mass]]es ({{solar mass}}), or possibly more for those that are especially rich in [[Metallicity|elements heavier than hydrogen and helium]].<ref>{{Cite journal |last1=Heger |first1=A.|last2=Fryer |first2=C. L.|last3=Woosley |first3=S. E.|last4=Langer |first4=N.|last5=Hartmann |first5=D. H.|year=2003|title=How Massive Single Stars End Their Life|journal=[[Astrophysical Journal]]|volume=591 |issue=1|pages=288–300|arxiv=astro-ph/0212469|bibcode=2003ApJ...591..288H|doi=10.1086/375341|s2cid=59065632}}</ref>

Once formed, neutron stars no longer actively generate heat and cool over time, but they may still evolve further through [[Stellar collision|collisions]] or [[Accretion (astrophysics)|accretion]]. Most of the basic models for these objects imply that they are composed almost entirely of [[neutron]]s, as the extreme pressure causes the [[electron]]s and [[proton]]s present in normal matter to combine into additional neutrons. These stars are partially supported against further collapse by [[Degenerate matter#Neutron degeneracy|neutron degeneracy pressure]], just as [[white dwarf]]s are supported against collapse by [[electron degeneracy pressure]]. However, this is not by itself sufficient to hold up an object beyond {{Solar mass|0.7|link=y}}<ref>{{cite journal |first=R. C. |last=Tolman |date=1939 |title=Static Solutions of Einstein's Field Equations for Spheres of Fluid |journal=[[Physical Review]] |volume=55 |issue=4 |pages=364–373 |doi=10.1103/PhysRev.55.364 |bibcode=1939PhRv...55..364T |url=https://authors.library.caltech.edu/4362/1/TOLpr39.pdf |access-date=2019-06-30 |archive-date=2018-07-22 |archive-url=https://web.archive.org/web/20180722071018/https://authors.library.caltech.edu/4362/1/TOLpr39.pdf |url-status=live }}</ref><ref>{{cite journal |first1=J. R. |last1=Oppenheimer |first2=G. M. |last2=Volkoff |date=1939 |title=On Massive Neutron Cores |journal=[[Physical Review]] |volume=55 |issue=4 |pages=374–381 |doi=10.1103/PhysRev.55.374 |bibcode=1939PhRv...55..374O}}</ref> and repulsive nuclear forces increasingly contribute to supporting more massive neutron stars.<ref>{{cite web |title=Neutron Stars |url=https://www.astro.princeton.edu/~burrows/classes/403/neutron.stars.pdf |website=www.astro.princeton.edu |access-date=14 December 2018 |archive-date=9 September 2021 |archive-url=https://web.archive.org/web/20210909160017/https://www.astro.princeton.edu/~burrows/classes/403/neutron.stars.pdf |url-status=live }}</ref><ref>{{Cite journal|last1=Douchin|first1=F.|last2=Haensel|first2=P.|date=December 2001|title=A unified equation of state of dense matter and neutron star structure|journal=Astronomy & Astrophysics|volume=380|issue=1|pages=151–167|doi=10.1051/0004-6361:20011402|issn=0004-6361|arxiv=astro-ph/0111092|bibcode=2001A&A...380..151D|s2cid=17516814}}</ref> If the remnant star has a [[mass]] exceeding the [[Tolman–Oppenheimer–Volkoff limit]], which ranges from {{nowrap|2.2–2.9 {{solar mass}},}} the combination of degeneracy pressure and nuclear forces is insufficient to support the neutron star, causing it to collapse and form a [[black hole]]. The most massive neutron star detected so far, [[PSR J0952–0607]], is estimated to be {{val|2.35|0.17|u=solar mass}}.<ref name=blackwidow/>

Newly formed neutron stars may have surface temperatures of ten million K or more. However, since neutron stars generate no new heat through fusion, they inexorably cool down after their formation. Consequently, a given neutron star reaches a surface temperature of one million K when it is between one thousand and one million years old.<ref name="Chandra">"[https://Chandra.harvard.edu/resources/faq/sources/snr/snr-39.html Q&A: Supernova Remnants and Neutron Stars"], ''Chandra.harvard.edu'' (September 5, 2008)</ref> Older and even-cooler neutron stars are still easy to discover. For example, the well-studied neutron star, {{nowrap|[[RX J1856.5−3754]],}} has an average surface temperature of about 434,000 K.<ref>{{cite journal|arxiv=astro-ph/0612145 |doi=10.1111/j.1365-2966.2006.11376.x |doi-access=free |title=Magnetic hydrogen atmosphere models and the neutron star RX J1856.5-3754 |date=2007 |last1=Ho |first1=W. C. G. |last2=Kaplan |first2=D. L. |last3=Chang |first3=P. |last4=Van Adelsberg |first4=M. |last5=Potekhin |first5=A. Y. |journal=Monthly Notices of the Royal Astronomical Society |volume=375 |issue=3 |pages=821–830 |bibcode=2007MNRAS.375..821H }}. The authors calculated what they considered to be "a more realistic model, which accounts for magnetic field and temperature variations over the neutron star surface as well as general relativistic effects," which yielded an average surface temperature of {{val|4.34|e=5|+0.02|-0.06|u=K}} at a confidence level of 2𝜎 (95%); see §4, ''Fig.&nbsp;6'' in their paper for details.</ref> For comparison, the Sun has an effective surface temperature of 5,780 K.<ref>{{cite journal |quote=The Sun is less active than other solar-like stars |arxiv=2005.01401 |doi=10.1126/science.aay3821 |title=The Sun is less active than other solar-like stars |date=2020 |last1=Reinhold |first1=Timo |last2=Shapiro |first2=Alexander I. |last3=Solanki |first3=Sami K. |last4=Montet |first4=Benjamin T. |last5=Krivova |first5=Natalie A. |last6=Cameron |first6=Robert H. |last7=Amazo-Gómez |first7=Eliana M. |journal=Science |volume=368 |issue=6490 |pages=518–521 |pmid=32355029 |bibcode=2020Sci...368..518R }}</ref>

Neutron star material is remarkably [[Density|dense]]: a normal-sized [[Phillumeny#Matchbox|matchbox]] containing neutron-star material would have a weight of approximately 3 [[billion]] tonnes, the same weight as a 0.5-cubic-kilometer chunk of the Earth (a cube with edges of about 800 meters<!-- Do not change this to 500 metres: note that (800m)³ = (0.8 km)³ = 0.512 km³-->) from Earth's surface.<ref>{{cite web|url=https://heasarc.gsfc.nasa.gov/docs/xte/learning_center/ASM/ns.html|website=heasarc.gsfc.nasa.gov|title=Tour the ASM Sky|access-date=2016-05-23|archive-date=2021-10-01|archive-url=https://web.archive.org/web/20211001133602/https://heasarc.gsfc.nasa.gov/docs/xte/learning_center/ASM/ns.html|url-status=live}}</ref><ref>{{Cite web| url=http://www.universetoday.com/26771/density-of-the-earth/| title=Density of the Earth| date=2009-03-10| access-date=2016-05-23| archive-date=2013-11-12| archive-url=https://web.archive.org/web/20131112145139/http://www.universetoday.com/26771/density-of-the-earth/| url-status=live}}</ref>

As a star's core collapses, its rotation rate increases due to [[conservation of angular momentum]], so newly formed neutron stars typically rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars, and the discovery of pulsars by [[Jocelyn Bell Burnell]] and [[Antony Hewish]] in 1967 was the first observational suggestion that neutron stars exist. The fastest-spinning neutron star known is [[PSR J1748−2446ad]], rotating at a rate of 716 times per second<ref>{{Cite journal |last1=Hessels |first1=Jason |display-authors=4 |last2=Ransom |first2=Scott M. |last3=Stairs |first3=Ingrid H. |last4=Freire |first4=Paulo C. C. |author5-link=Victoria Kaspi |last5=Kaspi |first5=Victoria M. |last6=Camilo |first6=Fernando |title=A Radio Pulsar Spinning at 716 Hz |journal=[[Science (journal)|Science]] |volume=311 |issue=5769 |pages=1901–1904 |date=2006 |doi=10.1126/science.1123430 |pmid=16410486 |bibcode=2006Sci...311.1901H|arxiv = astro-ph/0601337 |citeseerx=10.1.1.257.5174 |s2cid=14945340 }}</ref><ref>{{Cite news |last=Naeye |first=Robert |date=2006-01-13 |title=Spinning Pulsar Smashes Record |periodical=[[Sky & Telescope]] |url=http://www.skyandtelescope.com/news/3311021.html?page=1&c=y |access-date=2008-01-18 |archive-url=https://web.archive.org/web/20071229113749/http://www.skyandtelescope.com/news/3311021.html?page=1&c=y |archive-date=2007-12-29 |url-status=dead }}</ref> or 43,000 [[revolutions per minute]], giving a linear (tangential) speed at the surface on the order of 0.24''c'' (i.e., nearly a quarter the [[speed of light]]).

There are thought to be around one billion neutron stars in the [[Milky Way]],<ref>{{cite web| url = https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html| title = NASA.gov| access-date = 2020-08-05| archive-date = 2018-09-08| archive-url = https://web.archive.org/web/20180908042354/https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html| url-status = live}}</ref> and at a minimum several hundred million, a figure obtained by estimating the number of stars that have undergone supernova explosions.<ref>{{cite book |last1=Camenzind |first1=Max |title=Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes |date=24 February 2007 |publisher=Springer Science & Business Media |isbn=978-3-540-49912-1 |page=269 |url=https://books.google.com/books?id=Nh68nl0abhMC&pg=PA269 |bibcode=2007coaw.book.....C |access-date=6 September 2017 |archive-date=29 April 2021 |archive-url=https://web.archive.org/web/20210429203757/https://books.google.com/books?id=Nh68nl0abhMC&pg=PA269 |url-status=live }}</ref> However, many of them have existed for a long period of time and have cooled down considerably. These stars radiate very little electromagnetic radiation; 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 a part of a binary system. Slow-rotating and non-accreting neutron stars are difficult to detect, due to the absence of electromagnetic radiation; however, since the [[Hubble Space Telescope]]'s detection of [[RX J1856.5−3754]] in the 1990s, a few nearby neutron stars that appear to emit only thermal radiation have been detected.

Neutron stars in binary systems can undergo accretion, in which case they emit large amounts of [[X-ray]]s. During this process, matter is deposited on the surface of the stars, forming "hotspots" that can be sporadically identified as [[X-ray pulsar]] systems. Additionally, such accretions are able to "recycle" old pulsars, causing them to gain mass and rotate extremely quickly, forming [[millisecond pulsar]]s. Furthermore, binary systems such as these continue to [[Stellar evolution|evolve]], with many companions eventually becoming [[Compact star|compact objects]] such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through [[ablation]] or collision.

The study of neutron star systems is central to [[gravitational wave]] astronomy. The [[Neutron star merger|merger of binary neutron stars]] produces gravitational waves and may be associated with [[kilonova]]e and [[gamma-ray burst|short-duration gamma-ray burst]]s. In 2017, the [[LIGO]] and [[Virgo interferometer|Virgo]] interferometer sites observed [[GW170817]], the first direct detection of gravitational waves from such an event.<ref>{{cite journal |year=2017 |title=Multi-messenger Observations of a Binary Neutron Star Merger |journal=The Astrophysical Journal Letters|volume=848 |issue=2 |pages=L12 |arxiv=1710.05833 |bibcode=2017ApJ...848L..12A |doi=10.3847/2041-8213/aa91c9 | last1 = Abbott | first1 = B. P. | last2 = Abbott | first2 = R. | last3 = Abbott | first3 = T. D. | last4 = Acernese | first4 = F. | last5 = Ackley | first5 = K. | last6 = Adams | first6 = C. | last7 = Adams | first7 = T. | last8 = Addesso | first8 = P. | last9 = Richard | last10 = Howard | last11 = Adhikari | first11 = R. X. | last12 = Huang-Wei |s2cid=217162243 |doi-access=free }}</ref> Prior to this, indirect evidence for gravitational waves was inferred by studying the gravity radiated from the orbital decay of a different type of (unmerged) binary neutron system, the [[Hulse–Taylor pulsar]].