Neutron Stars  

In General > s.a. astrophysics.
* Idea: Compact star remnants, produced as a result of core-collapse supernovae and supported by neutron degeneracy pressure; In some cases, a fast spinning neutron star is observed as a pulsar; Their existence was proposed by Zwicky in 1934, but at that time it was thought that they are to small to be observed; They may be the most compact form of matter other than black holes.
* History: The first one was discovered in 1967 (as a pulsar); 2014, About 2500 are known.
* Observation: Observations with X-rays, γ-rays, and hopefully gravitational waves can be used to constrain models.
* Ringing: There are many different modes, p-modes (with pressure as the restoring force), r-modes (with the Coriolis force as the restoring force), w-modes (pure spacetime modes present only in relativistic gravity), t-modes (torsional), g-modes (gravity), etc.
* With hyperons: At the high-density end, some matter may be in the form of hyperons; At even higher densities, baryons melt into a quark plasma; The hyperon puzzle is the difficulty to reconcile the measured masses of neutron stars with the presence of hyperons in their interiors.

Properties > s.a. models and types; pulsars [including super-Eddington].
* General properties: The observed range of masses is 1.2–2.0 MSun, their radii uncertain but probably in the range 8–13 km, rotational periods 1.4 ms to more than 12 s and extremely stable, their magnetic fields in the range 108–1015 G, the density at their centers 10 time nuclear density, their binding energy can be around 0.2M, the speed at the equator 0.15c, and the gravitational redshift at their surface 0.3; Some universal relations have been obtained among stellar observables that do not depend sensitively on the star's internal structure, such as the so-called I-Love-Q relations (relating the quadrupole moment, the moment of inertia and the tidal Love number), and relations among the star's multipole moments which resemble the well-known black hole no-hair theorems.
* Equation of state: Phenomenologically modelled as a piecewise polytrope, with phase transitions at certain values of the density.
* Crusts: They are not composed of neutrons, and can be modeled as nearly perfect crystal lattices; They are thought to shatter at deformations with ellipticity of about 0.1, the exact value depending on the equation of state.
* Maximum luminosity: A maximum value follows from the Eddington limit, but neutron stars exceed the limit.
@ Mass: Rezzolla et al ApJL(18) + news cosmos(18)jan [upper limit]; Köppel et al ApJL(19)-a1901 [threshold mass to prompt collapse].
@ Size: Özel & Psaltis ApJ(15)-a1505; news livesci(18)apr; news pw(18)oct [from GW170817].
@ Magnetic fields: Viganò a1310-PhD; Viganò et al a1501-proc; Lopes & Menezes JCAP(15)-a1411; Pili et al MNRAS(15)-a1412, Akgün et al MNRAS(16)-a1605 [twisted magnetosphere].
@ Other phenomenology: news sn(19)may [what a nearby kilonova would look like].

References > s.a. history of astronomy.
@ General references: Hartle & Thorne ApJ(68), ApJ(69); Hartle ApJ(70), PRP(78); Burrows PT(87)sep [birth]; Ipser & Lindblom PRL(89); Kutschera APPB(98)ap-ln; Bildsten & Strohmayer PT(99)feb; Bignami pw(03)sep [and X-ray astronomy]; Silbar & Reddy AJP(04)jul-nt/03; Lattimer & Prakash Sci(04)ap [rev]; Lai et al a0902-rp; Pizzochero a1001-ln [intro, based on fundamental physics]; Kaspi PNAS(10)-a1005 [types, properties]; van Leeuwen ed-a1212 [proc]; Yagi & Yunes PRP(17)-a1608 [universal relations among observables, rev]; > s.a. QCD phenomenology.
@ Reviews, books: Potekhin PU(10)-a1102; Reisenegger & Zepeda EPJA(16)-a1511 [properties, pedagogical]; Ekşi TJP(16)-a1511; Rezzolla et al 19.
@ And gravitational waves: Gärtig & Kokkotas PRD(11)-a1005 [asteroseismology]; Patruno et al ApJ(12)-a1109 [and the observed maximum spin-frequency]; Lasky PASA(15)-a1508 [rev]; Haskell IJMPE(15)-a1509 [r-mode instabilities]; news sn(17)oct [detection of neutron star collision]; Carney et al PRD(18)-a1805 [eos]; Sokol sa(18)jun [eos]; Silva et al a2004 [theoretical physics implications of multimessenger observations].
@ And hyperons: Lackey et al PRD(06)ap/05; Weber & Rosenfield ap/07-conf; Vidaña AIP(15)-a1509; Bombaci JPScp(17)-a1601 [the hyperon puzzle].
@ And other physics: Sedrakian et al MNRAS(97)ap [superconductivity]; Chakrabarty ApSS(07)ap/06 [upper limit on B field strength]; Graber et al IJMPD(17)-a1610 [low-temperature laboratory experiments on condensates]; Andersson JAA(17)-a1709; Alexander et al CQG(19)-a1810 [entropy].
@ Dark-matter core? Ciarcelluti & Sandin PLB(11)-a1005; Husaina & Thomas a2104.
@ Related topics: Ferrari & Kokkotas PRD(00)gq [particle scattering]; Güver et al JCAP(14)-a1201 [capture of dark matter]; Pérez-García a1205-proc [as laboratories for cosmology]; Taani et al ApSS(12)-a1205 [spatial distribution in the galaxy]; Ludwig & Ruffini JKPS(14)-a1402 [Gamow's critical mass calculation]; Pappas MNRAS(15)-a1506 [astrophysical properties independent of the equation of state]; > s.a. astronomical objects [strangelets].
> Online resources: see Wikipedia page.

main pageabbreviationsjournalscommentsother sitesacknowledgements
send feedback and suggestions to bombelli at – modified 5 apr 2021