View source for Compact object

{{Short description|Classification in astronomy}}
{{hatnote group|
{{For|the categorical notion in mathematics|Compact object (mathematics)}}
{{Distinguish|dwarf star|central compact object}}
}}
In [[astronomy]], the term '''compact object''' (or '''compact star''') refers collectively to [[white dwarf]]s, [[neutron star]]s, and [[black hole]]s. It could also include [[exotic star]]s if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high [[mass]] relative to their [[radius]], giving them a very high [[density]] compared to ordinary [[atom]]ic [[matter]]. The term is used as a generalization for cases where the exact nature of a significant gravitational effect isolated to a small radius is not known.

Since most compact object types represent endpoints of [[stellar evolution]], they are also called '''stellar remnants''', and accordingly may be called '''dead stars''' in popular media reports. The state and type of a stellar remnant depends primarily on the mass of its progenitor star. A compact object that is not a black hole may be called a '''degenerate star'''.

In June 2020, astronomers reported narrowing down the source of [[Fast radio burst|Fast Radio Bursts]] (FRBs), which may now plausibly include "compact-object mergers and [[magnetar]]s arising from normal core collapse [[supernova]]e".<ref name="SA-20200601">{{cite news |last=Starr |first=Michelle  |title=Astronomers Just Narrowed Down The Source of Those Powerful Radio Signals From Space |url=https://www.sciencealert.com/we-re-starting-to-figure-out-where-fast-radio-bursts-come-from |date=1 June 2020 |work=ScienceAlert.com |access-date=2 June 2020 }}</ref><ref name="AJL-20200601">{{cite journal |author=Bhandan, Shivani |title=The Host Galaxies and Progenitors of Fast Radio Bursts Localized with the Australian Square Kilometre Array Pathfinder |date=1 June 2020 |journal=[[The Astrophysical Journal Letters]] |volume=895 |number=2 |pages=L37 |doi=10.3847/2041-8213/ab672e |arxiv=2005.13160 |bibcode=2020ApJ...895L..37B |s2cid=218900539 |doi-access=free }}</ref>

== Formation ==
The usual endpoint of [[stellar evolution]] is the formation of a compact star. Every active star will eventually evolve to a point where the outward radiation pressure from nuclear fusion in its interior can no longer counteract its own gravity. When this happens, the star collapses under its own weight and undergoes the process of [[stellar death]]. For most stars, this will result in the formation of a very dense and compact stellar remnant, also known as a compact star.

Compact objects have no internal energy production, but will—with the exception of black holes—usually radiate for millions of years with excess heat left from the collapse itself.<ref>{{cite book |last1=Tauris |first1=T. M. |last2=J. van den Heuvel |first2=E. P. |title=Formation and Evolution of Compact Stellar X-ray Sources |url=https://archive.org/details/arxiv-astro-ph0303456 |arxiv=astro-ph/0303456|date=20 Mar 2003 |bibcode=2006csxs.book..623T }}</ref>

According to the most recent understanding, compact stars could also form during the [[Chronology of the universe#Hadron epoch|phase separations]] of the early Universe following the [[Big Bang]].<ref>{{cite journal
 | title=Primordial black holes
 | last=Khlopov | first=Maxim Yu.
 | journal=Research in Astronomy and Astrophysics
 | volume=10 | issue=6 | pages=495–528 | date=June 2010
 | arxiv=0801.0116 | bibcode=2010RAA....10..495K 
 | doi=10.1088/1674-4527/10/6/001
| s2cid=14466173 }}</ref> Primordial origins of known compact objects have not been determined with certainty.

==Lifetime==
Although compact objects may radiate, and thus cool off and lose energy, they do not depend on high temperatures to maintain their structure, as ordinary stars do. Barring external disturbances and [[proton decay]], they can persist virtually forever. [[Black holes]] are however generally believed to finally evaporate from [[Hawking radiation]] after trillions of years. According to our current standard models of [[physical cosmology]], all stars will eventually evolve into cool and dark compact stars, by the time the Universe enters the so-called [[heat death of the universe|degenerate era]] in a very distant future.

A somewhat wider definition of ''compact objects'' may include [[Substellar object|smaller solid objects]] such as [[planet]]s, [[asteroid]]s, and [[comet]]s, but such usage is less common. There are a remarkable variety of stars and other clumps of hot matter, but all matter in the Universe must eventually end as dispersed cold particles or some form of compact stellar or substellar object, according to [[thermodynamics]].

==White dwarfs==
{{Main|White dwarf}}
[[Image:Ngc2392.jpg|upright|thumb|The [[Eskimo Nebula]] is illuminated by a white dwarf at its center.]]
The stars called [[white dwarf|white or degenerate dwarf]]s are made up mainly of [[degenerate matter]]; typically carbon and oxygen nuclei in a sea of degenerate electrons. White dwarfs arise from the cores of [[main-sequence star]]s and are therefore very hot when they are formed. As they cool they will redden and dim until they eventually become dark [[black dwarf]]s. White dwarfs were observed in the 19th century, but the extremely high densities and pressures they contain were not explained until the 1920s.

The [[equation of state]] for degenerate matter is "soft", meaning that adding more mass will result in a smaller object. Continuing to add mass to what begins as a white dwarf, the object shrinks and the central density becomes even greater, with higher degenerate-electron energies. After the degenerate star's mass has grown sufficiently that its radius has shrunk to only a few thousand kilometers, the mass will be approaching the [[Chandrasekhar limit]] – the theoretical upper limit of the mass of a white dwarf, about 1.4&nbsp;times the [[mass of the Sun]] ({{Solar mass|link=y}}).

If matter were removed from the center of a white dwarf and slowly compressed, electrons would first be forced to combine with nuclei, changing their [[proton]]s to [[neutron]]s by [[inverse beta decay]]. The equilibrium would shift towards heavier, neutron-richer nuclei that are not stable at everyday densities. As the density increases, these nuclei become still larger and less well-bound. At a critical density of about 4{{e|14}} kg/m<sup>3</sup> – called the [[neutron drip line]] – the atomic nucleus would tend to dissolve into unbound protons and neutrons. If further compressed, eventually it would reach a point where the matter is on the order of the density of an atomic nucleus – about 2{{e|17}}&nbsp;kg/m<sup>3</sup>. At that density the matter would be chiefly free neutrons, with a light scattering of protons and electrons.

==Neutron stars==
<!--
[[Image:V838 Mon HST.jpg|160px|right|thumb|Formerly a white dwarf, [[V838 Monocerotis]] has [[accretion theory|accreted]] enough material to become a [[red supergiant]].]]
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{{Main|Neutron star}}
[[Image:Chandra-crab.jpg|upright|thumb|The [[Crab Nebula]] is a [[supernova remnant]] containing the [[Crab Pulsar]], a [[neutron star]].]]
In certain [[binary stars]] containing a white dwarf, mass is transferred from the companion star onto the white dwarf, eventually pushing it over the [[Chandrasekhar limit]]. Electrons react with protons to form neutrons and thus no longer supply the necessary pressure to resist gravity, causing the star to collapse. If the center of the star is composed mostly of carbon and oxygen then such a [[gravitational collapse]] will ignite runaway fusion of the carbon and oxygen, resulting in a [[Type Ia supernova]] that entirely blows apart the star before the collapse can become irreversible. If the center is composed mostly of magnesium or heavier elements, the collapse continues.<ref>
{{cite journal
|last1=Hashimoto |first1=M.
|last2=Iwamoto |first2=K.
|last3=Nomoto |first3=K.
|date=1993
|title=Type II supernovae from 8–10 solar mass asymptotic giant branch stars
|journal=[[The Astrophysical Journal]]
|volume=414 |pages=L105
|bibcode=1993ApJ...414L.105H
|doi=10.1086/187007
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Ritossa |first1=C.
|last2=Garcia-Berro |first2=E.
|last3=Iben |first3=I. Jr.
|date=1996
|title=On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M<sub>sun</sub> Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse
|journal=[[The Astrophysical Journal]]
|volume=460 |page=489
|bibcode=1996ApJ...460..489R
|doi=10.1086/176987
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Wanajo |first1=S.
|date=2003
|title=The r-Process in Supernova Explosions from the Collapse of O-Ne-Mg Cores
|journal=[[The Astrophysical Journal]]
|volume=593 |issue=2 |pages=968–979
|arxiv=astro-ph/0302262
|bibcode=2003ApJ...593..968W
|doi=10.1086/376617
|s2cid=13456130
|display-authors=etal}}</ref> As the density further increases, the remaining electrons react with the protons to form more neutrons. The collapse continues until (at higher density) the neutrons become degenerate. A new equilibrium is possible after the star shrinks by three [[orders of magnitude]], to a radius between 10 and 20&nbsp;km. This is a ''neutron star''.

Although the first neutron star was not observed until 1967 when the first radio [[pulsar]] was discovered, neutron stars were proposed by Baade and Zwicky in 1933, only one year after the neutron was discovered in 1932. They realized that because neutron stars are so dense, the collapse of an ordinary star to a neutron star would liberate a large amount of [[gravitational energy|gravitational potential energy]], providing a possible explanation for [[supernova]]e.<ref>{{cite journal
|last1=Osterbrock |first1=D. E.
|date=2001
|title=Who Really Coined the Word Supernova? Who First Predicted Neutron Stars?
|journal=[[Bulletin of the American Astronomical Society]]
|volume=33 |page=1330
|bibcode=2001AAS...199.1501O
}}</ref><ref>
{{cite journal
|last1=Baade |first1=W.
|last2=Zwicky |first2=F.
|date=1934
|title=On Super-Novae
|journal=[[Proceedings of the National Academy of Sciences]]
|volume=20 |issue=5 |pages=254–9
|bibcode=1934PNAS...20..254B
|doi=10.1073/pnas.20.5.254 |pmid=16587881 |pmc=1076395
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Baade |first1=W.
|last2=Zwicky |first2=F.
|date=1934
|title=Cosmic Rays from Super-Novae
|journal=[[Proceedings of the National Academy of Sciences]]
|volume=20 |issue=5 |pages=259–263
|bibcode=1934PNAS...20..259B
|doi=10.1073/pnas.20.5.259
|pmid=16587882
|pmc=1076396
|doi-access=free
}}</ref> This is the explanation for supernovae of types [[Type Ib and Ic supernovae|Ib, Ic]], and [[Supernova#Type II|II]]. Such supernovae occur when the iron core of a massive star exceeds the Chandrasekhar limit and collapses to a neutron star.

Like electrons, neutrons are [[fermions]]. They therefore provide [[neutron degeneracy pressure]] to support a neutron star against collapse. In addition, repulsive neutron-neutron interactions{{Citation needed|date=July 2008}} provide additional pressure. Like the Chandrasekhar limit for white dwarfs, there is a limiting mass for neutron stars: the [[Tolman–Oppenheimer–Volkoff limit]], where these forces are no longer sufficient to hold up the star. As the forces in dense hadronic matter are not well understood, this limit is not known exactly but is thought to be between 2 and {{Solar mass|3}}. If more mass accretes onto a neutron star, eventually this mass limit will be reached. What happens next is not completely clear.

==Black holes==
{{Main|Black hole|Stellar black hole}}
[[Image:Black Hole Milkyway.jpg|upright|thumb|A simulated black hole of ten solar masses, at a distance of 600&nbsp;km]]
As more mass is accumulated, equilibrium against gravitational collapse exceeds its breaking point. Once the star's pressure is insufficient to counterbalance gravity, a catastrophic gravitational collapse occurs within milliseconds. The [[escape velocity]] at the surface, already at least {{frac|1|3}}&nbsp;light speed, quickly reaches the velocity of light. At that point no energy or matter can escape and a [[black hole]] has formed. Because all light and matter is trapped within an [[event horizon]], a black hole appears truly [[black]], except for the possibility of very faint [[Hawking radiation]]. It is presumed that the collapse will continue inside the event horizon.

In the classical theory of [[general relativity]], a [[gravitational singularity]] occupying no more than a [[point (geometry)|point]] will form. There may be a new halt of the catastrophic gravitational collapse at a size comparable to the [[Planck length]], but at these lengths there is no known theory of gravity to predict what will happen. Adding any extra mass to the black hole will cause the radius of the event horizon to increase linearly with the mass of the central singularity. This will induce certain changes in the properties of the black hole, such as reducing the tidal stress near the event horizon, and reducing the gravitational field strength at the horizon. However, there will not be any further qualitative changes in the structure associated with any mass increase.

===Alternative black hole models===
* [[Fuzzball (string theory)|Fuzzball]]<ref name="small-dark-and-heavy">{{cite arXiv |last1=Visser |first1=M. |last2=Barcelo |first2=C. |last3=Liberati |first3=S. |last4=Sonego |first4=S. |date=2009 |title=Small, dark, and heavy: But is it a black hole?  |class=gr-qc |eprint=0902.0346}}</ref>
* [[Gravastar]]<ref name="small-dark-and-heavy"/>
* [[Dark-energy star]]
* [[Black star (semiclassical gravity)|Black star]]
* [[Magnetospheric eternally collapsing object]]
* [[Dark star (Newtonian mechanics)|Dark star]]<ref name="small-dark-and-heavy"/>
* [[Primordial black hole]]s

==Exotic stars==
{{Main|Exotic star}}
An ''[[exotic star]]'' is a hypothetical compact star composed of something other than [[electron]]s, [[proton]]s, and [[neutron]]s balanced against [[gravitational collapse]] by [[degeneracy pressure]] or other quantum properties. These include [[strange star]]s (composed of [[strange matter]]) and the more speculative [[preon star]]s (composed of [[preon]]s).

Exotic stars are hypothetical, but observations released by the [[Chandra X-Ray Observatory]] on April 10, 2002, detected two candidate strange stars, designated [[RX J1856.5-3754]] and [[3C58]], which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than they should, suggesting that they are composed of material denser than [[neutronium]]. However, these observations are met with skepticism by researchers who say the results were not conclusive.{{citation needed|date=March 2014}}

===Quark stars and strange stars===
{{Main|Quark star}}
If neutrons are squeezed enough at a high temperature, they will decompose into their component [[quark]]s, forming what is known as a [[quark matter]]. In this case, the star will shrink further and become denser, but instead of a total collapse into a black hole, it is possible that the star may stabilize itself and survive in this state indefinitely, so long as no more mass is added. It has, to an extent, become a very large [[nucleon]]. A star in this hypothetical state is called a "[[quark star]]" or more specifically a "strange star". The pulsar [[3C58]] has been suggested as a possible quark star. Most neutron stars are thought to hold a core of quark matter but this has proven difficult to determine observationally.{{citation needed|date=January 2019}}

===Preon stars===
A ''preon star'' is a [[List of hypothetical astronomical objects|proposed]] type of compact star made of [[preon]]s, a group of [[:Category:Hypothetical elementary particles|hypothetical]] [[subatomic particle]]s. Preon stars would be expected to have huge [[density|densities]], exceeding 10<sup>23</sup> kilogram per cubic meter – intermediate between quark stars and black holes.
Preon stars could originate from supernova explosions or the [[Big Bang]]; however, current observations from particle accelerators speak against the existence of preons.{{citation needed|date=March 2014}}

===Q stars===
{{Main|Q star}}
''Q stars'' are hypothetical compact, heavier neutron stars with an exotic state of matter where particle numbers are preserved with radii less than 1.5 times the corresponding [[Schwarzschild radius]]. Q stars are also called "gray holes".

===Electroweak stars===
{{Main|Electroweak star}}
An ''electroweak star'' is a theoretical type of exotic star, whereby the gravitational collapse of the star is prevented by [[radiation pressure]] resulting from [[electroweak burning]], that is, the energy released by conversion of [[quark]]s to [[lepton]]s through the [[electroweak force]]. This process occurs in a volume at the star's core approximately the size of an [[apple]], containing about two Earth masses.<ref name="newscientist">
{{cite web
|last1=Shiga |first1=D.
|date=4 January 2010
|title=Exotic stars may mimic big bang
|url=https://www.newscientist.com/article/dn18334-exotic-stars-may-mimic-big-bang.html
|work=[[New Scientist]]
|access-date=2010-02-18
}}</ref>

===Boson star===
A [[boson star]] is a hypothetical [[astronomical object]] that is formed out of particles called [[boson]]s (conventional [[star]]s are formed out of [[fermion]]s). For this type of star to exist, there must be a stable type of boson with repulsive self-interaction. As of 2016 there is no significant evidence that such a star exists. However, it may become possible to detect them by the gravitational radiation emitted by a pair of co-orbiting boson stars.<ref>{{cite book
 | first=Bernard F. | last=Schutz
 | title=Gravity from the ground up | url=https://archive.org/details/gravityfromgroun00schu_469 | url-access=limited | edition=3rd
 | publisher=[[Cambridge University Press]] | date=2003
 | isbn=0-521-45506-5 | page=[https://archive.org/details/gravityfromgroun00schu_469/page/n170 143]
}}</ref><ref>{{cite journal
 | author=Palenzuela, C.
 | author2=Lehner, L.
 | author3=Liebling, S. L.
 | title=Orbital dynamics of binary boson star systems
 | journal=Physical Review D | volume=77 | issue=4
 | doi=10.1103/PhysRevD.77.044036
 | date=2008
 | article-number=044036 |bibcode = 2008PhRvD..77d4036P |arxiv = 0706.2435 | s2cid=115159490
 }}</ref>

==Compact relativistic objects and the generalized uncertainty principle==
Based on the [[generalized uncertainty principle]] (GUP), proposed by some approaches to quantum gravity such as [[string theory]] and [[doubly special relativity]], the effect of GUP on the thermodynamic properties of compact stars with two different components has been studied recently.<ref>Ahmed Farag Ali and A. Tawfik, [http://www.worldscientific.com/doi/abs/10.1142/S021827181350020X Int. J. Mod. Phys. D22 (2013) 1350020]</ref> Tawfik et al. noted that the existence of quantum gravity correction tends to resist the collapse of stars if the GUP parameter is taking values between Planck scale and electroweak scale. Comparing with other approaches, it was found that the radii of compact stars should be smaller and increasing energy decreases the radii of the compact stars.

==See also==
*[[Galaxy formation and evolution]]

==References==
{{Reflist|30em}}

== Sources ==
*{{cite journal
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*{{cite journal
|last1=Sandin |first1=F.
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|volume=40 |issue= 2|pages=15–22
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*{{cite thesis
|last1=Sandin |first1=F.
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|publisher=[[Luleå University of Technology]]
}}

{{White dwarf|state=collapsed}}
{{Neutron star}}
{{Black holes}}
{{Stellar core collapse}}
{{Supernovae}}
{{Star}}
{{Portal bar|Astronomy|Outer space}}
{{Authority control}}

{{DEFAULTSORT:Compact Star}}
[[Category:Star types]]
[[Category:Compact stars|*]]
[[Category:Exotic matter]]
[[Category:Concepts in astronomy]]