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In astronomy, the term compact object (or compact star) refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars 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 atomic matter.

Compact objects are often the endpoints of stellar evolution and, in this respect, are also called stellar remnants. The state and type of a stellar remnant depends primarily on the mass of the star that it formed from. The ambiguous term compact object is often used when the exact nature of the star is not known, but evidence suggests that it has a very small radius compared to ordinary stars. 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 Bursts (FRBs), which may now plausibly include "compact-object mergers and magnetars arising from normal core collapse supernovae".

Formation

The usual endpoint of stellar evolution is the formation of a compact star.

All active stars will eventually come to a point in their evolution when the outward radiation pressure from the nuclear fusions in its interior can no longer resist the ever-present gravitational forces. 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.

According to the most recent understanding, compact stars could also form during the phase separations of the early Universe following the Big Bang. 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 degenerate era in a very distant future.

A somewhat wider definition of compact objects may include smaller solid objects such as planets, asteroids, and comets, 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

The stars called white or degenerate dwarfs 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 stars and are therefore very hot when they are formed. As they cool they will redden and dim until they eventually become dark black dwarfs. 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 times the mass of the Sun (M_☉).

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 protons to neutrons 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 4×10 kg/m – 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 2×10 kg/m. At that density the matter would be chiefly free neutrons, with a light scattering of protons and electrons.

Neutron stars

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. 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 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 potential energy, providing a possible explanation for supernovae. This is the explanation for supernovae of types Ib, Ic, and 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 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 3 M_☉. If more mass accretes onto a neutron star, eventually this mass limit will be reached. What happens next is not completely clear.

Black holes

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 1⁄3 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 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
• Gravastar
• Dark-energy star
• Black star
• Magnetospheric eternally collapsing object
• Dark star
• Primordial black holes

Exotic stars

An exotic star is a hypothetical compact star composed of something other than electrons, protons, and neutrons balanced against gravitational collapse by degeneracy pressure or other quantum properties. These include strange stars (composed of strange matter) and the more speculative preon stars (composed of preons).

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.

Quark stars and strange stars

If neutrons are squeezed enough at a high temperature, they will decompose into their component quarks, 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.

Preon stars

A preon star is a proposed type of compact star made of preons, a group of hypothetical subatomic particles. Preon stars would be expected to have huge densities, exceeding 10 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.

Q stars

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

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 quarks to leptons 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.

Boson star

A boson star is a hypothetical astronomical object that is formed out of particles called bosons (conventional stars are formed out of fermions). 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.

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. 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

1. Starr, Michelle (1 June 2020). "Astronomers Just Narrowed Down The Source of Those Powerful Radio Signals From Space". ScienceAlert.com. Retrieved 2 June 2020.
2. Bhandan, Shivani (1 June 2020). "The Host Galaxies and Progenitors of Fast Radio Bursts Localized with the Australian Square Kilometre Array Pathfinder". The Astrophysical Journal Letters. 895 (2): L37. arXiv:2005.13160. Bibcode:2020ApJ...895L..37B. doi:10.3847/2041-8213/ab672e. S2CID 218900539.
3. Tauris, T. M.; J. van den Heuvel, E. P. (20 Mar 2003). Formation and Evolution of Compact Stellar X-ray Sources. arXiv:astro-ph/0303456. Bibcode:2006csxs.book..623T.
4. Khlopov, Maxim Yu. (June 2010). "Primordial black holes". Research in Astronomy and Astrophysics. 10 (6): 495–528. arXiv:0801.0116. Bibcode:2010RAA....10..495K. doi:10.1088/1674-4527/10/6/001. S2CID 14466173.
5. Hashimoto, M.; Iwamoto, K.; Nomoto, K. (1993). "Type II supernovae from 8–10 solar mass asymptotic giant branch stars". The Astrophysical Journal. 414: L105. Bibcode:1993ApJ...414L.105H. doi:10.1086/187007.
6. Ritossa, C.; Garcia-Berro, E.; Iben, I. Jr. (1996). "On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M_sun Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse". The Astrophysical Journal. 460: 489. Bibcode:1996ApJ...460..489R. doi:10.1086/176987.
7. Wanajo, S.; et al. (2003). "The r-Process in Supernova Explosions from the Collapse of O-Ne-Mg Cores". The Astrophysical Journal. 593 (2): 968–979. arXiv:astro-ph/0302262. Bibcode:2003ApJ...593..968W. doi:10.1086/376617. S2CID 13456130.
8. Osterbrock, D. E. (2001). "Who Really Coined the Word Supernova? Who First Predicted Neutron Stars?". Bulletin of the American Astronomical Society. 33: 1330. Bibcode:2001AAS...199.1501O.
9. Baade, W.; Zwicky, F. (1934). "On Super-Novae". Proceedings of the National Academy of Sciences. 20 (5): 254–9. Bibcode:1934PNAS...20..254B. doi:10.1073/pnas.20.5.254. PMC 1076395. PMID 16587881.
10. Baade, W.; Zwicky, F. (1934). "Cosmic Rays from Super-Novae". Proceedings of the National Academy of Sciences. 20 (5): 259–263. Bibcode:1934PNAS...20..259B. doi:10.1073/pnas.20.5.259. PMC 1076396. PMID 16587882.
11. ^ Visser, M.; Barcelo, C.; Liberati, S.; Sonego, S. (2009). "Small, dark, and heavy: But is it a black hole?". arXiv:0902.0346 .
12. Shiga, D. (4 January 2010). "Exotic stars may mimic big bang". New Scientist. Retrieved 2010-02-18.
13. Schutz, Bernard F. (2003). Gravity from the ground up (3rd ed.). Cambridge University Press. p. 143. ISBN 0-521-45506-5.
14. Palenzuela, C.; Lehner, L.; Liebling, S. L. (2008). "Orbital dynamics of binary boson star systems". Physical Review D. 77 (4): 044036. arXiv:0706.2435. Bibcode:2008PhRvD..77d4036P. doi:10.1103/PhysRevD.77.044036. S2CID 115159490.
15. Ahmed Farag Ali and A. Tawfik, Int. J. Mod. Phys. D22 (2013) 1350020

Sources

• Blaschke, D.; Fredriksson, S.; Grigorian, H.; Öztaş, A.; Sandin, F. (2005). "Phase diagram of three-flavor quark matter under compact star constraints". Physical Review D. 72 (6): 065020. arXiv:hep-ph/0503194. Bibcode:2005PhRvD..72f5020B. doi:10.1103/PhysRevD.72.065020. S2CID 119356279.
• Sandin, F. (2005). "Compact stars in the standard model – and beyond". European Physical Journal C. 40 (2): 15–22. arXiv:astro-ph/0410407. Bibcode:2005EPJC...40...15S. doi:10.1140/epjcd/s2005-03-003-y. S2CID 119495444.
• Sandin, F. (2005). Exotic Phases of Matter in Compact Stars (PDF) (Thesis). Luleå University of Technology.

v t e Black holes
Outline
Types	BTZ black hole Schwarzschild Rotating Charged Virtual Kugelblitz Supermassive Primordial Direct collapse Rogue Malament–Hogarth spacetime
Size	Micro Extremal Electron Stellar Microquasar Intermediate-mass Supermassive Active galactic nucleus Quasar LQG Blazar OVV
Formation	Stellar evolution Gravitational collapse Neutron star Related links Tolman–Oppenheimer–Volkoff limit White dwarf Related links Supernova Micronova Hypernova Related links Gamma-ray burst Binary black hole Quark star Supermassive star Quasi-star Supermassive dark star X-ray binary
Properties	Astrophysical jet Gravitational singularity Ring singularity Theorems Event horizon Photon sphere Innermost stable circular orbit Ergosphere Penrose process Blandford–Znajek process Accretion disk Hawking radiation Gravitational lens Microlens Bondi accretion M–sigma relation Quasi-periodic oscillation Thermodynamics Bekenstein bound Bousso's holographic bound Immirzi parameter Schwarzschild radius Spaghettification
Issues	Black hole complementarity Information paradox Cosmic censorship ER = EPR Final parsec problem Firewall (physics) Holographic principle No-hair theorem
Metrics	Schwarzschild (Derivation) Kerr Reissner–Nordström Kerr–Newman Hayward
Alternatives	Nonsingular black hole models Black star Dark star Dark-energy star Gravastar Magnetospheric eternally collapsing object Planck star Q star Fuzzball Geon
Analogs	Optical black hole Sonic black hole
Lists	Black holes Most massive Nearest Quasars Microquasars
Related	Outline of black holes Black Hole Initiative Black hole starship Black holes in fiction Big Bang Big Bounce Compact star Exotic star Quark star Preon star Gravitational waves Gamma-ray burst progenitors Gravity well Hypercompact stellar system Membrane paradigm Naked singularity Population III star Supermassive star Quasi-star Supermassive dark star Rossi X-ray Timing Explorer Superluminal motion Timeline of black hole physics White hole Wormhole Tidal disruption event Planet Nine
Notable	Cygnus X-1 XTE J1650-500 XTE J1118+480 A0620-00 Sagittarius A* Centaurus A Phoenix Cluster PKS 1302-102 OJ 287 SDSS J0849+1114 TON 618 MS 0735.6+7421 NeVe 1 Hercules A 3C 273 Q0906+6930 Markarian 501 ULAS J1342+0928 PSO J030947.49+271757.31 AT2018hyz Swift J1644+57
Category Commons

v t e Stellar core collapse
Stars	Formation Evolution Structure Core Metallicity Stellar physics Stellar plasma Supergiant Variable star Cataclysmic variable star Binary star X-ray binary Super soft X-ray source
Stellar processes	Nuclear fusion Surface fusion Nucleosynthesis R-process RP-process Supernova nucleosynthesis Accretion (Bondi accretion) Electron capture Carbon detonation / deflagration Gamma-ray burst Helium flash Orbital decay
Collapse	Gravitational collapse Chandrasekhar limit Tolman–Oppenheimer–Volkoff limit
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Compact and exotic objects	Neutron star Pulsar Quasar Magnetar Radio-quiet White dwarf Black hole Collapsar Shell collapsar Exotic star Quark star Electroweak star Observational timeline
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Quantum theory	Quantum mechanics Introduction Basic concepts Quantum electrodynamics Quantum hydrodynamics Quantum chromodynamics (QCD) Lattice QCD Color confinement Deconfinement
Degenerate matter	Neutron matter QCD matter Quark matter Quark–gluon plasma Preon matter Strangelet Strange matter
Related topics	Astronomy Astrophysics Nuclear astrophysics Physical cosmology Physics of shock waves
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v t e Supernovae
Classes	Type Ia Type Iax Type Ib and Ic Type II (IIP, IIL, IIn, and IIb) Hypernova Superluminous Pair-instability Calcium-rich Common envelope jets supernova
Physics of	Carbon detonation Foe/Bethe Near-Earth Phillips relationship Nucleosynthesis p-process r-process γ-process Neutrinos
Related	Failed Fast blue optical transient Fast radio burst Gamma-ray burst Gravitational wave Kilonova Luminous red nova Micronova Nova Pulsar kick Quark-nova Soft gamma repeater Imposter pulsational pair-instability Symbiotic nova
Progenitors	Hypergiant yellow Red Luminous blue variable Supergiant blue red yellow White dwarf Wolf–Rayet star Super-AGB star Population III star
Remnants	Supernova remnant Pulsar wind nebula Neutron star pulsar magnetar Stellar black hole Compact star electroweak exotic quark Zombie star Local Bubble Superbubble Orion–Eridanus
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Lists	Candidates Notable Massive stars Most distant Remnants In fiction
Notable	Barnard's Loop Cassiopeia A SN 1054 Crab Nebula iPTF14hls SN 1000+0216 Tycho's Kepler's SN 1885A SN 1987A SN 1994D SN 185 SN 1006 SN 2003fg Remnant G1.9+0.3 SN 2007bi SN 2011fe SN 2014J SN Refsdal Vela Remnant SN 2006gy ASASSN-15lh SN 2016aps SN 2018cow SN 2022jli SN H0pe
Research	ASAS-SN Calán/Tololo Survey High-Z Supernova Search Team Katzman Automatic Imaging Telescope Monte Agliale Supernovae and Asteroid Survey Nearby Supernova Factory Sloan Supernova Survey Supernova/Acceleration Probe Supernova Cosmology Project SuperNova Early Warning System Supernova Legacy Survey Texas Supernova Search
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