Compact star

In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to 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 stars are often the endpoints of stellar evolution, and are in this respect 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 star 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 star that is not a black hole may be called a degenerate star.


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

Most 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 stars 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.[1]

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.


Although compact stars 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.

The somewhat wider definition of compact objects often includes smaller solid objects such as planets, asteroids, and comets. There is a remarkable variety of stars and other clumps of hot matter, but all matter in the Universe must eventually end as some form of compact stellar or substellar object, according to the theory of thermodynamics.

White dwarfs

The Eskimo Nebula is illuminated by a white dwarf at its center.

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 is now a white dwarf, the object shrinks and the central density becomes even larger, with higher degenerate-electron energies. The star's radius has now shrunk to only a few thousand kilometers, and the mass is approaching the theoretical upper limit of the mass of a white dwarf, the Chandrasekhar limit, about 1.4 times the mass of the Sun (M).

If we were to take matter from the center of our white dwarf and slowly start to compress it, we would first see electrons 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 4×1014 kg/m3), called the neutron drip line, the atomic nucleus would tend to fall apart into protons and neutrons. Eventually we would reach a point where the matter is on the order of the density (c. 2×1017 kg/m3) of an atomic nucleus. At this point the matter is chiefly free neutrons, with a small amount 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.[2][3][4] 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.[5][6][7] 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

Black Hole Milkyway
A simulated black hole of ten solar masses, at a distance of 600km.

As more mass is accumulated, equilibrium against gravitational collapse reaches its breaking point. The star's pressure is insufficient to counterbalance gravity and a catastrophic gravitational collapse occurs in milliseconds. The escape velocity at the surface, already at least 1/3 light speed, quickly reaches the velocity of light. No energy nor matter can escape: a black hole has formed. All light will be trapped within an event horizon, and so a black hole appears truly black, except for the possibility of Hawking radiation. It is presumed that the collapse will continue.

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

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, as long as no extra mass is added. It has, to some extent, become a very large nucleon. A-type 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 it has proven hard 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 1023 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.[9]

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.[10][11]

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.[12] 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.


  1. ^ Tauris, T. M.; J. van den Heuvel, E. P. (20 Mar 2003). "Formation and Evolution of Compact Stellar X-ray Sources".
  2. ^ 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.
  3. ^ 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 Msun 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.
  4. ^ Wanajo, S.; et al. (2003). "Ther‐Process in Supernova Explosions from the Collapse of O‐Ne‐Mg Cores". The Astrophysical Journal. 593 (2): 968. arXiv:astro-ph/0302262. Bibcode:2003ApJ...593..968W. doi:10.1086/376617.
  5. ^ 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.
  6. ^ 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.
  7. ^ Baade, W.; Zwicky, F. (1934). "Cosmic Rays from Super-Novae". Proceedings of the National Academy of Sciences. 20 (5): 259. Bibcode:1934PNAS...20..259B. doi:10.1073/pnas.20.5.259.
  8. ^ a b c Visser, M.; Barcelo, C.; Liberati, S.; Sonego, S. (2009). "Small, dark, and heavy: But is it a black hole?". arXiv:0902.0346 [hep-th].
  9. ^ Shiga, D. (4 January 2010). "Exotic stars may mimic big bang". New Scientist. Retrieved 2010-02-18.
  10. ^ Schutz, Bernard F. (2003). Gravity from the ground up (3rd ed.). Cambridge University Press. p. 143. ISBN 0-521-45506-5.
  11. ^ 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.
  12. ^ Ahmed Farag Ali and A. Tawfik, Int. J. Mod. Phys. D22 (2013) 1350020


AM Herculis

AM Herculis is a binary variable star located in the constellation Hercules. This star, along with the star AN Ursae Majoris, is the prototype for a category of cataclysmic variable stars called polars, or AM Her type stars.

Astronomical object

An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body or celestial body is a single, tightly bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.

Examples of astronomical objects include planetary systems, star clusters, nebulae, and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, and an object when describing the entire comet with its diffuse coma and tail.


Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:

The Compact, the agreement between the government and the voluntary and community sector in England

Interstate compact

Blood compact, an ancient ritual of the Philippines

Compact government, a type of colonial rule utilized in British North America

Compact of Free Association whereby the sovereign states of the Federated States of Micronesia, the Republic of the Marshall Islands and the Republic of Palau have entered into as associated states with the United States.

Mayflower Compact, the first governing document of Plymouth Colony

United Nations Global Compact

Global Compact for Migration, a UN non-binding intergovernmental agreement


DCO may refer to: Diploma in Community Ophthalmology

Exotic star

An exotic star is a hypothetical compact star composed of something other than electrons, protons, neutrons, or muons, and balanced against gravitational collapse by degeneracy pressure or other quantum properties. Exotic stars include quark stars (composed of quarks) and perhaps strange stars (composed of strange quark matter, a condensate of up, down and strange quarks), as well as speculative preon stars (composed of preons, which are hypothetical particles and "building blocks" of quarks, should quarks be decomposable into component sub-particles). Of the various types of exotic star proposed, the most well evidenced and understood is the quark star.

Exotic stars are largely theoretical – partly because it is difficult to test in detail how such forms of matter may behave, and partly because prior to the fledgling technology of gravitational-wave astronomy, there was no satisfactory means of detecting cosmic objects that do not radiate electromagnetically or through known particles. So it is not yet possible to verify novel cosmic objects of this nature by distinguishing them from known objects. Candidates for such objects are occasionally identified based on indirect evidence gained from observable properties.

F-type main-sequence star

An F-type main-sequence star (F V) is a main-sequence, hydrogen-fusing compact star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K.Tables VII and VIII. This temperature range gives the F-type stars a yellow-white hue. Because a main-sequence star is referred to as a dwarf star, this class of star may also be termed a yellow-white dwarf. Famous examples include Procyon A, Gamma Virginis A and B, and KIC 8462852.

Interacting binary star

An Interacting binary star is a type of binary star in which one or both of the component stars has filled or exceeded its Roche lobe. When this happens, material from one star (the donor star) will flow towards the other star (the accretor). If the accretor is a compact star, an accretion disk may form. The physical conditions in such a system can be complex and highly variable, and they are common sources of cataclysmic outbursts.

A common type of interacting binary star is one in which one of the components is a compact object which is well within its Roche lobe, while the other is an evolved giant star. If the compact object is a white dwarf, then accretion of material from the evolved star onto the white dwarf's surface may result in its mass increasing to beyond the Chandrasekhar limit. This can lead to runaway thermonuclear reactions and the massive explosion of the star in a Type I supernova.

An example of such a binary star is R Canis Majoris, in which it is thought that the secondary star has exceeded its Roche lobe and transferred mass to the primary star. This has resulted in the early evolution of the secondary star onto the subgiant star branch, and in exposure of helium-rich material on the surface of the primary, causing it to burn brighter and have a higher effective temperature than would usually be expected for a star of its mass.

Intermediate polar

An Intermediate Polar (also called a DQ Herculis Star) is a type of cataclysmic variable binary star system with a white dwarf and a cool main-sequence secondary star. In most cataclysmic variables, matter from the companion star is gravitationally stripped by the compact star and forms an accretion disk around it. In intermediate polar systems, the same general scenario applies except that the inner disk is disrupted by the magnetic field of the white dwarf.

The name "intermediate polar" is derived from the strength of the white dwarf's magnetic field, which is between that of non-magnetic cataclysmic variable systems and strongly magnetic systems. Non-magnetic systems exhibit full accretion disks, while strongly magnetic systems (called polars or AM Herculis systems) exhibit only accretion streams which directly impact the white dwarf's magnetosphere.

There were 26 confirmed intermediate polar systems as of 14 April 2006. This represents about 1% of the 1,830 total cataclysmic variable systems presented by Downes et al. (2006) in the Catalog of Cataclysmic Variables. Only two of them are brighter than 15th magnitude at minimum: the prototype DQ Herculis and the unusual slow nova GK Persei.

Iron star

In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.

The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.

Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.

Outline of astronomy

The following outline is provided as an overview of and topical guide to astronomy:

Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).

PSR J1719−1438

PSR J1719-1438 is a millisecond pulsar with a spin period of 5.8 ms located about 4,000 ly from Earth in the direction of Serpens Cauda, one minute from the border with Ophiuchus. Millisecond pulsars are generally thought to begin as normal pulsars and then spin up by accreting matter from a binary companion.

Preon star

A preon star is a theoretical type of compact star made of preons, which are "point-like" particles conceived to be subcomponents of quarks and leptons. Their existence was first theorized in

2005 by Fredrik Sandin and Johan Hansson, both from the Luleå University of Technology, Sweden. The theory behind them was that the sub-subatomic particles would have come before subatomic particles, which came before particles, and that the original stars were made of these sub-subatomic particles, with most gradually becoming made of sub-particles, and then particles. However, the theory postulates that it is possible for some stars made of the sub-subatomic particles to remain. It is believed that they may also form out of massive stars that collapse too unstably to become neutron stars, but not enough to become black holes.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.


The rp-process (rapid proton capture process) consists of consecutive proton captures onto seed nuclei to produce heavier elements. It is a nucleosynthesis process and, along with the s-process and the r-process, may be responsible for the generation of many of the heavy elements present in the universe. However, it is notably different from the other processes mentioned in that it occurs on the proton-rich side of stability as opposed to on the neutron-rich side of stability. The end point of the rp-process (the highest mass element it can create) is not yet well established, but recent research has indicated that in neutron stars it cannot progress beyond tellurium. The rp-process is inhibited by alpha decay, which puts an upper limit on the end point at 104Te, the lightest observed alpha decaying nuclide, and the proton drip line in light antimony isotopes. At this point, further proton captures result in prompt proton emission or alpha emission and thus the proton flux is consumed without yielding heavier elements; this end process is known as the tin-antimony-tellurium cycle.

SN 2004dj

SN 2004dj was the brightest supernova since SN 1987A at the time of its discovery.

This Type II-P supernova was discovered by Koichi Itagaki, a Japanese astronomer on July 31, 2004. At the time of its discovery, its apparent brightness was 11.2 visual magnitude; the discovery occurred after the supernova had reached its peak magnitude. The supernova's progenitor is a star in a young, compact star cluster in the galaxy NGC 2403, in Camelopardalis. The cluster had been cataloged as the 96th object in a list of luminous stars and clusters by Allan Sandage in 1984; the progenitor is therefore commonly referred to as Sandage 96. This cluster is easily visible in a Kitt Peak National Observatory image and appears starlike.

Stellar black hole

A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma ray burst. These black holes are also referred to as collapsars.

Symbiotic binary

A symbiotic binary is a type of binary star system, often simply called a symbiotic star. They usually contain a white dwarf with a companion red giant. The cool giant star loses material via Roche lobe overflow or through its stellar wind, which flows onto the hot compact star, usually via an accretion disk.

Symbiotic binaries are of particular interest to astronomers as they can be used to learn about stellar evolution. They are also vital in the study of stellar wind, ionized nebulae, and accretion because of the unique interstellar dynamics present within the system.

In binary
Single pulsars
Binary pulsars
Stellar core collapse
Stellar processes
Compact and exotic objects
Particles, forces, and interactions
Quantum theory
Degenerate matter
Related topics
Physics of
Luminosity class
Star systems
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