In astronomy, the term compact star (sometimes compact object) is used to refer collectively to white dwarfs, neutron stars, other exotic dense stars, and black holes. These objects are all small for their mass. The term compact star is often used when the exact nature of the star is not known, but evidence suggests that it is very massive and has small radius, thus implying one of the above-mentioned possibilities. A compact star which is not a black hole may be called a degenerate star. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ... This article or section does not adequately cite its references or sources. ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... An exotic star is a compact star composed of something other than electrons, protons, and neutrons, balanced against gravitational collapse by degeneracy pressure. ... For other uses, see Black hole (disambiguation). ... This article or section is in need of attention from an expert on the subject. ... Remote Authentication Dial In User Service (RADIUS) is an AAA (authentication, authorization and accounting) protocol for applications such as network access or IP mobility. ...

Compact stars as the endpoint of stellar evolution

Compact stars form the endpoint of stellar evolution. A star shines and thus loses energy. The loss from the radiating surface is compensated by the production of energy from nuclear fusion in the interior of the star. When a star has exhausted all its energy and undergoes stellar death, the gas pressure of the hot interior can no longer support the weight of the star and the star collapses to a denser state: a compact star. The difference between a white dwarf or neutron star and an ordinary star is analogous to the difference between and solids and gases. If you waited until a white dwarf or neutron star was sufficiently cold, and if you had a rocket which could survive the enormous gravitational and tidal forces, you could land on the surface of the star. Typical cooling times for white dwarfs, however, are much larger than the present age of the Universe. In astronomy, stellar evolution is the sequence of radical changes that a star undergoes during its lifetime (the time in which it emits light and heat). ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... For other uses, see Solid (disambiguation). ... This article does not cite any references or sources. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

Compact stars last forever

Although compact stars may radiate, and thus cool off and lose energy, they do not depend on high temperatures to maintain their pressure. Barring external perturbation or baryon decay, they will persist forever. Eventually, given enough time (when we enter the so-called degenerate era of the universe)[1], all stars will have evolved into dark, compact stars. The heat death is a impossible final state of the universe, in which it has run down to a state of no free energy to sustain motion or life. ...

A somewhat wider class of compact objects is sometimes defined to contain, as well as compact stars, smaller solid objects such as planets, asteroids, and comets. These compact objects are the only objects in the universe that could exist at low temperatures. There is a remarkable variety of stars and other clumps of matter, but all dense matter in the universe must eventually end in one of only five classes of compact objects. The eight planets and three dwarf planets of the Solar System. ... 253 Mathilde, a C-type asteroid. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ...

Thought experiment in building compact objects

Suppose we do a thought experiment and build a cold object by adding mass and ignoring thermal pressure. How will it stand the gravitational pull? In this experiment, we will find the five possible types of object: planet-like, white dwarf, neutron star, exotic star, and black hole. In philosophy, physics, and other fields, a thought experiment (from the German Gedankenexperiment) is an attempt to solve a problem using the power of human imagination. ...

Planets

At low density (planets and the like) the object is held up by electromagnetic forces. These forces constrain electrons to occupy orbitals around nuclei, which give rise to chemical bonds and thus allow stiff objects such as rocks to exist. These objects are so stiff that they do not compress very much when mass is added. Adding more (cold) mass therefore makes the object larger: radius increases with mass. This agrees with our intuitions. In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... In physics, the electromagnetic force is the force that the electromagnetic field exerts on electrically charged particles. ... e- redirects here. ... In chemistry, an atomic orbital is the region in which an electron may be found around a single atom. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ...

Eventually a point is reached where the central pressure is so large that all matter is ionized so that the electrons are stripped from the nuclei and move freely. No chemical bonds now exist to hold up the object. This point is reached at the center of the planet Jupiter. Add more mass to Jupiter and the increase of pressure is smaller than the increase of gravity, so the radius will decrease with increasing mass. The object will shrink. This article is about the electrically charged particle. ...

The largest cold mass in the universe

A planet such as Jupiter has about the largest volume possible for a cold mass. Add mass to Jupiter and the planet's volume, somewhat counter-intuitively, becomes smaller. The central density now is large enough that the free electrons become degenerate. This term means that the electrons have fallen into the lowest-energy states available. Since electrons are fermions, they obey the Pauli exclusion principle, and no two electrons can occupy the same state. The electrons thus occupy a wide band of low-energy states. Compressing the mass forces this band to widen, creating the quantum-mechanical force of electron degeneracy pressure which now holds the center of the planet apart. (The ions present contribute almost no force.) Image File history File links Download high-resolution version (779x773, 493 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Image File history File links Download high-resolution version (779x773, 493 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Image File history File links Download high-resolution version (779x773, 493 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Image File history File links Download high-resolution version (779x773, 493 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Adjectives: Jovian Atmosphere [4] Surface pressure: 20–200 kPa[9] (cloud layer) Scale height: 27 km Composition: Jupiter (IPA: or ) is the fifth planet from the Sun and the largest planet within the solar system. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... A quantum state is any possible state in which a quantum mechanical system can be. ... In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ... Fig. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

White dwarfs

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

Main article: White dwarf

If we continue to add mass in our thought-experiment, we will find that more and more of our object becomes degenerate. The stars called degenerate dwarfs or, more usually, white 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. Image File history File links Download high-resolution version (1500x1500, 402 KB)The Eskimo Nebula (NGC 2392) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high-resolution version (1500x1500, 402 KB)The Eskimo Nebula (NGC 2392) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... NGC 2392, the Eskimo Nebula. ... This article or section does not adequately cite its references or sources. ... A Degenerate dwarf is type of star, an alternative name for what is commonly called a White dwarf (see this reference for a more complete article). ... This article or section does not adequately cite its references or sources. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. ... A black dwarf is a hypothetical astronomical object: a white dwarf so old that it has cooled down so that it no longer emits significant heat or light. ...

The equation of state for degenerate matter is "soft", meaning that adding more mass will result in a smaller object. If in our thought experiment we keep adding mass to what is now a white dwarf, the object therefore 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[2], 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. In physics and thermodynamics, an equation of state is a constitutive equation describing the state of matter under a given set of physical conditions. ... A kilometer (Commonwealth spelling: kilometre), symbol: km is a unit of length in the metric system equal to 1,000 metres (from the Greek words χίλια (khilia) = thousand and μέτρο (metro) = count/measure). ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ...

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, more neutron-rich nuclei which 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 (~2·10¹⁷ kg/m³) of an atomic nucleus. At this point the matter is chiefly free neutrons, with a sprinkling of protons and electrons. Objects with these central densities will be formed if in our thought experiment we continue to add mass to a white dwarf until the Chandrasekhar limit is exceeded. They form our third class of compact objects. In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... This article or section does not adequately cite its references or sources. ... Electron capture is a decay mode for isotopes that will occur when there are too many protons in the nucleus of an atom, and there isnt enough energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron... The kilogram or kilogramme (symbol: kg) is the SI base unit of mass. ... The cubic meter (symbol m³) is the SI derived unit of volume. ... Neutron drip line is a concept in particle and nuclear physics. ...

Neutron stars

The Crab Nebula is a supernova remnant containing the Crab Pulsar, a neutron star.

Main article: Neutron star

We have reached a point where nature takes over from our thought experiment, as addition of matter to a white dwarf actually happens in nature. 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. The star will 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 which 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.[3],[4],[5] 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. Image File history File links Download high-resolution version (2400x2400, 314 KB) A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. ... Image File history File links Download high-resolution version (2400x2400, 314 KB) A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. ... The Crab Nebula (catalogue designations M 1, NGC 1952, Taurus A) is a supernova remnant in the constellation of Taurus. ... The Crab Nebula is an expanding cloud of gas created by the 1054 supernova. ... Categories: Astronomy stubs | Pulsars ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... A binary star system consists of two stars both orbiting around their barycenter. ... This article or section does not cite its references or sources. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ...

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.[6][7][8] 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. It has been suggested that Radio pulsar be merged into this article or section. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... Type Ib and Ic supernovae are categories of stellar explosions. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...

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 times the mass of the Sun. If more mass accretes onto a neutron star, eventually this mass limit will be reached. What happens next is not completely clear. Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... This article is in need of attention from an expert on the subject. ...

Exotic stars

Main article: Exotic star

An exotic star is a compact star composed of something other than electrons, protons, and neutrons, balanced against gravitational collapse by degeneracy pressure. ...

Strange stars

Main article: Quark star

It is possible that the neutrons will decompose into their component quarks. In this case, the star will shrink further and become more dense, but it may survive in this new state indefinitely if no extra mass is added. It has become a very large nucleon. A star in this hypothetical state is called a quark star or strange star. The pulsars RX J1856.5-3754 and 3C58 have been suggested as possible quark stars. A strange star or quark star is a hypothetical type of star composed of strange matter, or quark matter. ... This article or section does not adequately cite its references or sources. ... The six flavours of quarks and their most likely decay modes. ... In physics a nucleon is a collective name for two baryons: the neutron and the proton. ... A strange star or quark star is a hypothetical type of star composed of strange matter, or quark matter. ... RX J1856. ... an X-Ray image of 3C58 from the orbiting Chandra X-Ray Observatory. ...

Preon stars

Main article: Preon star

If we go beyond the standard model of particle physics and assume that quarks and leptons are not the fundamental elementary particles but are themselves composed of preons, then even denser objects, preon stars, would not be unthinkable. A star may collapse to one ten-thousandth of its size, bringing its radius to one metre or less. It would be a sort of giant quark whose density might exceed 10²³ kg/m³, and might even approach 10³³ kg/m³. A preon star is a hypothetical compact star made of preons, a group of theoretical subatomic particles that may compose quarks and leptons. ... The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ... The six flavours of quarks and their most likely decay modes. ... In physics, a lepton is a particle with spin-1/2 (a fermion) that does not experience the strong interaction (that is, the strong nuclear force). ... In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. ... In particle physics, preons are postulated point-like particles, conceived to be subcomponents of quarks and leptons. ... A preon star is a hypothetical compact star made of preons, a group of theoretical subatomic particles that may compose quarks and leptons. ...

Q stars

Main article: Q star

Q-Stars are compact, heavier neutron stars with an exotic state of matter. Definition A Q-Star is a compact object with an exotic state of matter. ...

The stellar-mass objects we have seen so far (white dwarfs, neutron stars, and presumably the more exotic possibilities of quark and preon stars) have all been held up wholly or partially by degeneracy pressure. Collectively we may therefore call them degenerate stars. We now come to a different possibility.

Black holes

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

Main article: Stellar black hole

As we add more mass, 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 or matter can escape: a black hole has been created. 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 will be created occupying no more than a point. 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. Image File history File links Download high resolution version (2560x2048, 1172 KB) Summary Description: A Black Hole of ten solar masses as seen from a distance of 600km with the Milky Way in the background (horizontal camera opening angle: 90°) Source: Gallery of Tempolimit Lichtgeschwindigkeit Date: 14. ... Image File history File links Download high resolution version (2560x2048, 1172 KB) Summary Description: A Black Hole of ten solar masses as seen from a distance of 600km with the Milky Way in the background (horizontal camera opening angle: 90°) Source: Gallery of Tempolimit Lichtgeschwindigkeit Date: 14. ... A stellar black hole is a black hole formed by the gravitational collapse of a massive star (3 or more solar masses) at the end of its lifetime. ... Space Shuttle Atlantis launches on mission STS-71. ... For other uses, see Black hole (disambiguation). ... For the science fiction film, see Event Horizon (film). ... Black cat, thought by some to cause bad luck (see superstition) Black is the shade of objects that do not reflect light in any part of the visible spectrum. ... In physics, Hawking radiation (also known as Bekenstein-Hawking radiation) is a thermal radiation thought to be emitted by black holes due to quantum effects. ... For a less technical and generally accessible introduction to the topic, see Introduction to general relativity. ... A gravitational singularity (sometimes spacetime singularity) is, approximately, a place where quantities which are used to measure the gravitational field become infinite. ... A spatial point is an entity with a location in space but no extent (volume, area or length). ... The Planck length, denoted by , is the unit of length approximately 1. ...

References

• D. Blaschke, S. Fredriksson, H. Grigorian, A. M.Oztas, and F. Sandin, The phase diagram of three-flavor quark matter under compact star constraints. (arXiv:hep-ph/0503194)
• Johan Hansson and Fredrik Sandin, Preon stars: a new class of cosmic compact objects. Phys. Lett. B 616, 1, 2005. (arXiv:astro-ph/0410417)
• Fredrik Sandin, Compact stars in the standard model - and beyond, Eur. Phys. J. C.
• Fredrik Sandin, Exotic Phases of Matter in Compact Stars. (May 8, 2005)