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. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type An exotic star is a Compact star composed of something other than Electrons Protons and Neutrons balanced against Gravitational collapse A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e 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. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers A compact star which is not a black hole may be called a degenerate star.
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Compact stars as the endpoint of stellar evolution
Compact stars form the endpoint of stellar evolution. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime 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. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus 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. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The difference between a white dwarf or neutron star and an ordinary star is analogous to the difference between solids and gases. A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. This page is about the physical properties of gas as a state of matter 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. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced Typical cooling times for white dwarfs, however, are much larger than the present age of the Universe.
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. In Particle physics, proton decay is a hypothetical form of Radioactive decay in which the Proton decays into lighter Subatomic particles 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 possible final state of the universe, in which it has " run down " to a state of no Thermodynamic free energy to sustain
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. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — 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.
Thought experiment in building compact objects
Suppose we do a thought experiment and build a cold object by adding mass and ignoring thermal pressure. A thought experiment (from the German Gedankenexperiment) is a proposal for an Experiment that would test a Hypothesis or Theory 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.
Planets
At low density (planets and the like) the object is held up by electromagnetic forces. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different In Physics, the electromagnetic force is the force that the Electromagnetic field exerts on electrically charged particles 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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic 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.
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. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge 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.
The largest cold mass in the universe
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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. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion This term means that the electrons have fallen into the lowest-energy states available. In Quantum physics, a quantum state is a mathematical object that fully describes a quantum system. Since electrons are fermions, they obey the Pauli exclusion principle, and no two electrons can occupy the same state. In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925 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. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Electron degeneracy pressure is a consequence of the Pauli exclusion principle, which states that two Fermions cannot occupy the same Quantum state at the (The ions present contribute almost no force. )
White dwarfs
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If we continue to add mass in our thought-experiment, we will find that more and more of our object becomes degenerate. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. 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. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion White dwarfs arise from the cores of main-sequence stars and are therefore very hot when they are formed. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness As they cool they will redden and dim until they eventually become dark black dwarfs. A black dwarf is a hypothetical Star, created when a White dwarf becomes sufficiently cool to no longer emit significant Heat or Light 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. In Physics and Thermodynamics, an equation of state is a relation between state variables More specifically an equation of state is a thermodynamic 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. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The Chandrasekhar limit limits the mass of bodies made from Electron-degenerate matter, a dense form of matter which consists of nuclei immersed in a gas of Electrons 4 times the mass of the Sun.
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 proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. Electron capture (sometimes called inverse beta decay) is a Decay mode for Isotopes that will occur when there are too many Protons in the 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. CM3 redirects here If you were looking for the 3rd game in the Cooking Mama series abbreviated as CM3 see here. The neutron drip line is a concept in particle and Nuclear physics. 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.
Neutron stars
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. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type 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. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. 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. Gravitational collapse in Astronomy is the inward fall of a massive body under the influence of the force of Gravity. A Type Ia supernova is a sub-category of cataclysmic Variable 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. 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 This is a neutron star. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type
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. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves 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. A supernova (plural supernovae or supernovas) is a stellar Explosion. [6][7][8] This is the explanation for supernovae of types Ib, Ic, and II. Types Ib and Ic supernovae are categories of stellar explosions A supernova (plural supernovae or supernovas) is a stellar Explosion. 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. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion 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. The Tolman-Oppenheimer-Volkoff ( TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter ( Neutron stars. 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.
Exotic stars
Strange stars
It is possible that the neutrons will decompose into their component quarks. An exotic star is a Compact star composed of something other than Electrons Protons and Neutrons balanced against Gravitational collapse A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. 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. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth It has become a very large nucleon. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. A star in this hypothetical state is called a quark star or strange star. A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. The pulsars RX J1856.5-3754 and 3C58 have been suggested as possible quark stars. RX J18565-3754 (also called RX J185635-3754, RX J185635-375, and various other designations is a nearby Neutron star. 3C58 (aka 3C 58) is a Pulsar and surrounding synchrotron nebula within the Milky Way that is possibly associated with the Supernova
Preon stars
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 preon star is a hypothetical Compact star made of Preons a group of Theoretical Subatomic particles that may compose Quarks The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made In Particle physics, preons are postulated "point-like" particles conceived to be subcomponents of Quarks and Leptons The word was coined A preon star is a hypothetical Compact star made of Preons a group of Theoretical Subatomic particles that may compose Quarks 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.
In general relativity, if the star collapses to a size smaller than its event horizon, it will become a black hole. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot For a one solar mass object, the event horizon has a radius of 3 km; so, to be consistent with general relativity, any exotic Preon star state would have to have a radius larger than this size.
Q stars
Q-Stars are compact, heavier neutron stars with an exotic state of matter. A Q-Star, also known as a Gray hole, is hypothetical type of a compact heavy neutron star 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
As we add more mass, equilibrium against gravitational collapse reaches its breaking point. A stellar black hole is a Black hole formed by the Gravitational collapse of a massive Star (20 or more Solar masses, though the exact amount 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. In Physics, escape velocity is the speed where the Kinetic energy of an object is equal to the magnitude of its Gravitational potential energy No energy or matter can escape: a black hole has been created. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e All light will be trapped within an event horizon, and so a black hole appears truly black, except for the possibility of Hawking radiation. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot Black is the Color of objects that do not emit or Reflect Light in any part of the Visible spectrum; they absorb all such frequencies of Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes 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. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 A gravitational singularity (sometimes spacetime singularity) is approximately a place where quantities which are used to measure the Gravitational field become In Geometry, Topology and related branches of mathematics a spatial point describes a specific point within a given space that consists of neither Volume 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. The Planck length, denoted by \scriptstyle\ell_P \, 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)
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