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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. Most compact stars are at the endpoint of stellar evolution and thus are called stellar remnants; the form of the remnant depends primarily on the mass of the star when it formed. These objects are all small in volume 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 a 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.
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.
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 virtually forever, although black holes are generally believed to finally evaporate from Hawking radiation. Eventually, given enough time (when we enter the so-called degenerate era of the universe), all stars will have evolved into dark, compact stars.
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.
Rocky planets (and objects like them), made of primarily of silicates and some of period 4 metals, are only slightly compressible, therefore their density is almost independent of mass (in their known mass range) and their radii increase proportionally to the cubic root of mass. Jovian planets, made of fluid matter like hydrogen and helium, are more compressible.
A planet such as Jupiter has about the largest volume possible for a cold mass. In objects of several masses of Jupiter the pressure becomes high enough that electrons become degenerate, which makes its radius less dependent on the chemical composition and even temperature. Heavy gas giants and brown dwarfs even demonstrate decreasing radius with increasing mass.
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.
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.
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 small amount of protons and electrons.
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 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. 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 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.
An exotic star is a 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 largely theoretical, 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
It is possible that the neutrons will decompose into their component quarks. In this case, the star will shrink further and become denser, 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 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 are hypothetical compact, heavier neutron stars with an exotic state of matter where particle numbers are preserved. Q stars are also called "gray holes".
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.
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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 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. 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
- Dark energy star
- Black star
- Magnetospheric eternally collapsing object
- Dark star
- Primordial black holes
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