An exotic star is a compact star composed of something other than electrons, protons, and neutrons; and balanced against gravitational collapse by degeneracy pressure or other quantum properties. These include quark and strange stars (composed of quark or 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 quark 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.[who?]
Quark stars and strange stars
It is possible that the neutrons will decompose into their component up and down quarks when sufficient gravitational pressure is applied. 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. If quark stars contain strange matter, then they are called strange stars. The pulsar 3C 58 has been suggested as a possible quark star.
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.
The stage of life of a star that produces an electroweak star is theorized to occur after a supernova collapse. Electroweak stars are denser than quark stars, and may form when quark degeneracy pressure is no longer able to withstand gravitational attraction, but may still be withstood by electroweak burning radiation pressure.  This phase of a star's life may last upwards of 10 million years.
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 kg/m3. They may have greater densities than quark stars and neutron stars although they are smaller and lighter than white dwarfs and neutron stars. Preon stars could originate from supernova explosions or the big bang. Such objects could be detected in principle through gravitational lensing of gamma rays. Preon stars are a potential candidate for dark matter. However, current observations from particle accelerators speak against the existence of preons.
In general relativity, if the star collapses to a size smaller than its Schwarzschild radius, an event horizon will appear at that radius and the star will become a black hole. To avoid a collapse at its density, a preon star's radius would need to be less than about 40 m with mass less than about 0.013 solar masses.
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 that possesses a small mass. As of 2002 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.
Boson stars may have been formed through gravitational collapse during the primordial stages of the big bang. At least in theory, a supermassive boson star could exist at the core of a galaxy, which might explain many of the observed properties of active galactic cores. Boson stars have also been proposed as a candidate dark matter object.
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