harry wrote:Neutron stars is there a limit to their size?
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How small can they get?
http://en.wikipedia.org/wiki/Free_neutron
<<A free neutron is a neutron that exists outside of an atomic nucleus. While neutrons can be stable when bound inside nuclei, free neutrons are unstable and have a mean lifetime of 886 s (about 15 minutes), decaying by emission of a negative electron and antineutrino to become a proton>>
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----------- Rest mass
Neutron : 939.566 MeV + kinetic energy - binding energy
Proton : 938.272 MeV + kinetic energy - binding energy
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_______ 1.294 MeV + Δkinetic energy - Δbinding energy
Electron : 0.511 MeV+ escape kinetic energy
Antineutrino : 0 MeV+ kinetic energy
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http://en.wikipedia.org/wiki/Dineutron
<<A dineutron is a hypothetical particle consisting of two neutrons that was suggested to have a transitory existence in nuclear reactions produced by helions that result in the formation of a proton and a nucleus having the same atomic number as the target nucleus but a mass number two units greater. A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.>>
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http://en.wikipedia.org/wiki/Tetraneutron
<<A tetraneutron is a hypothesised stable cluster of four neutrons. This cluster of particles is not supported by current models of nuclear forces. However, there is some empirical evidence which suggests this particle does exist, based on an experiment by Francisco-Miguel Marqués and co-workers at the Ganil accelerator in Caen using a novel detection method in observations of the disintegration of beryllium and lithium nuclei. Subsequent attempts to replicate this observation have failed.
Confirmation of the existence of a tetraneutron would be a significant discovery because current nuclear theory suggests that these clusters should not be stable, and thus should not exist. If it does, then it has been suggested that the substance be considered an "element", and be placed on the Periodic Table of the Elements, with an atomic number of 0 (zero).
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http://en.wikipedia.org/wiki/Neutronium
<<Neutronium is a term originally used in science fiction and in popular literature to refer to an extremely dense phase of matter composed primarily of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (i.e. before the discovery of the neutron itself) for the conjectured 'element of atomic number zero' that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been used legitimately to refer to extremely dense phases of matter resembling the neutron-degenerate matter postulated to exist in the cores of neutron stars.>>
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It is not exactly clear how small a large ball of neutronium there could be that would be gravitationally stable against either beta decay or the quantum degeneracy pressure of neutrons. However,
"In general, compact stars of less than 1.38 solar masses,
the Chandrasekhar limit, are white dwarfs.""
harry wrote:How big can they get?
http://en.wikipedia.org/wiki/Neutron_stars
<<A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 20 and 10 km, respectively — in contrast, the Sun is 30,000 to 70,000 times larger. Thus, neutron stars have overall densities of 8.4×1016 to 1×1018 kg/m³, which compares with the approximate density of an atomic nucleus of 3×1017 kg/m³. The neutron star's density varies from below 1×109 kg/m³ in the crust increasing with depth to above 6 or 8×1017 kg/m³ deeper inside.
In general, compact stars of less than 1.38 solar masses, the Chandrasekhar limit, are white dwarfs; above 2 to 3 solar masses (the Tolman-Oppenheimer-Volkoff limit), a quark star might be created, however this is uncertain. Gravitational collapse will always occur on any star over 5 solar masses, inevitably producing a black hole.>>
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http://en.wikipedia.org/wiki/Tolman-Opp ... koff_limit
<<The Tolman-Oppenheimer-Volkoff (TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter (neutron stars). It is analogous to the Chandrasekhar limit for white dwarf stars.
The limit was computed by J. Robert Oppenheimer and George Michael Volkoff in 1939, using work of Richard Chace Tolman. Oppenheimer and Volkoff assumed that the neutrons in a neutron star formed a cold, degenerate Fermi gas. This leads to a limiting mass of approximately 0.7 solar masses. Modern estimates range from approximately 1.5 to 3.0 solar masses. The uncertainty in the value reflects the fact that the equations of state for extremely dense matter are not well-known.
In a neutron star lighter than the limit, the weight of the star is supported by short-range repulsive neutron-neutron interactions mediated by the strong force and also by the quantum degeneracy pressure of neutrons. If a neutron star is heavier than the limit, it will collapse to some denser form. It could form a black hole, or change composition and be supported in some other way (for example, by quark degeneracy pressure if it becomes a quark star). Because the properties of hypothetical more exotic forms of degenerate matter are even more poorly known than those of neutron-degenerate matter, most astrophysicists assume, in the absence of evidence to the contrary, that a neutron star above the limit collapses directly into a black hole.
A black hole formed by the collapse of an individual star must have mass exceeding the Tolman-Oppenheimer-Volkoff limit. Theory predicts that because of mass loss during stellar evolution, a black hole formed from an isolated star of solar metallicity can have mass no more than approximately 10 solar masses. Observationally, because of their large mass, relative faintness, and X-ray spectra, a number of massive objects in X-ray binaries are thought to be stellar black holes. These black hole candidates are estimated to have masses between 3 and 20 solar masses>>
