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COMPACT STARS AS THE ENDPOINT OF STELLAR EVOLUTION Compact stars form the endpoint of es and Solid s. On the hard surface one could land with a rocket, if one waited long enough for the object to cool and if the rocket could survive the enormous gravitational forces (particularly the Tide ). Note that typical cooling times are much longer than the present age of the universe. COMPACT STARS LAST FOREVER The structure of compact stars is independent of temperature. They could just sit there forever, shining and cooling (hence terminology such as "the endpoints of stellar evolution"). The pressure is supplied by other means, which (as long as the hydrogen atom remains stable) does not change over time. Eventually, given enough time (when we enter the so called Degenerate Era Of The Universe ), all stars will stop shining and evolve into compact stars. One sometimes defines a somewhat wider class of compact objects as compact stars plus smaller solid objects such as Planet s, Asteroid s, and Comet s. 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 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. How will it stand the gravitational pull? In doing so, we find the five possible objects: 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 Force s ( Chemical Bond s between Atom s and repulsion between Electron s), which allow stiff objects such as rocks. The objects are so stiff that they can deal easily with the increased gravity from the added mass. So adding more (cold) mass means making larger objects (radius increases with mass). This corresponds with our intuitive thinking. Eventually a point is reached where all matter is (pressure) Ion ized, the electrons are stripped from the nuclei and are free. No chemical bonds can hold up the object. This point is reached at the center of the planet Jupiter. Add more mass to Jupiter and the pressure increase 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 A planet such as Jupiter is about the largest cold mass that can exist. Add mass to Jupiter and the planet, somewhat counter-intuitively, becomes smaller. The central density now is large enough that the free electrons become Degenerate . Quantum Mechanical forces hold the center of the planet apart, the ions hardly contributing at all. The matter has become "soft", in that adding more mass will result in a still smaller object. As an increasing part of the interior contains degenerate electrons, such objects are called White Dwarf s. Massive white dwarfs are smaller than less massive ones. WHITE DWARFS is illuminated by the white dwarf at its center.]] See Also: white dwarf In continuing our thought-experiment we keep adding mass to what is now a white dwarf, the star shrinks and the central density becomes even larger, with higher degenerate-electron energies. A point is reached where the electrons have sufficient energy to combine with Proton s in Atomic Nuclei ( Inverse-beta Decay ). As a result, Neutron s are formed and electrons disappear. Odd neutron-rich nuclei become possible, which would not be stable at lower density. Such nuclei are less well-bound and at a certain density, called the Neutron Drip Point , the atomic nucleus falls apart into many neutrons and as many protons as there are electrons. This stage is reached at a mass slightly below the theoretical upper limit of the mass of a white dwarf, the Chandrasekhar Limit , about 1.4 times the mass of the Sun. The star's radius has shrunk to 10,000 Kilometer s. NEUTRON STARS is a Supernova Remnant containing the Crab Pulsar , a Neutron Star .]] See Also: neutron star We have reached a point where nature takes over our thought experiment: addition of matter to a white dwarf actually happens in nature. In certain , to a radius between 20 and 10 km. This is a Neutron Star . More commonly, neutron stars form from the collapse of stars too massive to form white dwarfs. In any case, some of the energy of collapse is released in a Supernova of Type Ib Or Ic or Type II . EXOTIC STARS Neutron stars also have a maximum mass, called the Tolman-Oppenheimer-Volkoff Limit . It is currently thought to be about 3 times the mass of the Sun. The exact value depends on the forces between neutrons at high density that in addition to the degenerate neutron-pressure could add to the overall pressure. If more mass accretes onto a neutron star, eventually this mass limit is reached, and new equilibriums may be found. Strange stars See Also: quark star Quark Star s or ''strange stars'' are thought to lie between the density of Neutron Star s and Stellar Black Hole s. It is possible that the neutrons will decompose into their constituent Quark s. The star will shrink further, but it may survive in this new state indefinitely if no extra mass is added. It has become the largest Nucleon in the universe. Preon stars See Also: preon star If Quark s and Lepton s are not the fundamental Elementary Particle s but are themselves composed of Preon s, then even denser objects, Preon Star s, would not be unthinkable. Our star may collapse to one ten-thousandth of its size, bringing its radius to one meter or less. Its density will exceed 1020 G / Cm³ , and may approach 1030 g/cm³. BLACK HOLES See Also: stellar black hole Continuing our experiment, added mass pushes equilibrium to its breaking point. The star's pressure is insufficient to counterbalance gravity, and a catastrophic gravitational collapse occurs in milliseconds. The has been created. All light will be trapped within an Event Horizon , and so a black hole appears truly Black (but see '' Hawking Radiation ''). It is presumed that the collapse will continue, forming a Gravitational Singularity occupying no more than a Point . One expects a new "halt" of the catastrophic gravitational collapse at a size according to the Planck Length , but at present there is no theory of gravity at such densities to predict that. REFERENCES
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