What prevents a neutron star from collapsing and becoming a black hole?
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As Kris says, its neutron degeneracy pressure.  Particles with half-integer spin such as neutrons obey Fermi-Dirac statistics and the Pauli Exclusion principle which states two particles cannot have the same quantum mechanical state.  Particles with integer spin (such as photons) have no such restriction.\n\nIn degenerate matter, the extreme density means all the lower energy levels available for particles to occupy are filled up, meaning it takes a lot of energy to boost a particle to a high enough energy to reach an available level.  When you compress a material, you do work on the material and therefore change the velocity and energy level of particles.  Degenerate matter greatly resists such compression since changing the velocity of a particle is so difficult.  There are no levels available at lower energies, and the available levels at higher energies are so far off.  So the material is nearly incompressible.\n\nNearly.
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A (type II) supernova is the natural end of a massive star atleast 8 times more massive than the sun. The energy for the explosion is generated by a catastrophic gravitational crunch (as opposed to a type I supernova which is an actual nuclear detonation. They are probably caused by two white dwarfs colliding.)\n\nAs a massive star runs out of hydrogen in its core it  contracts and gets hotter until heliumfusion begins. When an average mass star, like the sun, enters this stage it is the end. The core doesn´t have enough mass to fuse elements heavier than helium and a white dwarf remains.\n\nIn heavier stars the fusion processes goes through increasingly heavier elements with the core getting smaller and hotter in each step. When silicone has fused into iron the end has come. Iron is too tighly bound in its nucleus to allow more fusion despite a temperature of 3 billion degrees. With no more energy keeping the core up it collapses with a quarter the speed of light. It shrinks from the size of earth to a sphere only 100 km in diameter in less than one tenth of a second. The gravitational energy released in that instant is 100 times greater than all the energy the sun has put out in all its 4.6 billion year lifetime. Needless to say the star explodes. A super nova is born.\n\nThe core remain however and it contracts so much that, when the core is only 8 km in diameter, protons and electrons cannot exist anymore and you have a neutron star.\n\nIf the mass of the core is greater then 3 solar masses then not even the neutrons can keep the neutron star from contracting further. In stead it shrinks and shrinks and shrinks until the density of the object is infinite. It is now a singularity. We call such singularities black holes because we can´t actually see them. The escape velocity is greater than the speed of light and since nothing can travel faster than light they are truly black. We can only see the effect the gravity of a black hole has on nearby stars.
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All stars eventually die because gravity pulls the gas to the inside where it implodes, sometimes causing a black hole. Most just burn out, though and are just a big floating mass.
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one of the properties of a neutron star (as far as i can tell) is that the electrons and the protons are so tightly packed that they form in to neutrons. But they are not bound together like they are in the nucleus of an atom.  \n\nThere is actualy a great statmech problem where you are suposed to calulate the radius of a neutron star using only the pauli exclution principle (basicly that fermions have the tendency to repell each other)\n\nat the moment there are two known things that are even denser... a black hole for one, and the neuclus of an atom. \n\nIf there was a free existing small part of a black hole, it would just explode in a matter of moments (~10^-14 sec i think).  as for a neutron star they would also fly appart because for the pauli exclution principle.  But if we have a huge mass of protons and neutron we would have a very very very very heavy atom.  It would attract electrons just like a normal atom.  But it would decay in just moments(problably very violently).   \n\nIt can be made, but it would exist in such small quantity and for so short of a time that it would seemingly have no properites
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no, but i can tell you what we think it is:\n\nit is a star whose gravity is too great for the nuclear fusion in its core to negate.  it collapses in on itself and goes supernova.  then it is a neutron star.  if the gravity is still too great, it collapses some more and becomes a black hole.  its gravity is so strong that light itself cannot escape it (thus it is 'black').  they are suspected to be at the center of many galaxies, especially quasars and other active galaxies.  one is believed to be at the center of our own galaxy, the milky way.\n\nonce again, no one has really seen one, just the indirect evidence that one exists (strange star orbits, gas being sucked into nothing, strange bursts of some kind of rays (can't remember what kind) coming out of the poles, etc.\n\neverybody is pretty convinced that they exist.  its just kinda hard to prove.\n\nps the idiot down there who said black holes are dead galaxies has no clue what he is talking about. please ignore him.  and its not the size of an atom, its an infinitely small point, the swartzchild point.
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The current scientific theory holds that black holes are formed when stars of sufficient mass (about 3x the mass of our Sun) reach the end of their life and collapse down into a singularity, which is, in essence, an infinitely small point with a huge amount of gravity - gravity so powerful that not even light can escape it's pull if it gets too close. It is also thought that black holes can be formed from smaller masses if external energy/pressure is exerted to squeeze the mass into a singularity.\nNot sure if there's any direct experimental evidence of creation, but there is evidence that they exist. Cosmologists use several different methods to detect blacks holes, such as by looking for light being bent and/or disappearing as it travels from remote stars towards us, and also by looking for bursts of x-rays that are given off as matter is sucked into the back hole and destroyed.\nFun fact: current evidence and theory suggests that the center of the Milky Way is a giant black hole that was formed as the densly packed stars in the center collapsed and sucked each other in.
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black hole \n\nIn astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution) by the American physicist John A. Wheeler. \n\nGravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova. If the mass is less than three times that of the sun, it will then form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the "singularity."
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A black hole is formed by a star, most likely a neutron star, collapsing into a space a few kilometers across. It is made up of the same thing a neutron star is, mainly hydrogen, helium, oxygen, and a few other elements. Once the black hole forms, it can absorb large amounts of matter from surrounding stars.
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a neutron star or pulsar
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After a star dies, there is still some residual heat left over. That heat makes the star (white dwarf or neutron star) glow, even though it is not producing any energy. Eventually, the star cools off and does indeed simply become a hunk of ash, which we call a "black dwarf." \n\npeace
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ALL STARS HAVE A GRAVITATIONAL PULL FROM THE OUTSIDE IN, WHEN A STAR LOSSES ALL ITD HYDROGEN THE GRAVITY PULLS THE STAR INWARDS(BASICALLY IT IMPLODES) WHNE IT INPLODES IT CAUSES A BLACK HOLE, WICH SUCKS IN THE STAR AND ALL LIGHT AROUND IT. THE INSIDE OF A BLACK HOLE IS CALLED THE EVENT HORIZON, NOTHING CAN EXCAPE THE EVENT HORIZON ONES ITS IN IT. NOT EVEN LIGHT! THATS WHY ITS BLACK! CAUSE ITS SUCKS UP ALL THE LIGHT. KINDA SCARY IF YOU ASK ME!
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Eric Jaffe-Magnetic Thrust: Fields force matter into black holes\n\nLike a child scared to dive into a pool, cosmic matter needs one last push to plunge into a black hole. New observations confirm that magnetic fields provide this final galactic shove. \n\n The observations come from a system in the Milky Way called GRO J1655-40, which consists of a black hole and a normal star. Gas from the star is pulled toward the black hole, where it forms what's called an accretion disk. Angular momentum—the same property that keeps Earth from diving into the sun—keeps the disk revolving around the black hole instead of falling into it. \n\nUnless something knocked them off course, the gases in the disk would continue to circle the black hole forever. "To get matter in toward the black hole, we have to change the orbits in the disk," says study leader Jon Miller of the University of Michigan in Ann Arbor. Until now, scientists weren't sure whether magnetic fields, radiation pressure, or heat alter the orbit and trigger that fall. \n\nUsing NASA's Chandra satellite observatory, Miller's team collected data on X rays emitted from the J1655 system. They found that the X rays came from a wind blowing from the disk, so some force must propel the wind past the black hole's gravitational pull. After simulating the wind on a computer, the researchers conclude that only magnetic fields could create such a force, they report in the June 22 Nature. \n\nThe team had ruled out a wind fueled by heat from the core of the disk or by pressure from radiation blasting out of the disk. A heat-driven wind would have been hotter than the one that the researchers observed, and radiation pressure is too weak to drive that wind. \n\n"Magnetic pressure really was the only viable means remaining," Miller says. \n\nSuch pressure could upset the gas disk's orbit in two ways. The magnetic fields could push the wind outward like a spring, or the force of the spinning disk could fling the wind away from the disk's center. Either disruption of the disk's orbit would cause some matter to spiral downward. \n\nMiller's team appears to prefer the first scenario, says Roger Blandford of Stanford University. Blandford prefers the second, which he describes as an organized magnetic slingshot. "I suspect there are different answers in different types of systems," he says. "But on the face of it, this looks like a big advance observationally." \n\nThough most astrophysicists expected magnetic fields to play a role in black holes, the finding is "very important evidence that what we thought before makes sense," says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "It's the first time there's clear evidence for wind coming off an accretion disk." \n\nThe finding could help astrophysicists understand the complex give-and-take between galaxies and black holes,
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There are theoretically two ways in which black holes can form.  Stellar (star) sized black holes have to be above the Chandresekhar limit of 3.2 solar masses, and are formed through the gravitational collapse of stars. They can form whenever conditions are right. These are the types of black holes that astronomers infer from binary X-ray sources and the like.\n\nMicro black holes, on the other hand, can only have been formed in the initial moments of the Big Bang, when intense pressures during cosmic expansion could have collapsed masses smaller than our sun into singularities.  According to Dr. Stephen Hawking's theory of black hole evaporation, all singularities lose mass, through a type of evaporation (the actual mechanism, having to do with vaccum energy and particle/antiparticle pairs is a bit complicated) with smaller black holes evaporating faster than large ones.  By this logic, most micro-holes would have evaporated out of existence in the thirteen billion years the universe has been around, while the larger micro-holes (things with the mass of, say, Neptune) should still be around, though hard to spot.
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Stars spend most of their life fusing hydrogen to helium.  As a star gets near the end of its life it starts to fuse Helium into heavier elements going up to Carbon, Oxygen, Silicon, etc (taking progressively less time to fuse each of them).\n\nEventually the star will have an Iron core but while fusing elements lighter than Iron results in an energy release trying to fuse Iron into something heavier requires energy and so the fusion reaction can not proceed any further.\n\nNow throughout the life a star the star is in a hydrostatic equilibrium between gravity pulling the star together and the radiation pressure from the nuclear fusion pushing the star apart, with the fusion reaction stopping the gravity wins and the star collapses.\n\nThe sudden collapse causes the core to be crushed to very high density at which point protons and electrons combine to form a neutron and a neutrino according to p + e -> n + ν with the neutrino escaping (the neutrinos carry most of the energy).\n\nThe core of the star is now very small and the outer layers fall in and bounce off which is what causes the explosion.\n\nIt takes a couple of hours for the outer layers of the star to actually blow apart (which is what we'd see) so the first evidence that a supernova has occured is the neutrino burst.
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You don't need a black hole to be able hold itself together through gravity. Just look at the earth or the moon for example. They are nowhere near as dense as a black hole, yet they support themselves gravitationally.
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A black hole is what remains when a massive star dies. \nThe concept of an object from which light could not escape (e.g., black hole) was originally proposed by Pierre Simon Laplace in 1795. Using Newton's Theory of Gravity, Laplace calculated that if an object were compressed into a small enough radius, then the escape velocity of that object would be faster than the speed of light.\nIf you have read How Stars Work, then you know that a star is a huge, amazing fusion reactor. Because stars are so massive and made out of gas, there is an intense gravitational field that is always trying to collapse the star. The fusion reactions happening in the core are like a giant fusion bomb that is trying to explode the star. The balance between the gravitational forces and the explosive forces is what defines the size of the star. \nAs the star dies, the nuclear fusion reactions stop because the fuel for these reactions gets burned up. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the material and radiation blasts out into space. What remains is the highly compressed, and extremely massive, \ncore. The core's gravity is so strong that even light cannot escape. \nThis object is now a black hole and literally disappears from view. Because the core's gravity is so strong, the core sinks through the fabric of space-time, creating a hole in space-time -- this is why the object is called a black hole. When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated.The star Cygnus X-1 is a strong X-ray source and is considered to be a good candidate for a black hole. stellar winds from the companion star, HDE 226868, blow material onto the accretion disk surrounding the black hole.As this material falls into the black hole, it emits X-rays.In addition to X-rays, black holes can also eject materials at high speeds to form jets. Many galaxies have been observed with such jets. Currently, it is thought that these galaxies have supermassive black holes (billions of solar masses) at their centers that produce the jets as well as strong radio emissions.One such example is the galaxy M87.It is important to remember that black holes are not cosmic vacuum cleaners -- they will not consume everything. So although we cannot see black holes, there is indirect evidence that they exist. They have been associated with time travel and worm holes and remain fascinating objects in the universe. There are two types of black holes: \nSchwarzschild - Non-rotating black hole \nKerr - Rotating black hole \nThe Schwarzschild black hole is the simplest black hole, in which the core does not rotate. This type of black hole only has a singularity and an event horizon. \nThe Kerr black hole, which is probably the most common form in nature, rotates because the star from which it was formed was rotating. When the rotating star collapses, the core continues to rotate, and this carried over to the black hole (conservation of angular momentum). The Kerr black hole has the following parts: \nSingularity - The collapsed core \nEvent horizon - The opening of the hole \nErgosphere - An egg-shaped region of distorted space around the event horizon (The distortion is caused by the spinning of the black hole, which "drags" the space around it.) \nStatic limit - The boundary between the ergosphere and normal space \nIf an object passes into the ergosphere it can still be ejected from the black hole by gaining energy from the hole's rotation. \nHowever, if an object crosses the event horizon, it will be sucked into the black hole and never escape. What happens inside the black hole is unknown; even our current theories of physics do not apply in the vicinity of a singularity. \nEven though we cannot see a black hole, it does have three properties that can or could be meas
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As much as Sci-Fi wants it to be something exotic, I'm afraid there is no "other side" to a black hole.\n\nImagine an atom of normal matter. Most of it is empty space. But it takes a great deal of pressure to compress it --- more pressure than is usually available. But if we concnetrate enough mass, the gravitational force will be strong enough that the space can be compressed. Protons (positive charge) and electrons (negative charge) will be pressed together to form neutrons (neutral charge). This happens in the core of very large stars at the end of their life. Their last stage of fusion produced iron at the core. When enough iron exist at the core, it suddenly collapses as I've described here. It then forms what is called a neutron star. If it is oriented just right with respect to the earth, it also appears as a pulsar (Internet search: pulsar +star +radio).\n\nThe neutron star is extremely dense. But there is still a force exerted by the neutrons which keeps the compression from going any further. But what if the mass was large enough to overcome that force and squeeze it even smaller. It appears that once this happens, there is nothing left to stop the compression and it continues until the object takes up no space at all, which is why it is sometimes called a singularity. It is a point in space with a huge mass and therefore extremely strong gravity.\n\nAll objects exert a gravitational pull on other objects. And therefore all distort space and bend the path that light takes. But this new object is so compressed that near it, the gravity is so strong that the path of light beams are looped back and cannot leave the object. Since no light can escape from it and anything nearby falls into it and disappears, it is called a black hole.\n\nMoving out from the black hole, we finally find a distance where light can barely escape. The line that divides where light can and cannot escape from the black hole is called the event horizon. It is not a "surface" of the black hole as there is nothing there. It is merely the distance at which the gravity drops enough that light can escape. If you could build a camera and drop it though the event horizon, nothing would change as it went through. No difference would be recorded. However the radio transmission from the camera would cease as radio is the same as light, just a different frequency. Once the camera went past the event horizon, it would no longer be able to transmit the pictures to you. The radio waves would loop back and head into the black hole. There are also some strange things that happen to time as you cross the even horizon, but you can run an Internet search to find out more.\n\nBecause it has compressed to the point of not taking up space, conventional physics expressions for matter no longer make sense in this environment. You end up with fractions where the volume or distance is in the denominator (lower half of the fraction) and therefore the fraction has an infinite value --- which doesn't make sense. So we can't really project what is going on inside. But the idea that it is a portal to other dimensions is more SciFi than science. There are some esoteric theories that guess about such things, but they are guesses. \n\nHope this helps\n\nIf you are interested in astronomy, try \nhttp://groups.yahoo.com/group/lunar-observing/\n\nChuck