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What is a black hole?

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Majority of black holes are formed from the death of large stars (such bigger than the sun) that run out of fuel and cannot sustain its nuclear reaction. The star loses the force pushing itself outward and is weakened by the force of its own gravity pulling inward. The star has so much gravity and is so compressed that it shrinks to a hole. Does that make sense? Black holes emit x-ray radiation and get smaller and smaller until they disappear.

Comments

Could you add a bit more on their structure? 

Recent surveys have uncovered black holes at the center of almost every galaxy leading cosmologists to ponder whether black holes are antecedents of galaxy formation or precursors of galaxy formation.   Initially BH's were thought to be antecedents but current thinking is that the BH precedes to galaxy.

Also in the surveys, it was also discovered that there is a persistent, positive  correllation between the BH size (mass) and the size (mass) of the galaxy.  Cosmologists love this kind of relationship.  I think the 'tendency to correlate' many things in cosmology all started with the Herzbrung-Russel diagram of luminousity vs color.

BruceS

No one really knows what the inside of a black hole is like, or consists of.  Most astrophysicists believe that all molecular and atomic structure as we know it is destroyed, and instead we have sub-atomic particles, including quarks, and probably strings (if string theory is ever proven).  Some physicists believe that "strings" represent the essence of physical existence, of our entire universe:  all particles, including electrons and smaller subatomic ones, are just a version or form of energy vibrations (Einstein proved that matter and energy are really different versions of the same thing, with E=MC2).  As you know, vibrating strings can create sound and music, like on a guitar or in a piano.  Different vibration patters along a string, and different string lengths create differently-pitched sounds (higher of lower), and this may be how these super-tiny vibrating energy strings create basic subatomic particles - Therefore these subatomic strings may be the "music of the universe," in a very real way.

If we could travel near a black hole, we would see a point where energy and matter being pulled into the black hole is suddenly changed (pulled apart), and then very soon disappears forever.  This is known as the "event horizon," and would be around the black hole in all directions, like around a ball.

Stars are the battleground between the forces of nature.  Gravity wants to make stars collapse, the electromagnetic force wants to keep the atoms inside a star apart and not occupy the same space.  The strong nuclear force is overcome by a star's internal temperature allowing for nuclear fusion.  Fusion, a la E=mc^2, turns atoms of one element into another giving off energy too.  In our Sun, fusion converts approx 400million tons of hydrogen into helium every second.  When a star runs out of one type of nuclear fuel, the energy it was producing to fight gravity loses its battle for a bit.  As the star's volume gets smaller, its energy increases again allowing for fusion of heavier elements.  This process continues through various elements on the periodic table until iron.  The instant iron is fused in a star's interior, it dies.  Depending upon what type of star on the HR Diagram is was born as, it will die a certain way.  Our sun will one day turn into a red giant, give a fizzle of a minor nova, then it will end its life as a white dwarf.

But, what about stars more massive than our sun?  How will they end their days?  They will go supernova.  Sometimes what is called a brown dwarf will be left over, sometime a pulsar.  Even more massive stars will end up as neutron stars or possibly black holes.  A black hole is a sever warpage of spacetime.   Imagine a bowling ball on a trampoline [yes the trampoline is only a 2D surface, but it will work for visualization].  It will cause the trampoline to pucker or sag and be drawn down around the ball.  Imagine this sag being so severe and so steep you cannot climb out.  Around the "lip" of the sag you might just be able to slide across, around and beyond it.  But, get too far down the steep side and you're going down no matter how fast you zip by.  This is what a black hole does to its surrounding spacetime.  The point of no return is called the blackhole's event horizon.  That is not the surface of the black hole, it is simply the point where light cannot escape from its pull.  Without light being able to escape from beyond that point, no information from the other side of that horizon can get out either.

The limit for the amount of mass necessary for a star to remain stable or keep going to become a neutron star or black hole is called the Chandrasekhar limit. It was derived by Subrahmanyan Chandrasekhar using Einstein's equations.  He was around 20 at the time.  

A black hole is believed to be the result of the collapse of a massive star.  As a star ages, it uses up all of its available fuel source and begins to collapse under its own gravity and mass.  For a black hole to form, the star must be a massive star and is usually not in the main-sequence stars on the H-R diagram.  As the star collapses, it pulls in all matter around it and becomes an extremely dense point in space.  It becomes so powerful and so dense, that not even light can escape its gravitational pull.  Hence, the name "black hole".  It is not really a hole in space, just a location of a very dense point of matter that captures all around it.  The current theory holds that at the center of most galaxies is a black hole that is gradually pulling the galaxy towards it, causing the galaxy to spin.  

In order for a star to remain a star, it needs to be in this constant state of equilibrium, where there are some forces (mostly fusion reactions) that make it expand, and some other forces (gravity) that make it contract. When all forces are balanced, the star maintains its size. But the problem starts when the star runs out of fuel to continue to produce nuclear fusion reactions. What happens then is that the inward-pulling forces win over, and the star starts to collapse.

Now, as it becomes smaller and denser, new forces start playing important roles. These are more complicated ideas - things like electron degeneracy pressure - so I won't go into detail about those. The point is that, for some stars, these new forces become large enough that a new balance is found between the inward pulling and the outward pulling things.

In the case of black holes, even these new forces are not enough to balance things, so the star continues to collapse and become denser and denser. Mathematically, we say that it becomes as small as a single point. Of course, physically we would like some better explanation than that, but the problem is that we don't really know how the laws of physics are altered when you reach such intense density.

Now, you've probably heard that even light can't escape a black hole (which is why we call them black holes). The idea behind that is the following. If you want to orbit a star, you need to be moving pretty fast. In fact, the closer you get to the star, the faster you need to move if you don't want to fall in. I should add that this is the case with satellites orbiting Earth, too. As soon as they lose speed, they start spiraling onto Earth. Now, the speed at which you need to go in order not to fall it depends on how far you are from the star, and how much mass the star has. In the case of most stars and other objects, the speed of light is much much faster than the minimum required to not fall it, at any distance. Black holes, however, are so very very small (so you can get closer to them without being *at* them) and so very very massive, that the speed you'd need in order to not fall in (at a given distance) would be faster than the speed of light. But it's a law of nature that nothing goes faster than the speed of light! That means that, at a certain distance, everything would eventually fall into the black hole. Of course, you can always go further away from it, where the speed necessary to orbit is smaller, and you wouldn't fall in.