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  Black hole           
Black hole
作者:佚名 文章来源:不详 更新时间:2008-3-13 13:51:09

The answer involves the gravity and the internal pressure within the star. These two things oppose each other -- the gravitational force of the star acting on a chunk of matter at the star's surface will want to cause that matter to fall inward, but the internal pressure of the star, acting outward at the surface, will want to cause the matter to fly outward. When these two are balanced (i.e., equal in strength) the star will maintain its size: neither collapse not expand. Such is the case for the Sun at the moment, and even, for that matter, for the Earth.
However, when a star runs out of nuclear fuel, and therefore continues to lose energy from the surface (it is emitting light energy), while not replacing the lost energy through nuclear fusion (no more nuclear fuel), gravity will win out over internal pressure and the star will contract slowly or collapse quickly depending upon the details of the internal structure and composition. Gravity wins out over the internal pressure of the star, because that pressure was produced by a normal, hot gas, and that gas is losing energy as the star radiates energy from the surface.
The star may thus end up as a black hole. It just depends upon whether or not the collapse is stopped at some smaller size once another source of pressure (other than what is produced by a normal, hot gas) can become sufficiently strong to balance the inward gravitational force. There are other forms of pressure besides that produced by a hot gas. Pressing your hand upon a desk top will let you experience one of these other pressures --- the desk pushes up against you, indeed it can support your weight (gravitational force)! The pressure that keeps the desk rigid against your weight is caused by forces between the atoms in the desk.
Furthermore, electrons within atoms must avoid each other (for example, they cannot all be in the same atomic "orbit" --- this is called "the exclusion principle"). Therefore, if we had a collection of freely moving electrons they would also avoid each other: the harder you compress the collection (the smaller the volume they are confined in) the more they rebel against the squeeze --- a pressure opposes your confinement of the electrons.
This "electron avoidance" pressure can only become strong enough to oppose the gravitational forces within a star of about the mass of the Sun when the star is compressed by gravity to about the diameter of the Earth. Thus a star as massive as the Sun can be prevented from becoming a black hole when it collapses to the size of the Earth, and the internal "electron avoidance" pressure (called the "degenerate electron pressure") becomes strong enough to hold the star up. This sort of pressure does not depend upon the energy content of the star ---- even if the star continues to lose energy from its surface, the pressure will continue to hold the star up. Our Sun can never become a black hole.
However, if the star is more massive than something like 3 to 5 solar masses, its gravitational forces will be larger, and its internal degenerate electron pressure will never be sufficient to stop its collapse. It turns out that neutrons can also obey the exclusion principle and neutrons will be produced in abundance when a massive star collpses, but even neutron degeneracy cannot stop the collapse of massive stars --- anything over 3 to 5 solar masses cannot be stopped, it will become a black hole according to current thinking.
How is time changed in a black hole?
Well, in a certain sense it is not changed at all. If you were to enter a black hole, you would find you watch ticking along at the same rate as it always had (assuming both you and the watch survived the passage into the black hole). However, you would quickly fall toward the center where you would be killed by enormous tidal forces (e.g., the force of gravity at you feet, if you fell feet first, would be much larger than at you head, and you would be stretched apart).
Although your watch as seen by you would not change its ticking rate, just as in special relativity (if you know anything about that), someone else would see a different ticking rate on your watch than the usual, and you would see their watch to be ticking at a different than normal rate. For example, if you were to station yourself just outside a black hole, while you would find your own watch ticking at the normal rate, you would see the watch of a friend at great distance from the hole to be ticking at a much faster rate than yours. That friend would see his own watch ticking at a normal rate, but see your watch to be ticking at a much slower rate. Thus if you stayed just outside the black hole for a while, then went back to join your friend, you would find that the friend had aged more than you had during your separation.
Does the E=mc^2 equation apply to a black hole?
E=mc^2 is always true. In the case of a black hole, for instance, there has been some speculation that black holes can, through a q

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