Introduction
A black hole is a phenomena that occurs when an extremely dense star (it would have to be three times as dense as Earth) comes to the end of its life and collapses. Its gravitational attraction is so strong that nothing can escape its influence. Albert Einstein predicted their existence in his General Theory of Relativity long before scientist actually Œobserved' them.
How do they Form?
After its initial collapse, the matter that once made the star continues to be compressed, making it ever more dense. This matter becomes so super-dense that its gravity becomes tremendous. When this happens, the escape velocity of an object trying to move away from the black hole would have to faster than the speed of light.
The gravity of an object is determined by its mass and its density, thus the the more dense the object the larger the escape velocity must be. That is why we can not see a black hole. It attracts everything toward it, even light and consumes it. This makes positively identifying a black hole almost impossible. The way scientists do identify one is the the impact it has on the systems surrounding it.
Escape Velocity
Newton has taught us that all objects are attracted to each other by gravity. Gravity is what causes object to fall toward Earth. If the shuttles wants to leave Earth, than it most produce a force that causes it to have a greater velocity than the it would have falling toward Earth. This specific velocity is known as an objects escape velocity. In the situation with the shuttle the escape velocity is in reference to Earth, but the Earth also has an escape velocity from the Sun as does any body rotating around another.
The Size of a Black Hole
No one knows why but a black hole is in a state of so called continuous gravitational collapse, were the present laws of physics do not apply. Scientists describe the black hole as a single point in space that is infinitely dense. That is hard to grasp but think about the head of a pin. Now imagine everything on the Earth, the trees, the houses, the beaches, the oceans, everything fitting into the head of that pin. The point in space were a black hole exists is infinitely smaller than the head of that pin, yet anything can be compressed and fit in it. All matter that is attracted by its gravitational force will finally disappear into it and become part of it. The point where this occurs is called the event horizon, everything that happens behind this point is completely shielded from the outside world.
According to the General Theory of Relativity space is curved and time is delayed in the vicinity of matter. The greater the mass concentration, the greater the curvature. Near a black hole the gravitational power of attraction is so strong that the space is curved into an infinitely deep well. The outer limit of this curvature is the event horizon.
Everything that crosses this event horizon, will finally run into that one singular point and out of existence. For objects that have crossed the event horizon time will pass normally, but someone looking from the outside will see everything near the black hole in slow motion and eventually even frozen in time. Within the event horizon space-time stops completely.
How can we Observe Black Holes?
The black hole itself is beyond our observations, because light nor other radiation is able to escape form it. The presence of the black hole can only be demonstrated by its influence to its surroundings (the activities at the event horizon can be observed).
A black hole has such enormous gravity, that it can even pull matter from a nearby star. This matter spirals towards the black hole and finally disappears into it. During this process the matter from the star becomes superheated and emits a type of radiation known as Rontgen radiation. This radiation can be observed by astronomers and satellites, where this radiation is accelerating towards there is assumed to be a black hole (see picture to the left).
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