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There is evidence, primarily using radio astronomy, that there is at least one and possibly two black holes in the center of the galaxy. (Regular telescopes won't work because of the amount of dust that lies between us and the center of the galaxy...it would be similar to using a telescope from here to read the print on a newspaper in Baghdad...during a dust storm.) The stars are packed very dense in that area, as many as several thousand within a light year, as compared to were we sit where the nearest star to the sun is about 4.2 ly away, and the next closest is over 6 ly...in a different direction. Thought of in another way, if we were in that area, there would be no night because of all the brilliant light from nearby stars, many of which would also be visible in the daytime.
Those densely packed stars are moving around the objects that are thought to be black holes at a totally incredible speeds, just as one would expect if they really are black holes. That is also part of the answer to your question. A dense object or one with mass produces a gravitational well, and the denser the object or greater it's mass, the stronger or deeper that well. For instance, the gravitational well of the earth is such that in order for something to remain in orbit, that is, to not be pulled back into the Earth, it must travel at something like 17,000 miles per hour at an altitude of 160 miles. (This may be a little off since I didn't check the exact speed before posting this, but the point is to just give a ballpark figure.) The sun has a gravitational well that is much stronger because of the greater mass, so at 160 miles from it's surface, assuming that an object could survive the heat radiation, it would have to be travelling at an enormous rate of speed, perhaps 17,000 miles per SECOND. Even at where the Earth is, about 93 million miles away from the sun, it must rotate around the sun at great speed to keep from falling into the sun. This is the key. When an object moves in a curved path, there are two main forces at play...one that is continuing it on it's path, and the other that is pushing outward at right angles to it's path. (This is centrifugal force.) Think of twirling a string that has beads on it...the harder and faster you twirl it, the greater the force is to cause the beads to fly off the string in a direction away from you.
This brings us to the black hole. It is estimated that a black hole in the center of our galaxy would have the mass of several million suns. This is to say that it is an extremely deep and strong gravity well. If anything gets within a certain distance of it, it makes no difference how fast it is moving, it will fall into the black hole. (This is also known as the event horizon.) But beyond that limit, if objects are moving fast enough, they will not fall inward, they will orbit the black hole, just as the Earth orbits the sun and satellites orbit the Earth. So stars that are even 10-20 light years away can orbit for billions of years, if they are moving fast enough, and as I said earlier, the stars in the center of our galaxy are moving at speeds that are enormous. If there are 2 black holes, they must also revolve around each other at great speed, for the same reason, though as their mass continues to grow, eventually it would be expected that they'd merge.
There are still some people that don't believe that a black hole or black holes exist there, though.
I apologize if this was too long winded or wasn't interesting, I wanted to try to explain it in an understandable way, and do hope that I was successful.
Last edited by Rev. Rex : 19th September 2006 at 04:10 PM.
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