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 Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...The Space and Astronomy Agora RE: RE: Question Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response ToPosted by Dave Thayer on June 12, 2000 06:04:45 UTC

As a black hole sucks in more and more mass, it becomes more and more massive. The size of the singularity is still zero. But the Event Horizon gets larger and larger. What is the event horizon? It has to do with escape velocity.

Every mass has an escape velocity. If you want to escape from the surface of the earth, for example, you have to build a rocket that will go faster than earth`s escape velocity. Jupiter has a higher escape velocity, and the Sun has a much higher one. A black hole is an object with such a huge mass and density, and thus such a high escape velocity, that even light isn`t fast enough to get out.

Now imagine that you start your escape attempt from a certain distance above earth. If you are already way up high, earth`s gravity is less, and you won`t have to go as fast to get off. Likewise, if light starts its escape from way "above" the black hole, there will be a distance at which it can escape. That distance is the radius of the event horizon. Any closer, and light can`t escape because it can`t go fast enough. Any farther out, and light will escape because the attraction of the black hole is less, so the escape velosity is less. Since no light can escape from within the event horizon, the black hole is black--it can`t be seen or detected directly.

Now imagine that the black hole sucks in a few thousand more stars and becomes much more massive. Its singularity is still zero in size. But because its mass is greater, light has to originate farther out in order to escape. The black hole`s event horizon has grown. The black hole is now said to be larger since its mass is greater and its event horizon has a greater radius.