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Q&A: Black Holes

Q:
If, within a given radius {proportional to mass}, nothing escapes the attraction of the Black Hole, won't the Hole continue to attain mass, growing ever larger? And if this is so, the 'event horizon' will also grow ever larger, consuming more and more of anything passing by or'consumed' by the ever-expanding gravitational field? If so, then, eventually, why wouldn't the entire universe be 'consumed' by the presence of the millions of ever-expanding Black Holes?

A:
Yes, everything that passes within a certain radius (the Schwarzschild radius, also called the event horizon, a sort of "edge" of the black hole) will fall into the black hole, never to escape. However, the key fact is that the Schwarzschild radius is not very long in astrophysical distances.

For example, in the center of our galaxy we believe a massive black hole exists. We've watched the stars that are orbiting this central mass very carefully, and judging by their speed and their distance from the center, we guess that a black hole exists there. But the event horizon of this black hole is not big enough even to capture the stars that are closest to it, much less our own Sun way out in a spiral arm of the galaxy.

To find out what happens when a star does come too close to a massive black hole, look at the recent Chandra press release:
http://chandra.harvard.edu/press/04_releases/press_021804.html

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