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Chalnoth
#7
Feb7-09, 12:28 AM
Sci Advisor
P: 4,782
Quote Quote by dj1972 View Post
Couple of questions here;
I know they may sound dumb but here they go. I'm "new" with some questions, so be gentle.

1.) So, if a black hole has enough gravitational pull to pull in stars and what nots, and nothing can pull away from this force once it is to close, its consumed by the black hole. Why cant anything orbit this black hole? I know stars, matter, gas and such orbit while its being consumed, but what prevents it from being in a orbit around the black hole?
Far away from a black hole, its gravitational field is like that of any other object with the same mass. If, for example, our Sun were replaced by a black hole of equal mass, there would be no difference as far as the orbits of solar system objects are concerned.

However, if you get very close to a black hole, it turns out that there are no stable orbits, and anything in orbit inside that distance will rather quickly fall into the black hole.

Quote Quote by dj1972 View Post
2.) If the center of our galaxy is a super massive black hole, then why are we staying in orbit around it and not being consumed into the black hole? I would assume that stars and matter and gas are being pulled in when they are close enough towards the center, but as an effect would that not be pulling in the rest of the galaxy into itself? I'm assuming galaxies can and do expand over time, how can one not effect the other? (If the black hole is consuming, how can we be expanding?)
The supermassive black hole at the center of our galaxy is only a tiny fraction of a percent of the mass of the stars in our galaxy. The mass of this black hole is a few million solar masses, while there are around 400 billion stars in our galaxy. And yet from the movements of these stars we know that the stars themselves are not orbiting one another, but are instead mostly orbiting the unseen dark matter that exists within our galaxy.

And galaxies don't expand over time. They're quite stable.