Many people view black holes as a massive cosmic recycling centers. I know that, when I was younger, I always thought of a black hole as being the universe’s vacuum cleaner, sucking up everything within its path. As dramatic and as cool as it may sound, it is not quite true. I am afraid that this is a common misconception about black holes.
When we look out into the universe, we find that most galaxies have a supermassive black hole at their cores. The galaxies that don’t are mostly dwarf galaxies or irregular galaxies. Although there are plenty of cases where a dwarf galaxy has a black hole at its core, a general rule of thumb is that an irregular galaxy doesn’t, though there are some exceptions.
So should we expect to be eventually sucked into our supermassive black hole some day? Well, I wouldn’t hold your breath or wait around. Our sun and some 200 billion other stars have been in a pretty well defined orbit around the center of the Milky Way for billions of years and would continue for indefinitely if it weren’t for the eventual collision with Andromeda.
The reason for this is that the same physics that determines the orbit of planets around the sun also governs the movements of stars around the center of the Milky Way. Everything in our solar system orbits around the sun, and has been doing the same for billions of years. This is because the sun is pretty much the same mass as it was 4.7 billion years ago. If you were to replace our sun with a black hole of the same mass the Earth and all the other planets would continue to do what they’ve been doing, the only real difference would be that life wouldn’t be able to exist because it would be very cold without the sun’s warmth. In short, a black hole is just a mass like any other.
When two galaxies collide, however, the supermassive black holes at their centers eventually merge. It may take a billion or more years, but it’ll eventually happen. Two enormous masses just aren’t going to happily orbit one another. After the eventual merge, however, things settle down again, stars resume a stable orbit around the now larger supermassive black hole and the galaxy goes on.
Although stars and gas that aren’t near the black hole are quite safe, that is not always the case for things that are near the black hole. Stars can and do have quite stable orbits near the center of the galaxy, but the active supermassive black holes do have an accretion disk that is very hot and emits a lot of energy. We can generally say that anything in the accretion disk around a black hole is eventually going to fall into it. The reason for this is that it is losing gravitational energy, where as a star orbiting outside the accretion disk isn’t.
In the accretion disk atoms are moving very quickly and are constantly colliding into other atoms quite violently. This causes the accretion disk to become very hot. This heat means that the atoms are releasing a lot of energy, a lot in the form of X-rays and radio waves. When atoms release this energy they lose some gravitational energy and move in closer to the black hole.
As you can see, black holes in general are not giant vacuums that will eventually suck everything caught in their gravitational influence. Most of the galaxy is in a nice stable orbit and would continue to be if it weren’t for galactic collisions. Black holes are just a mass like any other, the difference being that if you get too close to them, you may lose too much gravitational energy to continue your nice stable orbit.