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I've recently learned that the general consensus is that several (if not, most) galaxies have super massive black holes in their center, in particular the Milky Way. This, at least to me, makes perfect sense seeing as we are in a spiral galaxy which means we need something to "spiral" around (a large body or a bunch of mass).

But seeing as we're rotating around this massive black hole, won't we inevitably end up sucked in by it? Aren't we spinning towards the center of the galaxy, or are we staying steady where we are?

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By the way, the spiral structure of some galaxies is a traveling density wave, and it does not reflect the movement of any particular stars, just as an ocean wave does not carry any water molecules with. The fact that stars are moving in (sometimes complicated) orbits holds in all galaxies, spiral pattern or not. –  Chris White Dec 17 '12 at 21:59

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Look at the question a different way: will the Earth get "sucking into" the sun? Answer: no, it's in orbit.

Now, black holes are a little different because inside 3/2 of the Schwartchild radius there are no stable orbits, but at very large distances gravity is gravity and orbits are orbits.

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It's not like the black hole is the dominant gravitational influence in the galaxy, except for the few million closest stars to it. It makes up only about a millionth of the mass of the Milky Way. The rest would be better described as orbiting the portion of the galaxy more central than itself. Self-gravitating is a nice descriptive term. –  Kyle Jul 26 at 17:34
    
If you like this answer you may also enjoy reading this Phys.SE post. –  Qmechanic Jul 26 at 22:26

It is a supermassive black hole but its mass is nothing compared to the mass of the whole galaxy. So if you remove the black hole, we will not feel any difference because we are so far away. It is not the central black hole that keeps the galaxy together, our sun is orbiting around the center of mass of the galaxy and the supermassive black hole just happens to be there(for reasons we don't fully understand yet)

If you are not very close to the black hole like in the inner parts of the accretion disk, it is just another mass to you. it won't matter if it is a black hole or a massive star.

However, there is some strong evidence which lead many astrophysicists to believe that supermassive black holes play an important role in the evolution (or maybe even birth) of galaxies. There seems to be a correlation between supermassive black hole masses and sigma velocities of stars in the galaxy. Wikipedia has good references on this M-Sigma relation.

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