Re: What if we hit another Galaxy?

You have some interesting concerns, some of which no one now knows the answer to, but I'll give it a shot.

You're correct that our solar system revolves around the center of our Galaxy (the Milky Way). Although there seems to be a large black hole at the center, it's actually not very massive compared to the rest of the Galaxy, so what we're revolving around is all the stuff of the Galaxy (stars & gas) interior to our orbit. The Milky Way is part of the Local Group, which is a loose conglomeration of a couple dozen galaxies of all shapes and sizes. We're the second largest galaxy in the Group, and the largest is the famous Andromeda Galaxy. That galaxy is moving towards us at about 100 kilometers per second, which sounds fast, but it's two million light years away, so it will take 4 billion years to get here. Once it does get here, it's not clear what will happen -- it may zoom on by as we all breathe a sigh of relief, or it may smack right into our Galaxy. This would not be as disastrous an event as you might imagine, because galaxies are mostly empty space, with the stars very far apart, so during a collision they will pass right through each other unmolested (at least, as far as individual solar systems are concerned). However, the overall structure of the galaxies will be quite altered, as their mutual gravity pulls each other apart. Depending on the speed of the impact, the galaxies will then either merge together into one big galaxy, or continue on their merry way, with some substantial revisions to their appearances. One of the most dramatic changes will be the advent of new star formation. The gas between the stars will collide, which will set off the birth of new stars by the millions. Some pictures of galaxies in collision can be seen here and here and here.

Nearer to the Milky Way are several small satellite galaxies, which orbit around the outskirts of our Galaxy. Some of these galaxies may in fact have already passed through our Galaxy, and will do so again, slowly being disrupted in the process. In fact, one of the leading theories of galaxy formation holds that our Galaxy was built up over time from the collision and merger of many progenitor galaxies (small and large).

The answers to the questions about black holes and wormholes are more speculative. Wormholes are essentially two black holes which are joined together to form a tunnel or "bridge". They are theoretically possible, but may not be stable -- i.e., the bridge would collapse before you got through to the other side. I have seen some different ideas on how to stabilize them, and even make a time machine out of them, but at the moment such theories are quite speculative. Even if a wormhole was stable, and actually was made somehow in nature, it would have to be a very big one, or else the tidal forces would tear anything to shreds that tried to traverse it. (These are similar to the forces from our own Moon that produce the ocean tides on the Earth. For a black hole, the larger it is, the smaller the tidal forces are.) Some more info on black holes can be found here.

Hope this helps,
Aaron