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So, it's theorized that Andromeda will collide with the Milky Way some time in the next 3 billion years...

I don't get how galaxies can collide with each other. What is the cause of their movement? All I can think of is that possibly one of the Supermassive Black Holes of these two galaxies has a greater gravitational pull than the other and is pulling the weaker member toward itself.

That being the case, at what point will the galaxy belonging to the weaker SBH enter the event horizon of the stronger? And what can be expected to happen to the solar systems belonging to the two galaxies when the collision occurs?

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That's t hree questions you've got there. Which one would you like to ask first? Please do just ask one question at a time, just as you would on StackOverflow –  EnergyNumbers Jan 10 '13 at 8:35
    
Hah sorry about that... I think I got carried away. Would it possible for each to be addressed in order? –  Ortund Jan 10 '13 at 8:36
    
@Ortund: You can split it into two questions, post them separately. –  Manishearth Jan 10 '13 at 9:23
    
"at what point will the galaxy belonging to the weaker SBH enter the event horizon of the stronger" The event horizon of a black hole is the point of no return, where even light can't escape, and even for a supermassive black hole it's still a small region of space, about the size of a solar system. Objects outside a black hole's event horizon are still attracted by the black hole, but they will just orbit it without falling in and becoming part of it. –  Mitchell Porter Jan 10 '13 at 11:04
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You could think of the SBH as a bit like a single special star. It's dark, and it's very heavy, and it's at the center, but it's still just one object among the billions of objects in the galaxy. It's not the master of the galaxy; mostly it is just along for the ride too. –  Mitchell Porter Jan 10 '13 at 11:07
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2 Answers

  1. The reason for the continued motion of a galaxy, or anything else, is inertia. It's expressed by the first Newton's law. A body will continue in a uniform motion in the same direction, by the same speed, unless forces are acting upon it. When the matter in galaxies were formed, there was a lot of chaotic energy everywhere and some of it got translated to the initial motion of galaxies, local clumps of matter, too. This ordinary motion of galaxies relatively to each other – "residual motion" – has to be combined with the motion due to the expansion of the Universe (Hubble expansion) but both terms are nonzero.

  2. The black holes at the centers of galaxies will almost certainly not directly heat each other and/or join. Note that the black hole at the Milky Way center is just 4 million solar masses heavy; its counterpart in the Andromeda is billions of solar masses which is much heavier but still much lighter than the whole galaxy. By the collision, we only mean that the stars will start to mix up with each other and the overall shape of both galaxies will be distorted and will merge. But this collision doesn't require any actual collision of stars or black hole or other celestial bodies.

  3. Some Milky Way stars (a tiny minority) may nevertheless be unlucky and hit the Andromeda's big black hole. The devouring of stars and matter by black holes obeys the usual rules. It doesn't matter that the devoured star comes from another galaxy.

  4. Most of the Solar Systems may continue pretty much unperturbed. The picture of the night sky will be changing rather "dramatically". One should still not overstate the rate of this change. It will still take millions of years for the picture of a night sky to change substantially.

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I could spend days on this stuff. It's very thought provoking... Just based on your answer, I've already thought up about a dozen new questions... –  Ortund Jan 10 '13 at 10:37
    
@Ortund, you might want to play around with the GalCrash simulation applet here ... burro.cwru.edu/JavaLab/GalCrashWeb –  Carl May 23 '13 at 20:23
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Re your first question, see my answer to how can galaxies collide if everything moving outwards. All galaxies have relative motions superimposed on their motion due to the Hubble expansion. This is because the distribution of matter was uneven follwing the end of inflation and the uneven distribution resulted in uneven gravitational attraction and hence uneven motion.

Actually this is quite a good thing, because if the motion of all particles in the universe had been exactly uniform we'd still have a thin soup of atoms instead of stars and galaxies, and I wouldn't be around to answer this question. The aggregation of matter into clumps and the relative velocities of the galaxies stem from the same source.

Re your second question, there is lots of info and videos on modelling galaxy collisions. Googling will find you many hours worth of browsing. In a galaxy collision it's likely that no stars will collide because stars are so much smaller than galaxies, so at least in the outer reaches of the galaxy solar systems shouldn't be greatly affected. Near the core there may be stellar approaches close enough to disrupt planetary orbits and/or trigger cascades of comet's from the Oort cloud. I don't know of any quantitative work on this.

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