But while our solar system may not be hit or directly affected by Andromeda, isn't our solar system held together to some degree by the other celestial bodies in the Milky Way? Wouldn't our sun be ejected from it's current orbit (not the Milky Way itself) and bring the planets with it? While our orbits around the now rouge sun may stay the same, couldn't it put our entire solar system on a collision path with other stuff in the Milky Way? If the two galaxies were to merge (that might not happen), wouldn't that fuck everything up? I understand the the vast distances between stars is so great in makes it unlikely, but I can't help but think that the energy contained in two approaching galaxies wouldn't cause mass devastation upon their collision. I would imagine all solar systems would be in a shooting gallery of space debris for millions of years.
I'm not challenging you. I am genuinely curious and like to learn about as much of this stuff as I can.
The fact is that there would be no collisions, not even in the dense core of the galaxies. Just think of this: the inner solar system is a couple of light hours across. Let's say one light day, to be optimistic. The volume would then be proportional to this lenght cubed. Now we do know that there are 45 stars within a radius of 17 light years, which means that the free volume between two stars in our galactic neighborhood is about 5.3 billions of light days. This means that the ratio between the volume of our solar system and the average volume between stars is 1 on 5 billions! Now, since we know that the merging of the galaxies will involve a number of grazing collision before the eventual merging of the black holes, it's clear that our little place will interact with the outer regions of the Andromeda disk, which sport similar values of stellar density. This being said, the chances of impact are about the same of two flies (1 cm across) flying on a reasonably straight line in a room that's 20 meters across.
And by the way, I read now (after writing all that's above) the "I'm not challanging you". I didn't feel challanged at all, it's a huge pleasure for me to be able to speak of this with other people! And to be able to have the chance of doing such interesting calculations on the fly to give a rough reason of plausibility to the refined theories that have been or are being produced by astrophysics and whose results and implication we can accept but still wish to comprehend on a deeper level.
Oh, by the way: the only possible negative consequence of the gravitational perturbation of a passing star is a wave of comets being pushed towards the inner solar system. Which would be quite nasty, but in the end minimally concerning for a simple reason: by the time the Andromeda collision happens, the sun's power output, that is nothing but its luminosity, will have increased because of the increase of the core temperature due to the enhancement of the mean particle-to-hydrogen mass ratio (if you take hydrogen and add helium, which is what happens every second in our star, the mean mass of the particles in the core increases, as the mass of a helium nucleum is 4 times that of a hydrogen's). Basically the augmented luminosity will have made at least multicellular life impossible on the earth barring any human intervention, that would be "shipping" the earth to a wider orbit to keep the incoming light flux (the so called solar constant) within the tolerable range. You understand that displacing a whole planet (and the moon too, as doing without the moon would require further eingineering to stabilize the true pole wander) would require a much greater endeavour and technological level than preventing a swarm of oversized iceballs to hit out planet. And by the way, another star passing close enough to influence the *still theoretical* Oort Cloud (the main repository of comets) is an event that would likely have happened naturally another couple of times, or maybe more than a couple, before the galactic event begins.
ETA: I think I may have failed to address your main point: yes, there's a reasonably high (literature says 12%) probability that the sun will be ejected during one of the grazing encounters, and it is also said that it's almost a certainty that the sun will be swept farther from the nucleus of our galaxy (and stay there until the new galaxy is born). This would bear no ill consequences for our solar system. The gravitational force is what keeps our system together, and it decays with the square of distance. Twice as far, one fourth as intense. The effects of the change of inner structure of our galaxy would be dramatic in terms of galactic dynamics of our system, as I said, but I'm quite positive that they will hardly be noticble considering only the planetary dynamics within the system
So that being said, my post was definitely too longm, but I hope not uniteresting enough to earn a tl;dr. Let's keep this thread running, it's amazing!