There is an interesting article in the most recent Sky and Telescope by John Dubinski about the upcoming collision of M31 and the Milky Way. The article looks more at some specific special cases than really giving the whole picture of current possibilities, but it is pretty clear that even if (as I have said in several threads) there is a large transverse motion to go with the head-on motion, it will not be very many (rapidly shrinking) orbits before the two galaxies would merge head on. So, three or four billion years, tops. Let's try and be living on enough worlds by then that some of us survive.

__________________
Forming opinions as we speak

Aaagghh we are all doomed

Where is Bruce Willice when we need him

Oh he got blown up on that asteroid

__________________
Fame, glory adventure, a cyber warrior craves not these things.

Funny I don't have any memory of that. In fact, I remember next to nothing about the movie, although I saw it in a theatre... Tells something about how "good" it was.

Back on the subject, our Solar system most likely will not be affected by the collision, even if the Sun gets expelled from the galaxy. Individual stars very rarely encounter each other. However, it's not so for gas and molecular clouds: They will collide and contract starting rigorous stellar formation period after which the new galaxy runs literally out of gas. The end result will be a giant elliptical galaxy with only old stars.

__________________
Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman

Have you done a calculation to see how many Oort clouds and Kuiper belts the Earth will travel through at roughly 400 km/sec? What is the chance that some solid body in our solar system will by hit by a large energetic object sufficient to rain large rocks on us? What's the chance that some object would hit the Earth directly? My OOM calculations say it is not an insignificant chance.

A 25 kilometer diameter comet from an Andromedian solar system hitting the Earth would release 6x10²⁶ Joules of energy, which is 150 billion megatons (if you're thinking in terms of nuclear destruction). Not much warning, not very survivable.

__________________
Forming opinions as we speak

Makes you wonder about how many times a scenario like this has played out in the distant past? How many civilizations have lived, had dreams of greatness, and for different reasons, finally died out.

I think it's highly unlikely that the Human Race will be a witness to this galactic merger. Perhaps another civilization that will appear well after we are gone.

The transverse velocity (proper motion) of M31 is unknown, but can be constrained, as demonstrated here:

Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33
Abraham Loeb, et al., Astrophysical Journal 633(2): 894-898, Nov 2005
Abstract: A major uncertainty in the dynamical history of the Local Group of galaxies originates from the unknown transverse speed of the Andromeda galaxy (M31) relative to the Milky Way. We show that the recent VLBA measurement of the proper motion of Andromeda's satellite, M33, severely constrains the possible values of M31's proper motion. The condition that M33's stellar disk has not been tidally disrupted by either M31 or the Milky Way over the past 10 billion years favors a proper motion amplitude of 100+/-20 km s^-1 for M31 with the quadrant of a negative velocity component along right ascension and a positive component along declination strongly ruled out. This inference can be tested by future astrometric measurements with SIM, Gaia, or the SKA. Our results imply that the dark halos of Andromeda and the Milky Way will pass through each other within the next 5-10 billion years.

I don't know how many orbits that amounts to, or if it even amounts to more than one. But 5-10 billion leaves a lot of wiggle room. In 5 billion the sun should be a red giant, or very close to it. So if the collision happens on the early side, that could be a real show. But if it takes 10 billion, then the sun will be a white dwarf, and maybe it will all be a tad less spectacular around here.

I also don't know of any such fine scale simulations of galaxy collisions. Usually these billion-body-problems are necesarily restricted to the gravitational interactions of the stars, or collisions between the large clouds of dust & gas (Iono, Yun & Ho, 2005; Iono, Yun & Mihos, 2004; Braine, et al., 2004; de Gris, 2003).

• Galaxy Crash webpage, for java simulations of colliding galaxies.
• Multiple galaxy collision video from HST.
• A series of galaxy collision images.
• Astrophysicists Run Largest Galactic Collision Simulations Ever on NPACI's Blue Horizon (from the National Partnership for Advanced Computational Infrastructure, now defunct, May 2000).
• Milky Way - Andromeda Galaxy Collision, video from Hayden Planetarium

Colliding spiral galaxies NGC6621 & NGC 6622 (Arp 81), or maybe the Antennae Galaxies, are probably as good an example of any as to how a collision between M31 and the Milky Way will look. There is a lot of vigorous star formation where the clouds collide. But the stars rarely collide, and often produce tidal streams like the one in the Antennae. I think it is reasonable that in our local neighborhood, the extreme tidal disturbances could easily collapse large portions of the Oort cloud, and give us a rain of comets. There are several papers already on the books, showing that galactic tides are enough to trigger Oort Cloud comets to fall into the inner solar system, especially as the solar system passes through denser arms of the Galaxy (i.e., Napier, 2006; Dybczynski, 2005; Matese & Lissauer, 2004; Dybczynski, 2002).

__________________
Don't try this at home - We're what you call "professionals" - MythBusters.

In the case of a single galaxy and no collision, I could see that galactic tides would be significant if spiral galactic rotation curves were keplerian, but of course they're not - they're more or less flat, so I don't see where such "tides" could be significant enough to disturb the Oort Cloud.... (?)

__________________
Everyone is entitled to his own opinion, but not his own facts.