[Arthalion]

When Andromeda and the Milky Way collide, when will those of us still around start to feel something?

Our descendants probably won't "feel" a thing since galaxies are mostly empty space and the collision is really just a combination of the gravity wells.

Best case scenario: The constellations shift in the sky as gravity scrambles the orbits of the stars, and other than having to remake our star charts every century or so, humanity is untouched.

Less good but more likely scenario: The sun and Earth are ejected from the galaxy and live out their existence in a material filament in intergalactic space. Except for the sky getting a lot darker at night, humanity is also likely to ride this one out without feeling anything.

Equally likely but far worse possibility: The colliding gravity wells cause the dust lanes in both galaxies to compress and begin a period of new star formation. The resulting gamma ray bursts would likely sterilize our planet. On the upside, it would be instant and we wouldn't feel anything. On the downside, we'd all be dead.

Very unlikely but even worse possibility: Our sun could be at just the right spot when they collide that it gets sucked into either our own galaxies or the Andromeda galaxies core. Why is this one worse than gamma ray extinction? Because it would take centuries to fall into either one, and we'd see it coming. We'd know we were doomed and would have no escape.

[Talking_Mouse]

Very unlikely but even worse possibility: Our sun could be at just the right spot when they collide that it gets sucked into either our own galaxies or the Andromeda galaxies core. Why is this one worse than gamma ray extinction? Because it would take centuries to fall into either one, and we'd see it coming. We'd know we were doomed and would have no escape.

Only upside to this is we would have absolutely beautiful night sky -- and probably day sky, too.

All of these scenarios presuppose that the human race is still on earth during these events. Somehow, in the intervening billions of years, I think we will have abandoned earth for other more resource rich planets or we will have destroyed ourselves (war, over population, mutating plague, whatever) and some other life form will be dominate then.

Personally, I go for the former choose, I'd like to think of my descendants living thought so much of the galaxy that the destruction of one star system will not impact the population density of the human race.

[Larry Lucido]

the collision is really just a combination of the gravity wells

(In my best George Costanza voice):
Of course, the, uh, gravity wells.

The sun and Earth are ejected from the galaxy

Interestingly, I wondered aloud about that exact scenario here a few months ago (and got an answer largely consistent with yours).

Thanks for the thoughtful and descriptive reply.

[P.O.E.]

Unless, of course, there's a black dwarf in our path.

[EternalVigilance]

Because it would take centuries to fall into either one, and we'd see it coming. We'd know we were doomed and would have no escape.

Ah, the 'Terri Schiavo scenario'...

[jpsb]

I believe our portion of the Milky Way is, at this very moment, colliding with a darf Galaxy. No good, but the effects are still hundreds of thousands of years away.