Expert: Tom Whiting - 12/2/2009

Question
what would be the effect upon impact, post impact and the long lasting effect if the collision of Andromeda and milky way galaxies occur?  

Answer
Hi Justine,
Well, there really is no "impact"...these aren't hard frisbee disks.  In fact, in all the
galaxy collisions we've seen, no two stars have ever been seen to collide, as the distance
between the individual stars (relative to their diameters) is so huge, like a factor of
10,000 or more.  But galaxies do collide frequently because their distance to size is like one to
30.  (Milky Way is 100,000 ly in diameter but the distance to Andromeda (M-31) is about
3 million ly, or 1 :: 30.
It's actually the dust and gas that collide, producing a new round of star formation as the
dust and gas heat up and condense.  In fact, the Milky Way is currently cannibalizing several
dwarf galaxies right now...or colliding, (or merging)... however you want to look at it.
As far as the Solar System is concerned, we don't really care what happens on the galaxy
scale because... we follow along with the sun. Whereever the sun goes, we go. If the sun
is eventually thrown out of the galaxy, then we just follow along, and could care less whether
we're in a galaxy, or not.
However, if the sun is thrown by gravity into the center of a galaxy, then we meet the same
fate, we die.  (Not much we can do about it, right?)
Of course all this colliding and merging with M-31 isn't for another 4-6 billion years, so
the Earth probably won't be around anyway, as that's just about the time that the sun enters
it's next stage, the red giant phase. And odds are that the Earth won't survive that event
anyway.
I would guess the end result of the completed merging in about 10 billion years is a giant
elliptical galaxy instead of two spiral galaxies.  That's the way the rest of the Universe
seems to work.
Clear Skies,
Tom Whiting
Erie, PA