Will we collide with Andromeda?

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Martin Symons

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Will we collide with Andromeda?
« on: 30/09/2011 23:30:03 »
Martin Symons asked the Naked Scientists:
What will come first Andromeda crashing into the Milky Way or our sun becoming a red giant and consuming our planet?

Where is the August and September Naked Astronomy? I am having withdrawal symptoms...

What do you think?
« Last Edit: 30/09/2011 23:30:03 by _system »


Offline CliffordK

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Will we collide with Andromeda?
« Reply #1 on: 03/10/2011 04:58:09 »
I will put my money on the idea that at some point in the future, the Sun will no longer support fusion, with likely entering a red giant phase and ending as a white dwarf. 

Whether or not the estimates that the sun is 4-5 billion years old, and the sun will enter a red giant phase in another 4-5 billion years are accurate, well, I guess none of us will ever know.

As far as a collision with Andromeda, I'd put my money against that happening.

There are many unknowns with the universe.  Is there a center of the universe?  If so, then where is it?  Are the galaxies orbiting around the center of the universe?  Perhaps multiple universe centers like super-galaxies?

Without answering some of these fundamental questions, any speculation about the motion of individual galaxies would be problematic at best.

Consider the following diagram:


In Frame A, you see the approach vectors (black) of Andromeda and the Milky Way intersecting at point (a).
In Frame B, you see the approach component (black) of the actual vector paths of the two galaxies (blue).  They will actually collide at point (b) at the predicted time.
In Frame C, if the timing is off...  say by a billion years, then while both galaxies eventually go through point B, they don't collide.
In Frame D, you could also have an approach component going through point A, but the actual paths of the two galaxies are not on a collision course, and they will each pass through separate points (b) and (c).

Considering that I believe only a single vector component of a very complicated 3-dimensional relationship has been calculated, my guess is that we will either miss in time (case C), or miss by separate vector paths (case D).  Also consider that all of these vectors likely represent curved trajectories.

Of course, gravity would tend to push the two galaxies towards each other.  However, the gravity would equally likely cause them to curve around each other.

That doesn't mean that small galaxies aren't merged into the Milky Way all the time.  I believe that the current theories indicate that a couple of small galaxies are currently being merged into the Milky Way.

The other thing you have to consider, not really having to do with galactic collisions, but...
The orbital speed of the spiral arms of galaxies is problematic, with the inner stars orbiting too quickly, and the outer stars orbiting too slowly.  One theory to solve this problem is that there is a certain amount of slippage between spiral arms.  So, the stars in the galaxy are constantly moving between areas of high star density and areas of low stellar density over the time course of a few million years.  This would mean that the stellar positions within the galaxy are likely to change, and it would not necessarily require a collision between galaxies to cause a collision between stars, or changes in the interstellar medium that the sun passes through that could affect Earth.\

At this point, I'm not going to loose any sleep over it.