Andromeda collision question

[rhetoric]

I'm an absolute novice so please forgive me if this is a stupid question. But there are a couple things I've heard that I can't reconcile. Either I heard them wrong, or there's just another piece to the puzzle I am not familiar with.

1: The universe is expanding (or stretching as the case may be) and all other galaxies are moving away from us.

2: The Milky Way will collide with the Andromeda galaxy in something like 3 billion years.


Assuming #1 is true, then how could #2 be true? I figure that I must be stating #1 incorrectly since I'm more certain about #2. Can someone clue me in?

Thanks!

 

[derekmcd]

It's probably a bit more accurate to state that galactic superclusters are receding from each other. Within the superclusters, smaller subclusters and local groups, the gravitation forces are strong enough to overcome the expansion and you will see blueshifted galaxies such as Andromeda. Galactic collisions do happen even on the scale of clusters:

http://en.wikipedia.org/wiki/Bullet_Cluster

 

[MeteorWayne]

What derekmcd said is correct as usual. On small scales (say a few billion light years) the attraction of gravity results in motion that is faster than the expansion of the Universe. The latest estimate of a collision with Andromeda is about 5 billion years, but that is only a rough estimate

Edit: Sorry, I forgot to add, Welcome to Space.com! Ask away

 

[crazyeddie]

I'm an absolute novice so please forgive me if this is a stupid question.

"The only stupid question is the one that goes unasked", as someone once said.

Please don't hesitate to ask us questions. You will find all of us here absolutely chomping at the bit to answer them!

 

[rhetoric]

Thanks for the responses and the welcome. Simple answer to the question that makes sense. Been reading the site updates for a couple years but only just now had a real reason to post something. Great and fascinating site!

 

[rhetoric]

It's probably a bit more accurate to state that galactic superclusters are receding from each other. Within the superclusters, smaller subclusters and local groups, the gravitation forces are strong enough to overcome the expansion and you will see blueshifted galaxies such as Andromeda. Galactic collisions do happen even on the scale of clusters:

http://en.wikipedia.org/wiki/Bullet_Cluster

Quick follow up to this. Just a "correct me if I'm wrong" sort of thing. I understand that 'red-shifted' means that galaxies are moving away from us, and the more red they appear, the faster they are moving away. Is that right so far?

I had not heard 'blue-shifted' before, but based on the context, I'm assuming a 'blue-shifted' galaxy is one that's moving toward us. Right? So does this mean the same is true for 'blue-shifted' galaxies in that the bluer they appear, the fast they are moving toward us?

 

[MeteorWayne]

That is correct. Blue shifted means the object (be it a galaxy or a star) is moving towards us.

 

[crazyeddie]

It's probably a bit more accurate to state that galactic superclusters are receding from each other. Within the superclusters, smaller subclusters and local groups, the gravitation forces are strong enough to overcome the expansion and you will see blueshifted galaxies such as Andromeda. Galactic collisions do happen even on the scale of clusters:

http://en.wikipedia.org/wiki/Bullet_Cluster

Quick follow up to this. Just a "correct me if I'm wrong" sort of thing. I understand that 'red-shifted' means that galaxies are moving away from us, and the more red they appear, the faster they are moving away. Is that right so far?

I had not heard 'blue-shifted' before, but based on the context, I'm assuming a 'blue-shifted' galaxy is one that's moving toward us. Right? So does this mean the same is true for 'blue-shifted' galaxies in that the bluer they appear, the fast they are moving toward us?

Correct on both accounts! You seldom hear of galaxies showing a blue-shifted spectrum because there are very few of them compared to the ones expressing a red-shirt.....only the ones in our Local Group can do this. The Andromeda galaxy and M-33 (the Pinwheel galaxy) are the two principle examples; the rest are dwarfs.

 

[rhetoric]

Neat stuff. Thanks guys.