# How far away is Andromeda Galaxy really?

If it was where it now appears to be 2.2 million years ago, where is it actually now? At what rate has it been moving away from us and us from it due to expansion of the universe? And in general, is the farthest observed celestial object actually much further away than it appears, and if so, does that mean that the universe is much older than the apparent present distance of the farthest object?

Here’s a challenge. In your above question, precisely define “now.”

Sidestepping curtc’s interjection and trying to get to the heart of your question…

‘relatively speaking, not very far from where it appears to be.’ I think. Not really an astrophysicist… (and I’m not going to bug the astrophysicist next to me because I’m supposed to be working on other stuff just now… ) the effects of the expansion of the universe, or proper motion, would not have a very great effect on the distance from andromeda to here, even over a 2.2 million year period. They might be 2.21 million light years away now.

I don’t know of a formula to calculate the rate of its travel or anything. Sorry.

here
80 km/sec = 2.52 X 10^9 km/year. Multiply by 2.2 million years. M31 is 585 light years closer to us than it appears in the sky. That’s likely within the margin of error of the distance measurements.
Being so close to us, the contribution of the expansion of space to M31’s velocity away from us is overwhelmed by ‘peculiar’ velocities: The Hubble constant is 72 kilometers per second per megaparsec, and M31 is 675,000 parsecs away, so the expansion of space should contribute 48.6 km/sec to Andromeda’s velocity.

It just snuck up and scared the bejeezus out of me!

Muttrox’s attention was diverted for a moment, when – WHAM!-- the Andromeda Galaxy struck him in the small of the back.

I am not an astrophysicist, so I did not understand a word Squink said. (which is my problem, not his). He may have said something different or the same thing in a different way. However, I believe that the expansion of the universe has little to do with the relative position of bodies within our galaxy. If I’m wrong please explain why in layman’s terms, please.

IANAA, but my father is a physicist and we are both avid amateur astronomers, so perhaps this might help. Imagine that the galaxies are raisins in a muffin that you just put in the oven. As the muffin expands, the raisins are going to move away from each other,

Got it?

Now throw that analogy away, because unlike the raisins which are stuck in the batter matrix, galaxies are free to move about the universe while it is expanding. Some will show gravitational attraction to each other, while others might have just had some initial direction in which they were headed. Hence the calculations above by Squink, i.e. how does the effect of expansion impact the velocity of the Andromeda Galaxy towards us.

But this begs the further question: If the Andromeda Galaxy is headed our way due to gravity, will we not see an increase in the velocity (gravity accelerates) with which it is approaching us?

1. Galactic clusters are gravitationally bound. That is, just like a coin in your pocket, a cluster is not going to expand apart over time. Forget expansion of the universe. That has nothing to do with galaxies in the same cluster.

2. Andromeda is within our “local group.” I.e., it is gravitationally bound with the Milky Way and 20+ other (mostly small) galaxies in our local group.

3. Andromeda’s current motion is somewhat towards the Milky Way. There was a report published early in the year where some research group claimed that it would collide with the Milky Way at some point. Others immediately objected, etc. The precise motions of such large objects, as well as predicting their future positions, is not an exact Science.

4. See Squink’s link for more info on one such claim. Note that they did a computer calculation suggesting 3Byrs to a galactic crunch.

1 redux. Expansion of the Universe has nothing to do with the motions of galaxies within a cluster (or even supercluster).

Not surprising, when all redshift gives you is the component of motion towards or away from you. M31 could be streaking across the bow of the Milky way at 1000 kps, and we’d have no way of knowing it.

I didn’t think it was an interjection. It was, however, addressed more at Liberal’s second question, having to do with how big the universe is now.

Ok, it’s very late and my eyes aren’t focusing right. Please reassure me that the character following the “3” is a “B,” not an “8.”

Now I’ve gotta clean up the mess I made, thinking a galactic crunch is only 38 years away.

Why so? Everything is gravitionally tied with everything. Why would them being “bound” make a difference? Why isn’t the set including this cluster and the closest other clusters not expanding, either, in this case? Why not the whole universe not expanding?

Gravity weakens greatly over distance, is what he meant. The Hubble Constant affects things of a truly titanic scale - megaparsecs, as Squink said (a parsec is a little more than three lightyears, so somewhere above 3 million lightyears) - and the gravity of galactic clusters is still strong enough, since they’re still close enough, that their gravitational attraction is still too strong for the universe’s expansion to pull them away.

Imagine putting two magnets on a piece of very stretchy cloth. If the magnets are a foot apart, the stretching of the cloth will pull them away from each other. But if they are placed right next to each other, they will stay connected no matter how much the cloth is stretched.

(I hope this is coherent.)

When gallaxies collide the matter in them doesn’t actually collide does it? You dont have stars smashing into one another because galaxies are very much empty space.

Thus the structures of the galaxies intersect and imping apon each other gravitationally, but since there is so much emptyness in a galaxy there is no actual collisions at an electrostatic level.

I understand that. But it means that expansion still takes place in a cluster, even if it can’t counterbalance motions caused by gravitation.

You’re right… if you can cause all galaxies in a cluster to stop having any gravitational influence on each other… well, first, you’d be God, but second, yes, the universe expansion will cause them to move away from each other.

Go back and re-read Squink’s first post… he took this into account in his second paragraph.

But remember as the individual stars near each other, they’ll start being attracted by each other’s gravity.

Here is a visual representation of two galaxies colliding.

Nope.

I shoulda read my own cite better. While it’s rare for stars to actually collide, you’ll get a lot of diffuse matter - dust clouds and the like - from the two galaxies that will just do it like rabbits when they get close.