Galaxy collisions

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wurf

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Astronomers say that from wherever you are in the universe, every galaxy would be observed to be moving away from you, as we see here on Earth. They also say that galaxies sometimes collide into each other, and that our Milky Way and Andromeda are on a collision path. Can someone explain this contradiction? Things moving away from each other don't collide, and if you were located in a galaxy which was on a collision course with another, wouldn't you observe the other galaxy moving towards you?
 
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weeman

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We are indeed witnessing Andromeda moving towards us at a high rate. I understand why you are confused <img src="/images/icons/smile.gif" /> Galaxies often exist within local groups. Our Milky Way is part of a large local group, which also contains Andromeda. <br /><br />Astronomers see the expansion on a very large, universal scale. Galaxies within their local groups can actually pull each other together through their enormous gravity. <br /><br />However, on large scales (beyond local groups), all galaxies are expanding away from each other. <br /><br />The Andromeda galaxy is speeding towards us, and may begin its collision with the Milky Way sometime in the next 3-4 billion years. This makes it sound like it's moving incredibly slow, but Andromeda is VERY far away, some 20 billion billion kilometers. <br /><br />When they collide, it will begin a sort of "cosmic cannibalism" where the larger galaxy often consumes the smaller galaxy. The two supermassive black holes at the galactic centers would collide (assuming they exist <img src="/images/icons/wink.gif" /> ) and a new galaxy is born. It's hard to say which one will "consume" the other in the Andromeda/Milky Way collision since they are very close in size. Recent estimates have suggested that Andromeda is slightly bigger, containing perhaps 150-200 billion stars. Of course, even MORE recently, I've read up on current research from the Spitzer Telescope, which shows evidence that as many as one trillion stars could exist within Andromeda.<br /><br />Whew! That's a LOT of stars <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>
 
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Saiph

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The contradiction disappears once one recognizes that there are two types of motion for galaxies.<br /><br />There's the motion caused by universal expansion, due to the stretching of space itself, between all objects. This motion is directly proportional to the amount of distance between the two objects in question. So, the further apart two objects are, the faster they receed from eachother due to this expansion. Of course, this means they're further apart after a while, and so they travel away even faster.<br /><br />Essentially expansion creates a repulsive "force" between galaxies that is directly proportional to the distance. **Note, this force is illusory, but it can be treated as a force in this case.<br /><br />However, there is another component to their motion, caused by gravitational attraction (I.e. newtonian motion). The force of gravity between galaxies pulls them towards eachother. This force is inversley proportional to the distance (square of the distance to be precise). So, the further apart things are, the weaker the force, and the smaller the induced motion is.<br /><br />Now, the motion of galaxies, like all objects, is the result of all the forces acting upon them, in this case both the "expansionary" force, and the gravitational force. Expansion pushes them apart, gravity pulls them together. <br /><br />If the objects are far apart, the expansionary force is very strong, and overcomes the weak gravitational force, and the objects move apart.<br /><br />If the objects are close, the expansionary force is very weak, and gravity is much stronger. In this case gravity wins, and the objects move towards eachother.<br /><br />So for colliding galaxies: The two colliding galaxies are close enough that gravity wins, but far enough from everything else that they're still receeding. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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MeteorWayne

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Very well put and lucid explanation Saiph.<br />I knew all that, but now I grok it!!<br />As refreshing as my Sunday morning coffee. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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dragon04

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<font color="yellow">If the objects are close, the expansionary force is very weak, and gravity is much stronger.</font><br /><br />Wow... I just had an epihpany or something. Seeing that gravity is the weakest force we know, then would not an expansionary force be weaker still?<br /><br />Were it always stronger, we shouldn't be blue shifted in regards to M31 or the Local Group, no? <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>
 
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Saiph

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Well, it would be the weakest force...if it really was a force. It doesn't act on the objects themselves, it's just a consequence of space stretching that gives the illusion of a force (departing an acceleration).<br /><br />Anyway, glad to be of service <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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weeman

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I'd say that sounds correct, Saiph. The expansion of space is not in relation to any of the three forces. <br /><br />We have yet to discover what is causing the expansion of space, correct? We don't know where new space is coming from, or why it is expanding. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>
 
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