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Bigger Galaxy Eats Smaller Neighbor
Mr.Happy3050 writes "CNN is reporting here that the large galaxy Centarus A absorbed a smaller neighboring galaxy 200-400 million years ago. The absorption created a line of blue stars thousand of light-years across."
As an Anonymous person here said, the stars interact because of gravity, so even though there is some space between the stars, the two [?] galaxies that come out of the collision will look nothing like they did when they entered it.
For a smaller scale comparision, imagine 2 solar systems colliding. The odds of the planets, and stars hitting are not 100%, but you can bet the orbits are going to be drastically changed.
Saskboy's blog is good. 9 out of 10 dentists agree.
"Now I can see a large galaxy colliding with a smaller cluster of dwarfs or whatever it was - but at the end of the article they talk as if it's a fairly well-known fact that the Milky Way and Andromeda will collide at some point in the distant future."
Yes, they will collide.
"Now, I seem to remember the classic marker-spots-on-a-balloon explanation that so long as the universe is expanding each galaxy continues to get further and further away from each at a high speed (near light speed but not quite?)."
Remember that in the real world galaxies aren't fixed marks drawn on a balloon. They are free to move around space any way they want to in addition to this "cosmic recession" due to expansion of space. So, in terms of the balloon analogy, the galaxies can move freely around the surface of the balloon while it's inflating; their velocity will be the sum of this recession velocity (which they would have if they weren't moving on the balloon) and the velocity at which they are moving on the balloon.
Another important thing to note is that not every galaxy is receding at the same velocity. If you measure the velocities from one galaxy, those galaxies which are further away from this one will appear to have a larger velocity. The recession speed is given by a formula known as Hubble's law:
v = H*d
where v is the velocity, H is a constant (known as the Hubble constant - Google will give you a numerical value) and d is the distance from the Milky Way (or any other galaxy where you could be measuring these things) to the other galaxy. In addition to this velocity (which won't nowhere near light speed unless the other galaxy is *very* far away), the galaxy can have any other velocity due "ordinary" movement through space, but these velocities are randomly determined (they are due to collisions with other galaxies and such "ordinary" things), so on average galaxies will tend to follow Hubble's law and recede from us. Also, since the recession velocity increases in proportion to distance, distant galaxies will follow this law much better than nearby galaxies, so there will be much fewer exceptions.
OK, so that should be clear by now. Now, forget everything I just said! The above is all true and nice to know, but it isn't at all relevant for the Milky Way and Andromeda. I just wanted to make a few other things clear before moving on to this one...
Andromeda and the Milky Way do not recede at all due to expansion of space. They are bound together by gravity, just like (for example) the Earth and the Moon. If you still like to think in terms of the balloon analogy, imagine that some of the marks (which would have to be something a bit more substantial than pen marks) would be tied together with a piece of rubber band. As the balloon inflates, the marks would try to separate from each other, but the band would pull them back together. So, things that have some kind of a force holding them together ("bound systems") don't expand while most of the universe does.
In fact, speaking about galaxies in the balloon analogy is somewhat misleading, since they are usually gravitationally bound to many other galaxies, so galaxies really aren't the objects Hubble's law is all about. For example, the Milky Way is bound to Andromeda, the Magellanic Clouds and a huge number of other galaxies which have much more cryptic names. Together these are all known as the "local group" of galaxies. Some of its members are much further away from us than Andromeda and still not receding from us.
Oh, and I must say one thing: the balloon analogy is nice and quite illustrative, but taking it too seriously can lead to some pretty horrible misconceptions. You shouldn't trust too much on any conclusions drawn from it.
"This is why the stars in galaxies don't fly apart as the universe expands. The gravitational force at those scales is much greater than the expansion."
Some nitpicking: this is not right. You can't do this kind of a "strength comparison" between some force and the expansion of the universe. In fact, whether or not some system holds together in the process has absolutely nothing to do with the strength of the attractive forces between its individual parts.
For example, consider yourself just standing on the surface of the Earth. You are bound to the Earth by its gravity. To travel to outer space you would need to somehow obtain enough energy kinetic to climb out of the Earths gravitational well. If you somehow managed to do this and would start traveling around in space with a huge speed (greater than the Earths "escape velocity"), you would still feel the effect of the Earths gravity, but the difference would be that you'd have enough energy to escape the well and travel where ever you'd want to go. Similarly, the Moon is bound to the Earth since it doesn't have enough kinetic energy to escape the Earths potential well.
For another example, consider an electron and a proton traveling around freely in space close to each other. There is of course an attractive force between the two of them. The most important question then is whether they have enough energy to overcome this attractive force and travel freely to infinity or will the attractive force be strong enough to keep them together. If they are moving too fast for the force to hold them together, they'll move around as independent free particles; if they are moving so slowly that the force will always be strong enough keep them together they will form a bound state. (This particular bound state is better known as the hydrogen atom.)
The essential question here is whether or not the system is bound or not: if it is, the distance between its parts will not increase due to the expansion of the universe; if the system isn't bound, the distance will increase. This has really nothing to do with the strength of the interaction, but depends entirely on whether the internal forces are strong enough to hold the system together, given the energies of the internal parts.
The Milky Way and Andromeda are gravitationally bound in this sense: they don't have enough energy to escape each others potential wells, so they don't recede from each other when the universe expands. While the space between them expands, gravity will still always have the same distance-dependence, so it will keep the galaxies together exactly the same way.
(The collision is likely to change all this, though. At least some parts of our two galaxies are going to acquire enough kinetic energy to escape the new system, whatever that is going to look like.)