The coming collision
Published on Oct 18, 2006 at 5:52 pm.
4 Comments.
Filed under galaxies.
In my last post, I mentioned that the Andromeda Galaxy and the Milky Way would one day collide. That day is only about 3 or 4 billion years into the future. You can’t really nail it down to a particular day, because galaxies are pretty big. How do you define just when the collision is? Is it when the outermost halos of the galaxies touch, when the disks first run into one another, when the centers of the two are closest together, or just what circumstance do you use to define collision? In fact, the extended halos (coronae) of these two galaxies will start to run into each other in just over a billion years.  Also, we can’t say for sure when this will happen, because we don’t know for sure just how far away the Andromeda Galaxy is. Various measurements range from 2.5 to 2.9 million lightyears. I normally just split the difference with my students and say 2.7 million lightyears. No matter how you cut it, though, you find that the collision will occur before the Sun leaves the Main Sequence, which is in about 5 billion years.
As the Andromeda Galaxy gets closer, you’d be able to see it better and better in the sky. But, unlike this drawing, it wouldn’t stand out so brilliantly in the sky. You’d see it only from dark skies, and it would look about as bright as the Milky Way. Well, duh. That is pretty much just what the Milky Way is — our own galaxy seen from the inside! The difference would be that the Andromeda Galaxy would look like a spiraled version of the Milky Way cutting across the rest of the Milky Way in the sky.
Most of a galaxy is empty space, so for the most part, the stars would pass right by one another. However, the varying gravitational fields would distort their orbits. Some stars would change orbit a little, some would drastically change orbit. Some would be flung completely out of the galaxies. The chance of a star colliding with the Sun is so tiny as not to even be worth thinking about. In fact, it would be unlikely that any stars would pass close enough to even alter the orbits of any but perhaps the outermost planets of the Solar System. However, several stars would likely pass through the Oort cloud, and they would be tossing comets our way. That would be pretty (until one hit the Earth). But, from Earth, the collision would be totally unimpressive at this stage. In fact, the collision would take so long that if someone were living then, they’d be able to live out their whole life without ever noticing any change at all happening in the sky.
Now, when the collision happens, it will be impressive to see from a long way away, even if not from Earth. Tidal forces would begin to distort the galaxies, and the Milky Way and Andromeda pair would begin to look a lot like NGC 2207, seen here. These are likely galaxies in the early stages of a collision. Also, one galaxy is about twice the size of the other, like with Andromeda and the Milky Way.
As I said, most of the galaxies are empty space, so they will pass through one another, with the exception of some of the interstellar gas, which would be compressed and new stars would begin to form at a fantastic rate, in what we call a starburst event (the Milky Way and Andromeda would become a starburst galaxy pair). After passing through one another, the two galaxies would then be brought to a halt by their mutual gravity, and then fall back towards one another. They’ll do this dance several times, 
with each passage causing some stars and gas to be accelerated to escape velocity, producing huge streamers of stars and gas shooting out from the pair, as with NGC 4676, seen here.
Eventually, what is left of the two galaxies would merge into one galaxy, mostly devoid of gas, so it would probably be either an elliptical or an S0 galaxy. John Dubinski of the University of Toronto has done computer simulations of this coming collision, and you can look at those movies at the galaxy dynamics page.
Many of the newly formed stars during the starburst would be very massive ones. Such stars die in a supernova, a massive explosion that tears the very star itself apart. The rate of supernovae in the galaxy might increase by a factor of 50 or so from what it is today.  According to Dubinski, the Sun will be slung through the core of the merged galaxy. Both the Andromeda Galaxy and the Milky Way have supermassive black holes in their cores. So, as the Milky Way goes shooting past, these beasts might actually be seen from Earth. Now, we won’t pass so close as to be in any danger, other than from the radiation emitted by the accretion disks of these black holes. 
But, there would likely be jets shooting out from from the poles of the accretion disk. If the Earth passed through one of those jets, then we’d have to deal with hard cosmic radiation for a while. The Earth might pick up enough speed on its plunge through the heart of the galaxy to achieve escape velocity, and if so it would continue onwards until it exits the galaxy on the other side. The Sun would then burn itself out sailing through intergalactic space.
Now, interestingly enough, this may not be the first time that the Andromeda Galaxy has been involved in a galactic collision. Some years back, the Hubble Space Telescope showed that there is actually a double core in M31, as seen below. It is believed that this may have resulted from a merger with a smaller galaxy. This would have been a collision with a much smaller galaxy than the Milky Way, so basically that galaxy simply merged with the Andromeda Galaxy rather than ripping both to bits.  Further evidence supporting this idea is found in meaurements of M31’s mass. Even though the Andromeda Galaxy is nearly twice the diameter of the Milky Way, it seems that it may have only about the same, or perhaps even a little less, mass. That may happen if a collision were to push stars and gas into bigger orbits. Also, there seems to have been a starburst event in Andromeda some time back, as many of the stars in the halo are much younger than the Milky Way’s halo stars (not true of the stars in the extended halo). Â
-Astroprof  Â
(Images courtesy of NASA)
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TIMMAAY on August 23, 2007 at 7:24 am: 1
cool
Astroprof’s Page » NGC 7252 on December 27, 2007 at 6:17 pm: 2
[…] For quite some time astronomers have knows that the streams of material around NGC 7252 result for a galaxy collision. In fact, this peculiar galaxy is most likely the result of two similar sized galaxies coming together slow enough to merge into one structure. This may be the future of the Milky Way and the Andromeda galaxy when they collide and merge. Astronomers believe that the collision leading to the merger of NGC 7252 may have been about one billion years ago. A closeup image of the core of the galaxy shows a spiral structure that mimics the shape of a classic spiral galaxy, only in miniature. The spiral feature may have been caused by the collision itself. Much of the galaxy’s interstellar medium is being ejected from the collision, but some of the material was compressed in the collision itself, leading to a burst of star formation. Hundreds of extra luminous star clusters containing stars only tens to hundreds of millions of years old have been identified. These unusual star clusters appear to be the progenitors of globular clusters. That is an interesting finding, since we seldom seem globular clusters forming. That globular clusters can be formed in a galaxy collision event is every exciting, since this would tend to explain how elliptical galaxies, most of which are believed to have formed in galaxy collisions, have far more globular clusters than could be explained by simply adding the globular clusters of colliding spiral galaxies. […]
Vishal on February 17, 2008 at 5:11 am: 3
thanks a lot 4 this informative article
Meeee. on April 5, 2008 at 1:40 am: 4
Awh. I wiash i was alive then.