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Milky Way and Andromeda Galaxies Merging
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The Milky Way and Andromeda galaxy are on a collision course! In about 3 billion years, the two galaxies will collide. After a very complex gravitational dance lasting about 1 billion years, they will merge to form an elliptical galaxy.

The Milky Way
Our home galaxy, the Milky Way, is a collection of about 400 billions stars spread out in a thin disk more than 100,000 light year across. Our sun is one of those stars sitting about midway out in the disk moving around with the others on nearly circular orbits. The Milky Way would look like an average looking spiral galaxy if we could see it from the outside.

Andromeda
The nearest big spiral galaxy to the Milky Way is the Andromeda galaxy. Appearing as a smudge of light to the naked eye in the constellation Andromeda, this galaxy is about four times as massive as the Milky Way but very similar in many ways. At the moment it is about 2.2 million light years away from us but the gap is closing at 500,000 km/hour. Andromeda is the only big spiral galaxy galaxy moving towards the Milky Way, and the best explanation is that the Milky Way and Andromeda are in fact a bound pair of galaxies in orbit around one another. Both galaxies are thought to have formed close to each other shortly after the Big Bang, initially moving apart with the overall expansion of the universe. But since they are bound to one another, they are now falling back together, and one very plausible scenario puts them on a collision course in 3 billion years.

Interacting Galaxies
Galaxies collide and interact occasionally and there are several well-known examples in the vicinity of the Milky Way. We see interacting pairs as snapshots in time and the results are often very dramatic. Long streams of stars thrown off in beautiful open spiral patterns are characteristic of these collisions and are known as tidal tails and bridges because of their origin in the strong mutual gravitational tides of the two interacting galaxies. Colliding galaxies also tend to merge with one and the final outcome after some violent convulsions lasting a few hundred million years is another kind of galaxy called an elliptical. During this period, the gas in these galaxies can be ignited violently in a starburst creating stars at rates hundreds of times greater than normal. Galaxy interactions are not that common an event in the local neighbourhood (maybe one in a hundred galaxies) but the rates of merging and interaction increase dramatically at early times in the universe. Galaxy merging is fundamental to building up structure in the universe and explains many of the peculiar features of young galaxies seen by the Hubble Space Telescope.

A Simulation of the Milky Way/Andromeda Collision
It is fun and instructive to see how the collision of the Milky Way with Andromeda might play itself out. John Dubinski of the University of Toronto generated a numerical simulation of the collision using Blue Horizon, a 1152 processor IBM SP3 at the San Diego Supercomputing Centre. Each spiral galaxy is represented by about 40M stars and is surrounded by a 10M particle dark matter halo for a total of more than 100M particles for the galaxy pair. Click on movie above to see a graphical representation of the simulation.  In the still image above, the Milky Way galaxy is the smaller galaxy near the top of the image.  In the movie, the Milky Way galaxy starts out near the bottom.

These simulations reveal a tremendous amount of detail in the destruction and unravelling of the galaxies as they collide and merge to form an elliptical galaxy. The Milky Way is shown face-on and moves from the bottom up to the left of Andromeda and the to the upper right. Andromeda is tilted from this perspective. The images are 1 million light years across. After the initial collision, a open spiral pattern is excited in the both the Milky Way and Andromeda and long tidal tails and a connecting bridge of stars form are apparent. The galaxies move apart and then fall back together for a second collision and then after a few convulsions which throw off more stars in complex ripple patterns they settle into something looking like an elliptical galaxy.

A View from the Inside
An interesting fact on the timing is that the Sun will still be burning brightly when this collision occurs and maybe life of some sort will still be around on Earth at that time. So what would people see in the night sky during this billion year galactic dance? As Andromeda approaches, it will grow in size and just before the collision the night sky will be filled by a giant spiral galaxy. When the two galaxies intersect, our familiar Milky Way arch over the sky will be joined by a second intersecting arch of stars but this will only last for 100 million years or so and will be a very confusing state of affairs for galactic astronomers. Finally, when the two galaxies merge our view will depend on which direction the Sun is thrown. There are two possible fates of the Sun which depend closely on the details of where it is in its galactic orbit at the time of the collision. In the first case the Sun may take a ride on a tidal tail and be ejected into the darkness of intergalactic space. In this case, our star would be all alone with few stellar neighbours so the night sky would be very dark with few stars to see—maybe like the view of the nightsky from downtown Toronto. In the second case, the Sun is thrown right into the centre of the merging pair where a great starburst will be underway. The huge number of stars forming will result in supernovae going off at a rate of a few per year in the new merged galaxy. While these will not present a direct hazard to the Earth, they will truly light up the sky letting you read at night but probably frustrating the endeavours of backyard astronomers!

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Examples of actual galaxies in collision: Mice Galaxies, Tadpole Galaxy, Antennae Galaxies, Bird's-Head Galaxy