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Crash Course: Simulating the Fate of Our Milky Way
By Tariq Malik
SPACE.com Staff Writer
posted: 07:00 am ET
07 May 2002
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Where we're headed
Most scientists agree that the Milky Way will cross paths with the Andromeda galaxy in about three billion years. Both galaxies are now spiral in shape, though Andromeda is about twice as large as the Milky Way.
The galaxies are separated by about 2.2 million light years (one light-year is about 6 trillion miles, or 10 trillion kilometers). That gap is closing at about 310,000 miles per hour (500,000 kph).
While a collision appears inevitable, astronomers admit that the sideways motion of Andromeda -- the galaxy’s speed perpendicular to its forward path toward the Milky Way -- could affect the encounter’s timing, but it has yet to be measured precisely. Dubinksi used an estimate of 12.4 miles per second (20 km per second) for his collision model.
"Even if the galaxies have a wider passage on the first pass, if they are on a bound orbit they are destined to merge eventually," Dubinski said. "If not on the first flyby, then within the second or third pass over the next 10 billion years, he added.
The clincher is gravity. Even if there’s enough space between the Milky Way and Andromeda to simply brush past each other at spiral arm’s length, their mutual gravity will ultimately win out, drawing the two galaxies together on successive flybys.
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 | | SCIENCE TUESDAY | | | | | |  |  | Images |
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| Our Future: A simulation of what might happen when the Andromeda Galaxy hits ours shows tidal forces of gravity creating long plumes of material. The central regions will relatively quickly fall back together and merge into a single remnant galaxy.
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| Merging Mice: Simulation shows how galaxy collision known to be going on today might have begun, and how it might end.
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| | |  | Related SPACE.com STORIES |
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|  | TODAY'S DISCUSSION | | | | | | |
Dubinski hopes to refine his model of the collision between the Milky Way and Andromeda galaxies in the future by modeling a system of about a trillion or so particles to match the number of stars in the two galaxies. But with the current growth in computer memory and speed, such computations won’t be possible for about 10 years, he said.
The Mice
Meanwhile, another pair of researchers has taken a pair of interacting galaxies called the Mice and worked backward to simulate what they figure has already taken place. The Mice, recently photographed by the Hubble Space Telescope's new camera, represent a collision in progress that could be very much like the pending crash of our own galaxy into Andromeda.
Joshua Barnes of the University of Hawaii worked with John Hibbard, now at the National Radio Astronomy Observatory, to animate a past that might have led to the present-day Mice.
Their computer animation shows two pinwheel galaxies falling together, swerving as they pass, and flinging out long tails of stars. At present the two galaxies have made one pass, and are coming back for a second and closer encounter. Eventually they will coalesce into a single galaxy, whose possibilities the simulation projects.
"Simulating colliding galaxies is a bit like investigating a car crash," Barnes says. "Suppose you had no witnesses, just a couple of wrecked cars. You might try different test crashes, varying things like speed and angle of impact, until you found a way to get the same damage as the original collision. That's basically what we did."
See the "Mice" Picture taken by Hubble's new camera | | | |
