John Dubinski (University of Toronto)

We live in the Milky Way Galaxy, a collection of gas, dust, and hundreds of billions of stars. About two million light years (20 billion billion kilometers) away lies the Andromeda Galaxy, a spiral galaxy similar in size and shape to our Milky Way. Current measurements suggest that, in about three billion years, the Milky Way and Andromeda galaxies may collide. What will happen? The stars in the galaxies, our Sun included, will probably not hit each other, but the galaxies' mutual gravity will probably pull, twist, and distort them until, about a billion years later, a new elliptical-shaped galaxy is born.

Gravity and tides at work

This movie shows a supercomputer simulation of one possible collision scenario between the Milky Way and Andromeda. Each spiral galaxy is represented by a disk of stars surrounded by a spherical dark matter halo. The simulation contains over 100,000,000 virtual particles. The Milky Way is shown face-on and is initially at the bottom of the frame while Andromeda moves from the top of the frame down and is tilted from this perspective. The movie's field of view is about one million light years (10 billion billion kilometers) across, and the total elapsed time of the movie is about 1 billion years. The complex patterns and structures created during the collisions are caused by tides, the same process that works on Earth's oceans. The gravitational pull of the each galaxy's stars and dark matter twist, tear, and distort their original disk-like structures, leaving a single elliptical galaxy and lots of tidal debris after all is said and done.

Gordon Myers

Chris Mihos (Case Western Reserve University)

Galaxies like the Milky Way often have a number of much smaller, satellite galaxies orbiting around them like moths around a campfire. What happens if one of these satellite galaxies comes too close to its giant neighbor? This movie shows one possible outcome; the satellite falls to the center of the large galaxy, puffing it up in the process.

Galactic acquisition

In this simulation, we begin by looking at the two galaxies from the side. The blue ball on the right is a spherical satellite galaxy; the yellow object is a large spiral galaxy as seen from its side, or edge-on. Over the course of several hundred million years, the satellite goes above, then below the large galaxy's disk, then above again. Finally, the hapless satellite is pulled into the galactic bulge, becoming part of the now-larger galaxy.

Tracing a galactic collision

According to this supercomputer simulation, the collision tosses parts of both galaxies into a scattered halo around the disk and bulge. This result suggests that astronomers can look for such galactic crumbs to see if one galaxy has recently consumed another. The latest theories of galaxy formation suggest that our own Milky Way Galaxy may have gobbled dozens, even hundreds, of small galaxies to attain the size and mass it has today.

Charles Liu

Chris Mihos (Case Western Reserve University)

When galaxies collide, a number of factors affect the outcome. This movie shows two large spiral galaxies colliding under a particular set of initial conditions: one galaxy is rotating clockwise, the other counterclockwise. The simulation follows the first 100 million years of the collision. The complexity of these calculations requires a powerful supercomputer and many hours (and sometimes days) to run.

Collisions create star formation

The blue material represents stars in these galaxies, while the yellow material represents clouds of hydrogen gas that could form new stars during the collision. Before the collision, both stars and gas are evenly distributed throughout each galaxy, but as the galaxies crash into each other, the stars spread out into a much larger volume. Conversely, the gas gathers into tight knots and thin threads. This behavior suggests that, because of the collision, new stars will form in massive, concentrated starbursts. The directions of the galaxies' spin affects how the stellar and gaseous matter move; different combinations of spins, collision speeds, and angles will produce markedly different results.

Tracing a galactic collision

According to this supercomputer simulation, the collision tosses parts of both galaxies into a scattered halo around the disk and bulge. This result suggests that astronomers can look for such galactic crumbs to see if one galaxy has recently consumed another. The latest theories of galaxy formation suggest that our own Milky Way Galaxy may have gobbled dozens, even hundreds, of small galaxies to attain the size and mass it has today.

Charles Liu