Kathryn Johnston (Wesleyan University)

In 1994, astronomers discovered the Sagittarius Dwarf, a small dwarf galaxy that looked like it had been stretched into spaghetti-like strands thousands of light-years long. It turns out that the Sagittarius Dwarf has literally been torn into shredded loops by our Milky Way Galaxy's gravity field. This remarkable object is a classic example of the power of tides, whether it moves Earth's oceans or pulls apart entire galaxies. Using a galaxy similar to the Sagittarius Dwarf, the video shows how tidal interactions work to change the forms of whatever they touch.

Stealing stars

In this simulation, the Milky Way is represented as blue in the center with the small satellite galaxy orbiting around it. We follow the evolution of these two galaxies over several billion years. The dwarf galaxy is shown much larger than it would normally be so that we can see detail during the interaction. In fact, the Milky Way appears to be unaffected by the interaction, which is accurate since the Milky Way is so much more massive than the small dwarf galaxy. The colors in the dwarf show the density of stars being stripped from the satellite galaxy.

Charles Liu

Joshua E. Barnes (Institute for Astronomy, University of Hawaii)

What happens when two galaxies merge? The answer to this question lies in the mass of each galaxy. If one galaxy is significantly larger and more massive than the other, the smaller galaxy could become completely absorbed by the larger one. If the masses are comparable, a number of outcomes are possible. The galaxies may pass right through one another, remaining, for the most part, intact as separate gravitational entities (albeit disrupted). Often these two galaxies will collide again, either forming a new structure from the merger or becoming two dwarf galaxies that are a fraction of their original sizes.

What happens inside a merging galaxy?

Inside galaxies, stars are relatively far apart. Therefore, when galaxies collide, the likelihood of the individual stars colliding are minimal. However, what fills the space between the stars is the gas and dust of the galaxy. When these clouds of gas and dust collide with one another, a great deal of turbulence is created which disrupts the gas clouds. As the particles are forced together at high speeds, areas of extreme pressure are created. These active pockets will eventually explode into regions of renewed stellar birth.

The Simulation

This simulation demonstrates a merging of two gas-rich disk galaxies. Only the gas particles are shown. The colors indicate the energy dissipated by shocks arising from the collision (blue is lower energy). Ultimately, the galaxies merge into one body giving birth to new star-forming regions and a new galaxy.

Eve Klein

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.

The Simulation

This simulation demonstrates a merging of two gas-rich disk galaxies. Only the gas particles are shown. The colors indicate the energy dissipated by shocks arising from the collision (blue is lower energy). Ultimately, the galaxies merge into one body giving birth to new star-forming regions and a new galaxy.

Gordon Myers