Galaxy Collisions

The above image shows two galaxies colliding. The collision will take tens of millions of years to evolve. During that time, although the stars themselves will almost never collide (The nearest star to Earth is 3 light years away, or 40 million times farther than the radius of the Sun. Even if another galaxy were to collide with ours, the density of stars near the Sun would not appear all that different!), the motions of stars within the galaxies will be disturbed. If enough collisions occur, the stars will no longer be concentrated into a disk, but will move in random directions.

In a collision between galaxies, the main drama does not occur with stars, but with the gas that fills the vast space between stars. The collision will compress this gas, triggering the formation of new stars. As the stars form and evolve, some will explode as supernovae, which will eject gas from the nascent elliptical galaxy.

Moreover, if black holes lie at the centers of these galaxies, matter could be channeled in their directions. As matter falls into the black hole, it will become heated, releasing an enormous amount of energy in the form of light. While these black holes are devouring matter, they are referred to (somewhat blandly) as "active galactic nuclei". In reality, this doesn't do justice to a on object the size of the solar system that is millions of times the mass of the Sun, and that is devouring a hundred Suns every year. The light that emerges from the matter falling into the black hole may be able to push yet more gas out of colliding galaxies.

Therefore, galaxy collisions are thought to be an enormous factor in how a galaxy evolves. The difficult task for astrophysicists is understanding how often these collisions occur, and what the galaxy will look like when the collision is done. We have a rough idea, but the details require making complex computer observations, and comparing the results to galaxies that we are lucky enough to see at various stages of their collisions.