What happens when galaxies collide?

This is an important question because Andromeda is going to crash into the Milky Way in approximately 4 billion years. Yeah, I know that none of us will be around, but I think it would be interesting to see just what would happen.

Fortunately, astronomers have actual galaxy collisions to rely on for ideas of what would happen. As it turns out, galaxy collisions are a part of galaxy formation and evolution. You can blame that on gravity, the force that attracts everything that has mass. And, galaxies have plenty of mass, not only in the stars, gas, black holes, and other exotic objects, they also have lots of dark matter.

NGC 2207 and IC 2163

There are lots of examples of actually occurring galaxy collisions out there in space. A good example of this is NGC 2207 and IC 2163. These two spiral galaxies are in the process of colliding, but it still at an early stage where they both still look like spirals. It's like looking at an image of two cars that have just barely touched in what will become a terrible accident. This particular collision will take another billion years to complete, which will end up merging the two and forming a large elliptical galaxy. This is the type of collision that Andromeda and the Milky Way will undergo.

NGC 4676

An example of two spiral galaxies in the second phase of collision is the Mice galaxies: NGC 4676. They started colliding and merging around 290 million years ago. The collision has produced a long tail in what is characterized as a galactic tide. Basically, stars get shoved out away from the collision point.

The Antennae galaxies

The Antennae galaxies, NGC 4036 and NGC 4039, are in the third phase of collision. In this case the two have merged and are no longer nice spirals. Two galactic tidal arms stretch out from what remains of both galaxies. This collision has been going on for 1.2 billion years. At this point in the collision process, the pressure of gas clouds has produced a starburst era where new stars are forming like crazy.
Another example of this third phase, perhaps much further into the process, is NGC 520.

NGC 520

Astronomers can use these examples to speculate what will happen when Andromeda and the Milky Way galaxies collide.

By the way, if we could see Andromeda if it were brighter in the sky, it would look very large, many times larger than the full moon.

One thing is fairly certain. The stars will not collide. That sounds crazy, but it's true. The Andromeda galaxy has around a trillion stars, and the Milky Way has approximately 400 billion. There is enough interstellar space between them that they will avoid collision. The main effect of the collision is to pull stars around into a new configuration, first shooting them out away from the collision point and then attracting them back again. This process could occur several times before the remnant stabilizes as a large elliptical galaxy.

Both the Milky Way and Andromeda contain supermassive black holes at their centers. These will merge eventually, but not until the very end of the collision process. Simulations show that when the two black holes come within a light year, they will merge. This is based on simulations.

What will happen to our solar system? There are two possibilities based on the simulation. One is that we will end up much further from the center of the new elliptical galaxy or be swept totally out of the galaxy. It won't matter much because our star will be a red giant by then and Earth will be swallowed up. However, if mankind moved out to say Titan, the large moon that orbits Saturn, it would be habitable then.
It would be a grand sky show to watch the collision. The Milky Way that we see now will be gone. In its place will be a luminous, glowing red swatch across the sky where ionized gas will be heated to millions of degrees from the collision. You won't need a flashlight at night. Stars would be forming rapidly, popping up as bright flashes constantly. The problem with this is that it will take a few billion years to complete. I wonder what version of the iPhone will be around then so that we can record it and post it on Facebook.

Thanks for reading.