The universe is a vast expanse of mostly pretty empty space, but our Milky Way galaxy does have some neighbors. In addition to several small “moon” galaxies that orbit the Milky Way, there are a few large ones relatively close by. The biggest is the Andromeda Galaxy, located about 2.5 million light-years away — at least for now.
Andromeda and the Milky Way are on a collision course, destined to fuse into one giant supergalaxy in about 4 billion years. It's going to go something like this...
It won't be a very collision-y sort of collision.
What you can't tell from the video simulation is that even though the Milky Way and Andromeda are home to hundreds of billions of stars, very few actual impacts will take place when the galaxies merge. Remember the first sentence of this blog? "The universe is a vast expanse of mostly pretty empty space" — and so are galaxies!
The closest star to our Sun is Proxima Centauri, which is about 4.2 light-years away. That may sound like a small number, but let's put it in perspective. Earth is 93 million miles from the Sun and it takes sunlight about eight minutes to reach us (so we could say the Sun is about eight light-minutes away from Earth). It takes light 4.2 years to reach us from Proxima Centauri, which is about 25,000,000,000,000 miles away. And that's our nearest neighboring star! (Still having trouble picturing it? If we scaled the universe down to where the Sun was the size of a Skittle, Proxima Centauri would be about 180 miles away!)
So the stars are super far apart. What's in between them? Not a whole lot! That's why the galaxy merger won't be like a galactic scale car crash. Each galaxy's stars will pass through mostly unscathed. But while the stars won't smash together, they will affect each other gravitationally.
Each galaxy will send a spray of stars out into the cosmos.
Both the Milky Way and Andromeda will go through some major changes during the two billion or so years it will take for them to fuse into one galaxy. You may have also noticed a tiny third galaxy (called Triangulum) in the video above — that little guy could make things even more complicated since it might join in the galactic merger. While there will be few (if any) actual collisions throughout all this, the stars in each galaxy will be flung into new orbits and some will be lost from the galaxies entirely.
But don't worry — our solar system should be safe!
What does all this mean for our solar system? According to NASA, "Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today." Earth isn't going to be destroyed, but if humans are still around then, their nighttime sky view will go through a lot of changes!
This image walks you through the metamorphosis the galaxies will go through. First, Andromeda will grow closer, eventually becoming brighter in the night sky than the band of our own Milky Way. Then things really get interesting!
New stars will blaze into life as the galaxies interact, followed by some major galactic stretching and warping. The bottom left image shows a point where the two galaxies' cores are visible and the bottom right image shows the cores merged together in one. A huge elliptical galaxy will emerge into the cosmos.