One of the tasks I get to do only rarely these days is to show a student a picture of the Andromeda Galaxy. Andromeda is the closest galaxy to us as seen from the northern hemisphere.
Students have a tendency to skip over Andromeda and ask to see Saturn again, until it is pointed out that this small, unimpressive gray smudge on the sky which we also call Messier 31 shines with the glow of 100 billion stars, and that the light we see traveled 2.2 million light years to reach us.
Andromeda is the closest approximation to our own Milky Way that we will every have, and we will see this sister galaxy to the Milky Way only for a limited time.
Andromeda is a large spiral galaxy with spiral arms that is roughly the same size as the Milky Way. The stars are forming in both galaxies at a good pace - not too fast and not too slow. And like the Milky Way, we think the center of Andromeda also is primarily composed of a material we cannot see called dark matter.
Both Andromeda and the Milky Way are heavyweights situated in the neighborhood of a mid-sized town of other galaxies called the Local Group. They are massive and pulled to each other by the gravitational force. In fact they are on a direct collision course.
That’s right - in another 5 billion years there will be a large crash. To trace what happens in this galaxy-galaxy collision, one will want to follow the most massive components. In both galaxies the dark matter dominates the mass and forms their backbones, so to speak.
Interestingly, the dark matter may be relatively unaffected by a head-on collision, or at least the dark matter is not expected to produce gigantic explosions. This is because dark matter appears to interact only weakly with itself and with the ordinary matter that you and I are made of. It is as if two football players set on a direct hit instead pass straight through each other.
The most dramatic explosions will take place with the gas. Here enormous amounts of hydrogen gas will be shock-heated and probably induced to form stars (think of super giant blimps crashing into each other at ultra-high speeds)! The end state will be a single larger galaxy.
The collision will be mind-bogglingly big, that’s true, and fortunately for us Earth will be just a tiny speck of dust in this big picture that we may come out of the 'merger' entirely unaffected. We will in fact have more to fear from the fact that in a bit less than 5 billion years our Sun will run out of fuel and start to shut down!