I've always been fascinated with the spectacle of galaxies running into each other. Think about it—these are giant stellar cities, with billions or trillions of stars in them. They are relatively far apart in space, but their motions can bring them in close proximity to each other. And, when they do, it sets off cosmic fireworks on a level that is truly astonishing.
This is an image from the Hubble Space Telescope of an object called Markarian 266. It looks like a really blobby single galaxy, but it turns out to be two galaxies. They're smashing into each other, creating a really chaotic scene. That's because both galaxies are exerting a gravitational pull on each other's stars and clouds of gas and dust. The force rips giant, long streamers of gas and flings them out to space. Shock waves move through the strings and set off waves of star birth in a cosmic fireworks display we can see across 350 million light-years of space. Such collisions usually result in whole new populations of stars flaring in the wake of the merger.
People often ask if stars in the colliding galaxies hit each other head-on. The answer is "mostly not". There's a lot of space between stars in most galaxies, so the chances of two stars colliding are pretty small. Even more scary to some folks is the idea that any planets around a star could be affected by a collision. That's also not very likely, although it could happen if conditions were just right.
The biggest stellar activity stemming from a merger is that starburst action I just described. It results in swarms of massive stars, plus collections of stars like the Sun. The most massive stars live shorter lives compared to the sun-like ones. Around ten million years after they're born in the starburst knot, those monster stars begin to destroy themselves in supernova explosions, lighting up the galaxy once again with energetic bursts. Shock waves from those events can ignite more starbirth if there's enough material left for the starbirth créches to operate. The sun-like and smaller stars live much longer and don't generally die in explosions.
Now, what can and does happen is that if both galaxies have supermassive black holes at their hearts, those monsters can collide to create a really massive one. Such cosmic behemoths settle into the core of the resulting elliptical galaxy that forms after the collision is done and things settle down. What astronomers are observing with this galaxy's supermassive black hole are x-ray emissions streaming from the region around the black hole.
Astronomers see mergers throughout the universe and know that the Milky Way will do it again someday. In about four or five billion years, it will collide with another spiral called the Andromeda Galaxy. When it does, the two will dance around each other for millions of years, eventually merging to become a giant elliptical. Astronomers even have some suggestions for names for the new future galaxy: Milkdromeda is one and the other is Milkomeda.
Chances are good that this future collision will look a lot like Markarian 266 does to us now. By the time it happens, the Sun will be evolved to become a white dwarf star. Earth will be gone, and who knows where humanity will be!
