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I was reading this article posted on /r/space about photographing black holes, and they showed a picture of what's believed to be two black holes formed from two galaxies colliding.

I didn't realize the collision of galaxies could result in black holes, but it got me wondering. If you lived in one of those galaxies, what would happen to you? Would your planet be destroyed in the blink of an eye at some point? If so, when?

Would the sky gradually fill with more and more stars? If your galaxy became a black hole, how would you experience that? All at once or very gradually?

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In the vast, vast majority of cases, not much happens. Galaxies are a whole lot of empty space.

If the Sun was a soccer ball in a field in California, the nearest star of similar size, Alpha Centauri, would be another soccer ball all the way across the continent, in Greenland. This is a scaled view, where everything is proportional. Think of all that empty space in between.

So the galaxies would merge, or keep swirling around each other, but for the most part stars and their systems would just continue as they were. It is extremely unlikely that any given star would collide. That's true for most of the volume of the galaxies.

In the center, however, things are a little different. Most galaxies already have very massive black holes at the center. These may collide and merge, or orbit each other a very long time, or continue separately.

The formation of new black holes as a result of galactic collisions must be a very rare event. Even then, the entire galaxy would not "become a black hole"; rather, a small fraction of it would be added to an existing black hole or, in very rare cases, produce a new black hole.

For most observers in such galaxies, there would not be dramatic changes, even over very long time intervals. This is because, again, galaxies are mostly empty space, and because the stars you see with the naked eye are very close to you, within a few dozen to a few hundred light years, which is a small portion of the whole galaxy.


EDIT: As shown in the comments, it's more accurate to think of Alpha Centauri as not a single soccer ball, but instead a larger soccer ball and a volleyball separated by a few hundred meters - it's a double star. Then Proxima Centauri would be a pingpong ball hovering 100 km away from Alpha.

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    $\begingroup$ "Very large black holes" is a bit misleading. While they have large masses they are very small in size. The SMB at the center of the Milky Way is about 4 million solar masses but "only" about 17 times the Sun's radius, which in practical terms is tiny - there are normal stars that are larger. $\endgroup$ – StephenG Jan 9 '18 at 19:54
  • $\begingroup$ Alpha Centauri would be two soccer balls in Greenland, wouldn't it? :) $\endgroup$ – user18017 Jan 10 '18 at 0:48
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    $\begingroup$ @Setrofew Alpha Centauri would be three (well maybe two soccer balls and a golf ball :-) ). $\endgroup$ – StephenG Jan 10 '18 at 7:12
  • $\begingroup$ @StephenG Indeed, I left Proxima Centauri out of the image. I guess one could argue there should be some marbles and sand grains in the mixture as well, but what the naked eye see of Alpha Centauri, when resolved closely enough (WAY beyond what the eye can see, I know), would be two soccer balls in the analogy. Proxima, at apparent magnitude 11, had to wait 1915 to be discovered. $\endgroup$ – user18017 Jan 10 '18 at 15:44
  • $\begingroup$ Good point. Alpha would be a large soccer ball and a volley ball separated by hundreds of meters. Proxima would be a ping-pong ball 100 km away. $\endgroup$ – Florin Andrei Jan 10 '18 at 19:23

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