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

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<hr>

<p><strong>EDIT</strong>: 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.</p>

CC BY-SA 3.0
2018-01-09T19:54:19.817Z
"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.
StephenG - Help Ukraine
CC BY-SA 3.0
2018-01-10T07:12:30.360Z
@Setrofew [Alpha Centauri](https://en.wikipedia.org/wiki/Alpha_Centauri) would be three (well maybe two soccer balls and a golf ball :-) ).
StephenG - Help Ukraine
CC BY-SA 3.0
2018-01-10T19:23:06.667Z
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.
Florin Andrei
CC BY-SA 3.0
2018-01-09T19:13:09.453Z
2018-01-10T20:27:15.113Z
2018-01-10T20:27:15.113Z
Florin Andrei
<p>I was reading <a href="https://www.vox.com/science-and-health/2018/1/8/16822272/black-hole-looks-like-what/" rel="nofollow noreferrer">this article</a> 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.</p>

<p>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?</p>

<p>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?</p>

<black-hole>
If one galaxy were to collide with another, what would it look like to an immortal inhabitant of one of their planets?
temporary_user_name
# If one galaxy were to collide with another, what would it look like to an immortal inhabitant of one of their planets?

(Asked by: username_0 on 2018-01-09)

I was reading [this article](https://www.vox.com/science-and-health/2018/1/8/16822272/black-hole-looks-like-what/) 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?

**Tags:** black-hole


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## 1 answers

### Answer by username_1 on 2018-01-09. Score: 5 | Accepted answer

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.

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**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.

> **Comments:**
> **username_2**: "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.  
> **username_2**: @Setrofew [Alpha Centauri](https://en.wikipedia.org/wiki/Alpha_Centauri) would be three (well maybe two soccer balls and a golf ball :-) ).  
> **username_1**: 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.  

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