What would be the possible and definite effects of two galaxies colliding with each other? (Milky Way & Andromeda)

Long-term and short-term effects are both important.

I'd like to know because it's planned to happen in the future of the story.

Two galaxies shown near each other

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    $\begingroup$ Stars colliding may be a better scope for a single story. $\endgroup$ – James Nov 17 '15 at 19:25
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    $\begingroup$ Specify your perspective. From a human size & time perspective - nothing happens. There's no change measurable to the unaided human eye over a human lifespan. $\endgroup$ – Jim2B Nov 17 '15 at 19:25
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    $\begingroup$ I strongly disagree with the close votes, and I'll point out that other "clear questions" like this have been accepted on Worldbuilding. $\endgroup$ – HDE 226868 Nov 17 '15 at 22:01
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    $\begingroup$ @JDługosz We haven't had time to establish any migration paths yet. (They don't happen automatically; we need to set them up manually, and in order to do that, we need to know which ones to set up.) Maybe you should post a Worldbuilding Meta question asking which ones we want? $\endgroup$ – a CVn Nov 17 '15 at 22:15
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    $\begingroup$ Sir Daws, part of the reason this question was put on hold is that it is too broad. As @Jim2B said, you'll need to specify both what timespan you want and what objects you want affected. $\endgroup$ – HDE 226868 Nov 17 '15 at 23:15

Honestly, it's only exciting if you speed up time to billions of years per second.

enter image description here

It's unlikely that any of the systems inside would actually collide, some systems may get flung out into intergalactic space, but it all takes a very very long time.

Short term, entire civilizations could rise and fall without ever realizing it's happening.

Long term, many systems are lost to the void, but the new galaxy wouldn't appear all that different from other galaxies. Even the systems flung out would not be very different locally, they'd just have dimmer stars in the sky at night.

  • $\begingroup$ Without ever realising it's happening? $\endgroup$ – Sir Daws Nov 17 '15 at 18:17
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    $\begingroup$ @SirDaws I mean the people won't be looking into the sky and screaming like "holy cow! that galaxy is going to collide with us!". The sky will look basically the same as 100 years before. They'd have to make very careful measurements over a long period of time to put together what's happening. $\endgroup$ – Samuel Nov 17 '15 at 18:22
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    $\begingroup$ That graphic is wonderful. $\endgroup$ – Dan Smolinske Nov 17 '15 at 18:27
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    $\begingroup$ Could a planet lose its parent star or at least change its orbital parameters significantly due to tidal effects during the collision? $\endgroup$ – John Dvorak Nov 17 '15 at 20:13
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    $\begingroup$ @JanDvorak Possible, but very highly unlikely. $\endgroup$ – Samuel Nov 17 '15 at 20:14

For anyone who wants to see a cool simulation of a galaxy, merger, I can recommend a neat little applet): Galaxy Crash JavaLab.1 Two more good sites with a bunch of different simulations are the GALMER website and this one (which turns out to have Samuel's simulation).

Cool, huh?

Anyway, let's look at some scientific work on galaxy collisions. Cox & Loeb (2008) simulated the future collision of the Milky Way and Andromeda. Here are some of their results:

  • There is a small chance that the Solar System will be flung out by tidal forces.
  • There is an equally small chance that the Solar System will be captured by Andromeda in the early stages of the collision, before the formation of the result of the merger.
  • Gas in the galaxies could be heated by an additional order of magnitude, from ~3$\times$105 Kelvin to ~3$\times$106 Kelvin. This alone could impact star-forming rates. Star formation is expected to increase anyway, due to other factors, although given that neither galaxy has a lot of gas, this will be minor compared with other collisions.
  • The result will most likely be an elliptical galaxy, leading to a different mass distribution and a slight change in the Solar System's orbit, even if most of the exotic tidal scenarios don't happen.

Van der Marel et al. (2012) brings up another good point: These two galaxies aren't the only players. The Triangulum Galaxy, M33, has interacted with Andromeda in the past and will do so in the future. This may be before, during, or after the Milky Way-Andromeda merger (which itself will take quite a long time). Both the Milky Way and Andromeda also have satellite galaxies, which could play important roles in determining the fates of a select few stars.

The addition of a third galaxy (which isn't as massive as the other two) has additional effects:

  • There is a 20% chance that the Solar System will go through M33 within 10 billion years, leading to a slightly higher chance of collision with other stars.
  • The Solar System will likely be pulled further away from the center of the result of the merger than it is now (although this may happen without the help of M33).

Finally, John Dubinski has written up an excellent overview (which is also quite non-technical) of the collision. He notes that a binary supermassive black hole will form during the merger, which could - depending on the precise path of the cores, eject the Solar System entirely.

Actually, I did find a cool simulation, and some images from it, each separated by approximately 170 million years:

1 Designed, programmed and developed by Chris Mihos, Greg Bothun, Dave Caley, Bob Vawter, and Cameron McBride.


As stated in Samuel's answer: over a single human lifespan, there will be no change to the star positions measurable by human perceptions. Our instruments and data recording devices will be able to measure the change but that will be of primary interest to scientists and not generally known to the population as a whole.

It will be boring to the common person.

No star collisions are expected from the Milky Way with Andromeda merger. Stars flying through each other's solar systems would be possible but exceedingly unlikely.

You are far more likely to have exciting sky stuff happening in a closed globular cluster.

If you still want to use the merger as a "historical setting" and in your story humans have not made it to the stars. You might make a series of solar system close approaches caused by the merger to make the human diaspora possible.

With computer simulation, the colonists could predict with some reasonable reliability the trajectory of the stars passing by (will get flung into intergalactic space, which will get flung into the core, and which will remain in the galaxy's habitable area - too close to the center and planets get cooked by supernovae - too far from the center and there's not enough heavier elements to sustain life).

Sun's trajectory through the Milky Way

One thing that will change is the collision of the two galaxies' gas and dust clouds. The collision will trigger massive and violent star forming areas. Some of these will form exceedingly large (e.g. $100 M_{\odot}$ - 100x the mass of the Sun). These stars burn hot, die quickly, and with a big bang (a Supernova).

Artist's impression of a supernova remnant

A close approach of one of these to the Earth might spell the doom for every living thing on Earth.

Currently in our galaxy we expect about 1 supernova per century. During the merger, Earth might see many more supernova (from a distance - because a near one would leave no witnesses) at a time.


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