<p>A full merger may not occur on that timescale. The encounter in 4.5 billion years will take place but the galactic centers may be far enough apart that it is a &quot;glancing blow&quot;. Regardless, the orbit will be diminished and the two <em>will</em> come together some billions of years later (e.g. <a href="https://arxiv.org/abs/2102.10938" rel="nofollow noreferrer">Schiavi et al. 2021</a>).</p>
<p>The basic physics here is that the Milk-dromeda system(!) has a certain amount of energy (the sum of its kinetic energy and gravitational potential energy), and the total is <em>negative</em>. This indicates that the system is bound.</p>
<p>As time goes on energy is lost from the system through radiation and also perhaps through the ejection of <em>some</em> gas and stars. This makes the system <em>more</em> bound. These processes are actually enhanced as the galaxies approach each other and their gas starts to interact (the stars are effectively point-like particles and won't collide).</p>
<p>Thus the net effect will always be to bind the galaxies closer together and they won't escape from each other.</p>
<p>That said, the fact that each of the galaxies contains billions of component stars, does mean the energy can be shared out in many different ways. It is quite likely that <em>some</em> (a small fraction) of the stars and gas will gain sufficient kinetic energy during the collision to escape from the system as a whole.</p>
<p>Here is a <a href="https://youtu.be/zhuMuiYDNmk" rel="nofollow noreferrer">simulation</a> based on the Schiavi et al. calculations. You can see the initial glancing blow followed by the merger sometime later. You can also see that a fair number (difficult to be quantitative based on this) of stars do get thrown out of the system during the merger.</p>

CC BY-SA 4.0
2022-10-03T13:38:17.200Z
Pretty amazing demo on how big space is -- stars seem huge to us yet the most likely outcome of throwing 100 billion stars at another 100 billion stars is that there isn't even 1 collision.
eps
CC BY-SA 4.0
2022-10-03T18:08:00.620Z
@eps Yeah I always thought it was interesting just how much space there is between the earth and the moon. We think of it as close, but you could fit 30 more earths in between the earth and the moon (ish). Though I do wonder how many systems will get disrupted from a "near miss". Doesn't take a sun splatting a sun to create some interesting scenarios.
JamieB
CC BY-SA 4.0
2022-10-03T19:00:46.643Z
"the sum of its kinetic energy and gravitational potential energy" does it not also have other kinds of potential energy? e.g. chemical and nuclear? is the contribution of these to the result negligable?
Peter Green
CC BY-SA 4.0
2022-10-03T20:21:30.863Z
@PeterGreen that would make an interesting question. I think the answer is that it is negligible except where you get a large number of high mass stars formed in a short period of time.
ProfRob
CC BY-SA 4.0
2022-10-04T03:17:00.980Z
The troubling aspect of Milkdromeda (Androlky Way? :D) is that I have been reading that it will be an elliptical galaxy which means essentially very few additional stars will ever form... :'-(
Michael
CC BY-SA 4.0
2022-10-04T04:24:58.780Z
@Michael Yikes, that sounds so cursed .
Nilay Ghosh
CC BY-SA 4.0
2022-10-04T16:24:07.893Z
@NilayGhosh Well, you only have about 4.5 billion years or so to come up with a better name, otherwise we're just going to have to go with one of those. :D
HiddenWindshield
CC BY-SA 4.0
2022-10-04T19:43:49.647Z
Feel free to correct the Wikipedia page if something else happens 4.5 billion years from now.
Darrel Hoffman
CC BY-SA 4.0
2022-10-04T20:48:19.750Z
@DarrelHoffman I don't suppose I'll be around in 4.5 billion years time.
ProfRob
CC BY-SA 4.0
2022-10-03T07:20:35.213Z
2022-10-05T09:35:13.147Z
2022-10-05T09:35:13.147Z
ProfRob
<p>The Milky Way and Andromeda <a href="https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision" rel="noreferrer">are destined to collide and merge within the next 4-5 billion years</a>, but I feel like there could be a chance that instead of merging, they could just eject each other. I dunno, but is it possible, and if so, how likely will it be?</p>

CC BY-SA 4.0
2022-10-03T06:51:13.673Z
Eject each other from where?
user438383
CC BY-SA 4.0
2022-10-03T15:14:31.480Z
What do you even mean be eject?
DKNguyen
<galaxy><milky-way><collision><future>
Is it possible for the Milky Way and Andromeda to get ejected upon colliding instead of merging?
Prince Pugs
# Is it possible for the Milky Way and Andromeda to get ejected upon colliding instead of merging?

(Asked by: username_0 on 2022-10-03)

The Milky Way and Andromeda [are destined to collide and merge within the next 4-5 billion years](https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision), but I feel like there could be a chance that instead of merging, they could just eject each other. I dunno, but is it possible, and if so, how likely will it be?

**Tags:** galaxy, milky-way, collision, future
> **Comments:**
> **username_1**: Eject each other from where?  
> **username_2**: What do you even mean be eject?  

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## 1 answers

### Answer by username_3 on 2022-10-03. Score: 29 | Accepted answer

A full merger may not occur on that timescale. The encounter in 4.5 billion years will take place but the galactic centers may be far enough apart that it is a "glancing blow". Regardless, the orbit will be diminished and the two *will* come together some billions of years later (e.g. [Schiavi et al. 2021](https://arxiv.org/abs/2102.10938)).

The basic physics here is that the Milk-dromeda system(!) has a certain amount of energy (the sum of its kinetic energy and gravitational potential energy), and the total is *negative*. This indicates that the system is bound.

As time goes on energy is lost from the system through radiation and also perhaps through the ejection of *some* gas and stars. This makes the system *more* bound. These processes are actually enhanced as the galaxies approach each other and their gas starts to interact (the stars are effectively point-like particles and won't collide).

Thus the net effect will always be to bind the galaxies closer together and they won't escape from each other.

That said, the fact that each of the galaxies contains billions of component stars, does mean the energy can be shared out in many different ways. It is quite likely that *some* (a small fraction) of the stars and gas will gain sufficient kinetic energy during the collision to escape from the system as a whole.

Here is a [simulation](https://youtu.be/zhuMuiYDNmk) based on the Schiavi et al. calculations. You can see the initial glancing blow followed by the merger sometime later. You can also see that a fair number (difficult to be quantitative based on this) of stars do get thrown out of the system during the merger.

> **Comments:**
> **username_4**: Pretty amazing demo on how big space is -- stars seem huge to us yet the most likely outcome of throwing 100 billion stars at another 100 billion stars is that there isn't even 1 collision.  
> **username_5**: @eps Yeah I always thought it was interesting just how much space there is between the earth and the moon. We think of it as close, but you could fit 30 more earths in between the earth and the moon (ish). Though I do wonder how many systems will get disrupted from a "near miss". Doesn't take a sun splatting a sun to create some interesting scenarios.  
> **username_6**: "the sum of its kinetic energy and gravitational potential energy" does it not also have other kinds of potential energy? e.g. chemical and nuclear? is the contribution of these to the result negligable?  
> **username_3**: @PeterGreen that would make an interesting question. I think the answer is that it is negligible except where you get a large number of high mass stars formed in a short period of time.  
> **username_7**: The troubling aspect of Milkdromeda (Androlky Way? :D) is that I have been reading that it will be an elliptical galaxy which means essentially very few additional stars will ever form... :'-(  
> **username_8**: @Michael Yikes, that sounds so cursed .  
> **username_9**: @NilayGhosh Well, you only have about 4.5 billion years or so to come up with a better name, otherwise we're just going to have to go with one of those. :D  
> **username_10**: Feel free to correct the Wikipedia page if something else happens 4.5 billion years from now.  
> **username_3**: @DarrelHoffman I don't suppose I'll be around in 4.5 billion years time.  

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