<blockquote>
  <p>What is the gravitational force between us and Andromeda?</p>
</blockquote>

<p>The masses of the Milky Way and Andromeda (depending on exactly whay you consider "part of" each) are <a href="http://adsabs.harvard.edu/abs/2014MNRAS.443.2204P" rel="nofollow noreferrer">estimated</a> as about 850 Billion solar masses and 1.7 Trillion solar masses respectively. Their mean distance is about 2.54 million light years, so it's easy to calculate the force using <span class="math-container">$$F = GMM'/R^2$$</span> We get a force of <span class="math-container">$$6.6\times 10^{29} N$$</span>.</p>

CC BY-SA 4.0
2018-09-27T11:42:51.033Z
And another way to frame that number is to ask what is the timescale for that force to significantly alter the location of Andromeda, as that timescale can be compared with the age of the universe.  The timescale is the square root of the quantity of distance over acceleration, which comes out to ten billion years.  Only if that were much larger than the age of the universe could we neglect the impact of that attraction on the overall dynamics.
Ken G
CC BY-SA 4.0
2018-09-27T09:56:22.620Z
2018-09-27T09:56:22.620Z
Steve Linton
<p>If the universe is expanding how come Andromeda is heading for us? 
In isolated spatial areas does gravity overcome universal expansion? 
What is the gravitational force between us and Andromeda?</p>

<gravity><expansion>
Expansion ofthe Universe
DaveW96
# Expansion ofthe Universe

(Asked by: username_0 on 2018-09-27)

If the universe is expanding how come Andromeda is heading for us?
In isolated spatial areas does gravity overcome universal expansion?
What is the gravitational force between us and Andromeda?

**Tags:** gravity, expansion


---

## 1 answers

### Answer by username_1 on 2018-09-27. Score: 0

> What is the gravitational force between us and Andromeda?

The masses of the Milky Way and Andromeda (depending on exactly whay you consider "part of" each) are [estimated](http://adsabs.harvard.edu/abs/2014MNRAS.443.2204P) as about 850 Billion solar masses and 1.7 Trillion solar masses respectively. Their mean distance is about 2.54 million light years, so it's easy to calculate the force using $$F = GMM'/R^2$$ We get a force of $$6.6\times 10^{29} N$$.

> **Comments:**
> **username_2**: And another way to frame that number is to ask what is the timescale for that force to significantly alter the location of Andromeda, as that timescale can be compared with the age of the universe.  The timescale is the square root of the quantity of distance over acceleration, which comes out to ten billion years.  Only if that were much larger than the age of the universe could we neglect the impact of that attraction on the overall dynamics.  

---

