<p>The answer to the question depends on precisely where you are in the Milky Way. According to <a href="https://en.wikipedia.org/wiki/Density_wave_theory" rel="nofollow noreferrer">density wave theory</a>, which appears to describe the formation and evolution of spiral arms quite well, the arms move relative to some static inertial frame at a global pattern speed $\Omega_{gp}$. At some defining radius $R_c$, known as the corotation radius, stars move at $\Omega_{gp}$. Inside $R_c$, stars move quicker and thus move ahead of the arms; outside $R_c$, stars move slower and thus "fall behind" the arms.</p>

<p>There is uncertainty surrounding the values of $R_c$ and $\Omega_{gp}$ in the Milky Way in all its spiral arms - especially since local stellar kinematics differ in each of the arms, and so too may $R_c$ and $\Omega_{gp}$ - but measurements place the Solar System slightly inside the corotation radius, meaning that we are moving ahead of the arms.</p>

<p>I have data from a review by <a href="http://adsabs.harvard.edu/abs/2011MSAIS..18..185G" rel="nofollow noreferrer">Gerhard (2011)</a>. Here are some basic average parameters, for the Milky Way as a whole:</p>

<ul>
<li><em>Bar</em> corotation radius ($R_{CR}$): $3.5$-$4.5\text{ kpc}$</li>
<li>Spiral arm corotation radius: $8.1$-$8.4\text{ kpc}$</li>
<li>Bar pattern speed ($\Omega_b$): $50$-$60\text{ km}\text{ s}^{-1}\text{ kpc}^{-1}$</li>
<li>Spiral arm pattern speed ($\Omega_{sp}$): $17$-$28\text{ km}\text{ s}^{-1}\text{ kpc}^{-1}$</li>
</ul>

<p>This also goes along with a local mean speed of stars in the solar neighborhood - the Sun included - of $\sim220\text{ km}\text{ s}^{-1}$.</p>

<hr>

<p>Hence for the three questions: (1) The pattern is rotating clockwise as seen in that diagram; (2) It rotates about once every 300 million years; (3) and at our radius, we're moving a little faster than the pattern so we are overtaking it.</p>

CC BY-SA 3.0
2016-11-28T15:24:10.667Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/49236/discussion-on-answer-by-hde-226868-do-we-know-if-we-are-overtaking-or-falling-be).
called2voyage
CC BY-SA 3.0
2016-11-25T15:47:18.497Z
2016-11-28T13:13:01.087Z
2016-11-28T13:13:01.087Z
HDE 226868
<p>Here's our galaxy as best we know, courtesy of this <a href="https://en.wikipedia.org/wiki/Milky_Way" rel="nofollow noreferrer">Wikipedia article</a>:</p>

<p><a href="https://i.sstatic.net/J5AKf.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/J5AKf.png" alt="enter image description here"></a></p>

<p>It seems that as best we know, each arm <strong>is a compressed or brighter area (rather like a wave)</strong>.</p>

<ol>
<li><p>Looking at that image, do we know/guess if the arm-patterns are rotating? Or do they just sit there statically, unchanging from that diagram? <strong>Which way</strong> does it rotate, cw or acw per that diagram? (If we know.)</p></li>
<li><p>If the pattern is rotating - how long does it take it to rotate 360 degrees? As far as we know?</p></li>
</ol>

<p>And hence ..</p>

<ol start="3">
<li><strong>Indeed, is the arm-pattern catching us and moving through us ... or is the arm pattern slower than us and we're moving through it?</strong></li>
</ol>

<galaxy><milky-way><spiral-arms>
Do we know if we are overtaking or falling behind our arm?
Fattie
# Do we know if we are overtaking or falling behind our arm?

(Asked by: username_0 on 2016-11-25)

Here's our galaxy as best we know, courtesy of this [Wikipedia article](https://en.wikipedia.org/wiki/Milky_Way):

[![enter image description here](https://i.sstatic.net/J5AKf.png)](https://i.sstatic.net/J5AKf.png)

It seems that as best we know, each arm **is a compressed or brighter area (rather like a wave)**.

1. Looking at that image, do we know/guess if the arm-patterns are rotating? Or do they just sit there statically, unchanging from that diagram? **Which way** does it rotate, cw or acw per that diagram? (If we know.)
2. If the pattern is rotating - how long does it take it to rotate 360 degrees? As far as we know?

And hence ..

3. **Indeed, is the arm-pattern catching us and moving through us ... or is the arm pattern slower than us and we're moving through it?**

**Tags:** galaxy, milky-way, spiral-arms


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## 1 answers

### Answer by username_1 on 2016-11-25. Score: 10 | Accepted answer

The answer to the question depends on precisely where you are in the Milky Way. According to [density wave theory](https://en.wikipedia.org/wiki/Density_wave_theory), which appears to describe the formation and evolution of spiral arms quite well, the arms move relative to some static inertial frame at a global pattern speed $\Omega\_{gp}$. At some defining radius $R\_c$, known as the corotation radius, stars move at $\Omega\_{gp}$. Inside $R\_c$, stars move quicker and thus move ahead of the arms; outside $R\_c$, stars move slower and thus "fall behind" the arms.

There is uncertainty surrounding the values of $R\_c$ and $\Omega\_{gp}$ in the Milky Way in all its spiral arms - especially since local stellar kinematics differ in each of the arms, and so too may $R\_c$ and $\Omega\_{gp}$ - but measurements place the Solar System slightly inside the corotation radius, meaning that we are moving ahead of the arms.

I have data from a review by [Gerhard (2011)](http://adsabs.harvard.edu/abs/2011MSAIS..18..185G). Here are some basic average parameters, for the Milky Way as a whole:

* *Bar* corotation radius ($R\_{CR}$): $3.5$-$4.5\text{ kpc}$
* Spiral arm corotation radius: $8.1$-$8.4\text{ kpc}$
* Bar pattern speed ($\Omega\_b$): $50$-$60\text{ km}\text{ s}^{-1}\text{ kpc}^{-1}$
* Spiral arm pattern speed ($\Omega\_{sp}$): $17$-$28\text{ km}\text{ s}^{-1}\text{ kpc}^{-1}$

This also goes along with a local mean speed of stars in the solar neighborhood - the Sun included - of $\sim220\text{ km}\text{ s}^{-1}$.

---

Hence for the three questions: (1) The pattern is rotating clockwise as seen in that diagram; (2) It rotates about once every 300 million years; (3) and at our radius, we're moving a little faster than the pattern so we are overtaking it.

> **Comments:**
> **username_2**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/49236/discussion-on-answer-by-hde-username_1-do-we-know-if-we-are-overtaking-or-falling-be).  

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