<p>The usual explanation of spiral arms involves <a href="https://en.wikipedia.org/wiki/Density_wave_theory" rel="nofollow">density waves</a>.
Nonuniform motion leads to matter alternately bunching up (boosting star formation) and spreading out.
Like cars on a congested road, stars move through regions of greater or lesser density over time.
The rotational smearing you anticipate is known as the "winding problem."
This <a href="https://www.scientificamerican.com/article/what-process-creates-and/" rel="nofollow">Scientific American article</a>
offers three explanations by professional astronomers.</p>

CC BY-SA 3.0
2016-10-07T18:42:01.547Z
I found that article very very interesting. Thanks for the post. I did not know that I had incidentally stumbled on one of the most interesting ongoing topics in astronomy.
Hal Clark
CC BY-SA 3.0
2016-10-07T23:47:20.363Z
It's really just gas that bunches up into arms, isn't it? What we see are the young large stars forming there. Most of them go supernova and disappear before they leave the high density regions in the arms. Small stars, which are invisible, are evenly distributed in the galactic disk, not concentrated to arms. Right?
LocalFluff
CC BY-SA 3.0
2016-10-13T20:15:32.767Z
@LocalFluff -- No, there are density waves in the general population of older stars as well, though they're not as strong as the gas spiral arms.
Peter Erwin
CC BY-SA 3.0
2016-10-07T02:41:03.433Z
2016-10-07T20:19:44.130Z
2016-10-07T20:19:44.130Z
Mike G
<p>A couple of years ago I started playing a game called Kerbal Space Program (KSP) and my understanding about how orbits actually work increased dramatically. Because of KSP I was looking at a picture of a galaxy with its spiral arms and something struck me as very odd. The stars closer in should orbit the galaxy center faster than the stars farther out and that would smear out these lanes of stars until they were no longer recognizable. Something weird is going on here!</p>

<p><strong>So why do galaxies have arms? What's bunching these stars together like that?</strong></p>

<p>I noticed that the center of galaxies are all smeared out and bulbous like I think that the rest should look like. So what if all that mass rotating around a common center is warping space around it, like the water draining down your sink, and this warping is what catching matter/stars into the lanes.</p>

CC BY-SA 3.0
2016-10-06T20:10:53.080Z
Welcome to the astronomy stack exchange and great first question! This was actually an unsolved problem in astronomy for a very long time, so you're not alone in wondering this.
zephyr
CC BY-SA 3.0
2016-10-07T01:56:47.567Z
I think I recall reading something somewhere a long time ago that the answer lies with the interaction with dark matter and/or dark energy. I could be wrong. Like you, I am very green when it comes to astronomy and astrophysics. I'm just a casual layman with a natural curiosity. I've not even played KSP (although I've seen a few KSP vids on YouTube in the past). I'm sure someone way smarter than me on here can expound on this or debunk me. They might even throw in one of those cool looking, ridiculously complex equations you or I won't have a clue what it means.
iMerchant
CC BY-SA 3.0
2016-10-07T05:17:56.867Z
I've cut out some of the "chat". There is no need to apologise for not being an "expert".
James K
CC BY-SA 3.0
2016-10-07T05:20:12.123Z
Possible duplicate of [Why do pictures of the Milky Way look like a spiral?](http://astronomy.stackexchange.com/questions/2521/why-do-pictures-of-the-milky-way-look-like-a-spiral)
James K
CC BY-SA 3.0
2016-10-07T11:57:20.647Z
Phil Plait over at slate.com/badastronomy wrote about this recently; danged if I can find the exact entry :-(
Carl Witthoft
CC BY-SA 3.0
2016-10-07T13:16:43.597Z
Just to point out something: Stars orbiting in galaxies do not following keplerian orbits, i.e., closer in stars don't orbit faster than farther out stars as you would expect for planets orbiting a Sun. The reason being that a galaxy is an extended object and the stars are embedded in this object and orbiting through it, rather than orbiting around a single large point mass at the center. So already, your expectations for how stars might orbit are dashed.
zephyr
CC BY-SA 3.0
2016-10-07T15:15:59.673Z
Is this what @CarlWitthoft was looking for? [Our Galaxy Just Got a Little More Interesting](http://www.slate.com/blogs/bad_astronomy/2013/06/06/milky_way_galaxy_two_new_discoveries_about_its_spiral_arms.html)
Mike G
CC BY-SA 3.0
2016-10-07T19:16:07.680Z
@zephyr thanks for the point.Doing some research it looks to me that for a keplerian orbit to work as I was assuming all stars in the galaxy would have to be in the sphere of influence of the super massive back hole which they are not. So yes this assumption was the biggest problem. Im going to have to get over that line of thinking.
Hal Clark
CC BY-SA 3.0
2016-10-07T19:34:39.500Z
@iMerchant if you do ply KSP get into the math side of it. I have planned missions on paper, calculated all the delta V I need, built the rocked to match the needs, and lastly flew the mission successfully. It is by far the most rewarding thing I have ever done with a computer game.
Hal Clark
CC BY-SA 3.0
2018-04-26T06:07:48.630Z
Similar question with a nice answer here:   https://astronomy.stackexchange.com/questions/6232/questions-about-spiral-galaxy-arms
userLTK
<orbit><galaxy>
Why do galaxies have arms?
Hal Clark
# Why do galaxies have arms?

(Asked by: username_0 on 2016-10-06)

A couple of years ago I started playing a game called Kerbal Space Program (KSP) and my understanding about how orbits actually work increased dramatically. Because of KSP I was looking at a picture of a galaxy with its spiral arms and something struck me as very odd. The stars closer in should orbit the galaxy center faster than the stars farther out and that would smear out these lanes of stars until they were no longer recognizable. Something weird is going on here!

**So why do galaxies have arms? What's bunching these stars together like that?**

I noticed that the center of galaxies are all smeared out and bulbous like I think that the rest should look like. So what if all that mass rotating around a common center is warping space around it, like the water draining down your sink, and this warping is what catching matter/stars into the lanes.

**Tags:** orbit, galaxy
> **Comments:**
> **username_1**: Welcome to the astronomy stack exchange and great first question! This was actually an unsolved problem in astronomy for a very long time, so you're not alone in wondering this.  
> **username_2**: I think I recall reading something somewhere a long time ago that the answer lies with the interaction with dark matter and/or dark energy. I could be wrong. Like you, I am very green when it comes to astronomy and astrophysics. I'm just a casual layman with a natural curiosity. I've not even played KSP (although I've seen a few KSP vids on YouTube in the past). I'm sure someone way smarter than me on here can expound on this or debunk me. They might even throw in one of those cool looking, ridiculously complex equations you or I won't have a clue what it means.  
> **username_3**: I've cut out some of the "chat". There is no need to apologise for not being an "expert".  
> **username_3**: Possible duplicate of [Why do pictures of the Milky Way look like a spiral?](http://astronomy.stackexchange.com/questions/2521/why-do-pictures-of-the-milky-way-look-like-a-spiral)  
> **username_4**: Phil Plait over at slate.com/badastronomy wrote about this recently; danged if I can find the exact entry :-(  
> **username_1**: Just to point out something: Stars orbiting in galaxies do not following keplerian orbits, i.e., closer in stars don't orbit faster than farther out stars as you would expect for planets orbiting a Sun. The reason being that a galaxy is an extended object and the stars are embedded in this object and orbiting through it, rather than orbiting around a single large point mass at the center. So already, your expectations for how stars might orbit are dashed.  
> **username_5**: Is this what @CarlWitthoft was looking for? [Our Galaxy Just Got a Little More Interesting](http://www.slate.com/blogs/bad_astronomy/2013/06/06/milky_way_galaxy_two_new_discoveries_about_its_spiral_arms.html)  
> **username_0**: @zephyr thanks for the point.Doing some research it looks to me that for a keplerian orbit to work as I was assuming all stars in the galaxy would have to be in the sphere of influence of the super massive back hole which they are not. So yes this assumption was the biggest problem. Im going to have to get over that line of thinking.  
> **username_0**: @iMerchant if you do ply KSP get into the math side of it. I have planned missions on paper, calculated all the delta V I need, built the rocked to match the needs, and lastly flew the mission successfully. It is by far the most rewarding thing I have ever done with a computer game.  
> **username_6**: Similar question with a nice answer here:   https://astronomy.stackexchange.com/questions/6232/questions-about-spiral-galaxy-arms  

---

## 1 answers

### Answer by username_5 on 2016-10-07. Score: 6 | Accepted answer

The usual explanation of spiral arms involves [density waves](https://en.wikipedia.org/wiki/Density_wave_theory).
Nonuniform motion leads to matter alternately bunching up (boosting star formation) and spreading out.
Like cars on a congested road, stars move through regions of greater or lesser density over time.
The rotational smearing you anticipate is known as the "winding problem."
This [Scientific American article](https://www.scientificamerican.com/article/what-process-creates-and/)
offers three explanations by professional astronomers.

> **Comments:**
> **username_0**: I found that article very very interesting. Thanks for the post. I did not know that I had incidentally stumbled on one of the most interesting ongoing topics in astronomy.  
> **username_7**: It's really just gas that bunches up into arms, isn't it? What we see are the young large stars forming there. Most of them go supernova and disappear before they leave the high density regions in the arms. Small stars, which are invisible, are evenly distributed in the galactic disk, not concentrated to arms. Right?  
> **username_8**: @LocalFluff -- No, there are density waves in the general population of older stars as well, though they're not as strong as the gas spiral arms.  

---

