<p>The Milky Way has arms that form due to <a href="https://astronomy.stackexchange.com/questions/18611/why-do-galaxies-have-arms/18620">density waves</a>. Like the majority of spiral galaxies, the arms are trailing. Individual stars orbit in circles (roughly), neither towards or away from the centre.</p>

<p>If you consider a common map of the Milky way (imagined from a point North of the Earth, Celestial North is not the same as Ecliptic North, which are both about 60 degrees off from <a href="https://www.physicsforums.com/attachments/there-planes-angles_no-earth-09oct2016-jpg.107279/" rel="noreferrer">Galactic North</a>)
<a href="https://i.sstatic.net/UkfQ2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/UkfQ2.jpg" alt="enter image description here"></a>
The stars in the galaxy would be moving in a clockwise fashion.</p>

<p>If you were to view from the other side, it would be as if you had made a mirror image of the galaxy, so the motion would be counterclockwise. If you look at the back of a clock, then from the wrong side, the hand would move counterclockwise. However the rotation of the galaxy is still with its arms trailing.</p>

<p>There are some galaxies that rotate with arms leading: <a href="http://hubblesite.org/news_release/news/2002-03" rel="noreferrer">NGC 4622</a> is one example.</p>

CC BY-SA 3.0
2017-07-02T23:11:57.857Z
Good answer but I think if your going to use North as a reference point, you should point out that Celestial North is not the same as Ecliptic North, which are both about 60 degrees off from Galactic North. Maybe a diagram like this would help https://www.physicsforums.com/attachments/there-planes-angles_no-earth-09oct2016-jpg.107279/
Dean
CC BY-SA 3.0
2017-07-03T12:47:11.433Z
Thank you so much! This answers all my puzzles. The question on clockwise/counterclockwise is that when I read those articles they seemed suggesting the galaxy could rotate in both directions. The galaxy rotates only in one direction as a system on it's own, constantly; only the observer got the opposing impression due to the reference point of observing, correct?
Mishu 米殊
CC BY-SA 3.0
2017-07-02T22:01:46.217Z
2017-07-03T05:43:22.810Z
2017-07-03T05:43:22.810Z
James K
<p>We don't have a photo of our Milky Way the whole, all the pictures are artists' interpretations showing a bulge, a central bar, and several spiral arms. The milky way is spinning. I would like to know: </p>

<p>a) Does it spin with the arms trailing or leading?</p>

<p>b) I read if the direction of <a href="http://www.sfaaa.com/index.php/read-our-answers/316-does-the-milky-way-rotate-and-or-revolve-clockwise-or-counterclockwise-by-james-goss" rel="noreferrer">spin clockwise or counterclockwise</a> depends on whether the Milky Way is observed from above or below. I don't understand how the direction of spin (<a href="http://curious.astro.cornell.edu/94-the-universe/the-milky-way/dynamics/482-does-the-milky-way-spin-counter-clockwise-if-so-do-all-spiral-galaxies-spin-in-this-direction-and-why-beginner" rel="noreferrer">clockwise &amp; counterclockwise</a>) depends on the point of view of the observer.</p>

CC BY-SA 3.0
2017-06-29T10:43:24.370Z
I dont really understand your question about spinning towards or away from the centre, there are forces going in both directions but generally stars follow circular orbits around the galaxies gravitational potential.
And as far as the direction of spin is concerned, its entirely a matter of perspective, depending on which way you view it, there is no right or wrong answer.
Dean
CC BY-SA 3.0
2017-06-29T10:49:53.583Z
thx. but if that's not "about spinning towards or away from the centre", why it's believed that eventually the black hole at the centre will consume all the stars in that galaxy? (i assumed it's towards the centre.) if there is no direction as towards or away from the centre, why there are spiral arms?
Mishu 米殊
CC BY-SA 3.0
2017-06-29T10:55:13.743Z
It's not believed that eventually a black hole will consume all the stars in the galaxy, this is just a myth. Black Holes have a very small [sphere of influence](https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole)) which means they wont be able to interact with 99% of the stars in the galaxy. And as for the spiral arms, I would encourage you to read up on [spiral density waves](https://en.wikipedia.org/wiki/Density_wave_theory#Galactic_spiral_arms) which are believed to be how spiral arms behave.
Dean
CC BY-SA 3.0
2017-06-29T11:03:21.553Z
@Dean if the characteristics of a black hole included that it exercised gravitational force in contrast to it's size but in proportion to it's mass, i.e., the more mass, the greater "pulling force", whilst, the smaller size, the denser the mass thus smaller but stronger "pull", if there is a black hole in the centre of a glalaxy that all the stars and materials will be consumed by the black hole is just a matter of time, although it doesn't interact with 99% of the stars... no? because the black hole is not constant it keeps growing in mass by consuming stars (more mass)...
Mishu 米殊
CC BY-SA 3.0
2017-06-29T11:43:15.193Z
Regarding the comment "why it's believed that eventually the black hole at the centre will consume all the stars in that galaxy" -- That is the bad pop sci version of a black hole. It is not generally believed to be the case.
David Hammen
CC BY-SA 3.0
2017-06-29T16:16:07.007Z
For (b) sub-question: the perceived direction of the rotation will indeed be different depending on your vantage point. For instance, have a look at this video: [Startrail & Timelapse 2015 - Northern vs Southern Hemisphere](https://youtu.be/UNiNJC3UHIo). The video is about earth's poles, but as you'd would probably notice, the basic idea is just the same for any object that's rotating.
Dhruv Saxena
CC BY-SA 3.0
2017-06-29T16:16:44.267Z
@Bhumishu米殊 as David says your approaching this question with a science fiction type interpretation of black holes, in reality they don't interact with that many stars, as I pointed out previously. Also it is not considered a given that black holes continuously keep amassing more mass indefinitely, there is evidence now that over long periods of time black holes loose mass via Hawking Radiation.
Dean
CC BY-SA 3.0
2017-07-02T22:02:49.347Z
I've done a rewrite. I think the question is, do the arms trail or lead in the rotation.
James K
CC BY-SA 3.0
2017-07-03T04:49:14.567Z
@JamesK Thx a millions! I'm not equipped with the technical terminologies that's hard to communicate accurately here.
Mishu 米殊
CC BY-SA 3.0
2017-07-03T04:52:56.210Z
@DhruvSaxena Thx. From your answer I now get to realize all those articles were talking about viewing the Milky Way from the earth, I would like to know if, in imagination the observer is the size of Kobe Bryant and the Milky Way the basket ball, how does it spin? Maybe I shall ask in a new question.
Mishu 米殊
CC BY-SA 3.0
2017-07-03T04:58:35.700Z
@Dean Ok. What I think is the Black Hole is a mass behaved in the negative way - negative mass, with a negative mass that means the totality of the mass of a system (a galaxy is a system) will get "lost" due to it's effect. Tiny or large has to be measured in proportion, like the earth is large but compared to the sun it's small, also by when considering the infinite nature of time, the proportion will not stand if an effect is taking place.
Mishu 米殊
CC BY-SA 3.0
2017-07-03T05:19:42.607Z
@Mishu米殊 Actually, it's not just about earth. In fact, even for basketball, the idea from the star trails video would apply as is. The direction of rotation is relevant to the _observer_ and not the object that itself is spinning. Imagine Kobe Bryant has held a basketball in his left hand and gives it a _clockwise_ flick to spin the ball atop his finger. From his point of view (north pole of the ball, if you will), it would appear that the ball is spinning clockwise. From the point of view of a kid, looking up at Kobe Bryant, the ball would _appear_ to spin counterclockwise. Does that help? :)
Dhruv Saxena
CC BY-SA 3.0
2017-07-03T12:54:52.573Z
@DhruvSaxena Ok, got it, thx. The thing bugged me is that the galaxy as a system if it rotates it should go in only one direction, although by observation it appeared in reference to the observer but the direction is only uniquely one on its own. I imagine in the universe there are many galaxies, one goes clockwise, the other goes counterclockwise but they will keep their tendency in constant unless some upheavals caused change of balance.
Mishu 米殊
CC BY-SA 3.0
2018-02-01T22:41:43.847Z
I understand that the direction of objects’ spin is relative to your frame of reference. However, one thing is not relative, and that is whether two objects are spinning in the same direction or not. For example in our solar system, the Sun, Earth, Luna, most planets (except Venus and Uranus), and the planets’ satellites spin all spin in approximately the same direction. Likewise, the orbits of these bodies is in the same direction. Does our galaxy rotate in approximately the same direction? I understand that the rotational plane of our solar system is tilted about 60 degrees relative to the g
John
CC BY-SA 3.0
2018-02-01T23:12:13.990Z
The majority of the galaxies rotate with trailing arms, but not all.
peterh
CC BY-SA 3.0
2018-02-02T00:04:16.403Z
`+1` The OP's question is reasonable, and responsiveness in comments is exemplary. I don't see why it was down voted.
uhoh
CC BY-SA 3.0
2018-02-03T14:14:44.213Z
@uhoh right, thx. this is bizarre, yeah. my 1st experience with this forum is... uhhhuu
Mishu 米殊
CC BY-SA 3.0
2018-02-03T14:20:53.283Z
@John i think there pull/push force exercised from the centre of the galaxy, the majority of stars in a system should spin in same direction due to the central force influence (a centralized force holding the items to form one system). planet like Venus or Uranus is weirdo, must get some accident to cause it behaved in a different way.
Mishu 米殊
CC BY-SA 3.0
2018-02-03T14:25:53.030Z
@peterh trailing arms meant the items getting towards the centre of galaxy, that is the galaxy will increase in mass but decrease in size; leading is opposite. not sure this has any explanation or just my hypothetical imagination
Mishu 米殊
CC BY-SA 3.0
2018-02-03T14:45:25.023Z
@Mishu米殊 No. It doesn't mean any similar.
peterh
<milky-way><galactic-dynamics><spiral-arms>
In which direction does the Milky Way rotate?
Mishu 米殊
# In which direction does the Milky Way rotate?

(Asked by: username_0 on 2017-06-29)

We don't have a photo of our Milky Way the whole, all the pictures are artists' interpretations showing a bulge, a central bar, and several spiral arms. The milky way is spinning. I would like to know:

a) Does it spin with the arms trailing or leading?

b) I read if the direction of [spin clockwise or counterclockwise](http://www.sfaaa.com/index.php/read-our-answers/316-does-the-milky-way-rotate-and-or-revolve-clockwise-or-counterclockwise-by-username_4-goss) depends on whether the Milky Way is observed from above or below. I don't understand how the direction of spin ([clockwise & counterclockwise](http://curious.astro.cornell.edu/94-the-universe/the-milky-way/dynamics/482-does-the-milky-way-spin-counter-clockwise-if-so-do-all-spiral-galaxies-spin-in-this-direction-and-why-beginner)) depends on the point of view of the observer.

**Tags:** milky-way, galactic-dynamics, spiral-arms

> **Comments:**
> **username_1**: I dont really understand your question about spinning towards or away from the centre, there are forces going in both directions but generally stars follow circular orbits around the galaxies gravitational potential. And as far as the direction of spin is concerned, its entirely a matter of perspective, depending on which way you view it, there is no right or wrong answer.  
> **username_0**: thx. but if that's not "about spinning towards or away from the centre", why it's believed that eventually the black hole at the centre will consume all the stars in that galaxy? (i assumed it's towards the centre.) if there is no direction as towards or away from the centre, why there are spiral arms?  
> **username_1**: It's not believed that eventually a black hole will consume all the stars in the galaxy, this is just a myth. Black Holes have a very small [sphere of influence](https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole)) which means they wont be able to interact with 99% of the stars in the galaxy. And as for the spiral arms, I would encourage you to read up on [spiral density waves](https://en.wikipedia.org/wiki/Density_wave_theory#Galactic_spiral_arms) which are believed to be how spiral arms behave.  
> **username_0**: @username_1 if the characteristics of a black hole included that it exercised gravitational force in contrast to it's size but in proportion to it's mass, i.e., the more mass, the greater "pulling force", whilst, the smaller size, the denser the mass thus smaller but stronger "pull", if there is a black hole in the centre of a glalaxy that all the stars and materials will be consumed by the black hole is just a matter of time, although it doesn't interact with 99% of the stars... no? because the black hole is not constant it keeps growing in mass by consuming stars (more mass)...  
> **username_2**: Regarding the comment "why it's believed that eventually the black hole at the centre will consume all the stars in that galaxy" -- That is the bad pop sci version of a black hole. It is not generally believed to be the case.  
> **username_3**: For (b) sub-question: the perceived direction of the rotation will indeed be different depending on your vantage point. For instance, have a look at this video: [Startrail & Timelapse 2015 - Northern vs Southern Hemisphere](https://youtu.be/UNiNJC3UHIo). The video is about earth's poles, but as you'd would probably notice, the basic idea is just the same for any object that's rotating.  
> **username_1**: @Bhumishu米殊 as username_2 says your approaching this question with a science fiction type interpretation of black holes, in reality they don't interact with that many stars, as I pointed out previously. Also it is not considered a given that black holes continuously keep amassing more mass indefinitely, there is evidence now that over long periods of time black holes loose mass via Hawking Radiation.  
> **username_4**: I've done a rewrite. I think the question is, do the arms trail or lead in the rotation.  
> **username_0**: @username_4 Thx a millions! I'm not equipped with the technical terminologies that's hard to communicate accurately here.  
> **username_0**: @username_3 Thx. From your answer I now get to realize all those articles were talking about viewing the Milky Way from the earth, I would like to know if, in imagination the observer is the size of Kobe Bryant and the Milky Way the basket ball, how does it spin? Maybe I shall ask in a new question.  
> **username_0**: @username_1 Ok. What I think is the Black Hole is a mass behaved in the negative way - negative mass, with a negative mass that means the totality of the mass of a system (a galaxy is a system) will get "lost" due to it's effect. Tiny or large has to be measured in proportion, like the earth is large but compared to the sun it's small, also by when considering the infinite nature of time, the proportion will not stand if an effect is taking place.  
> **username_3**: @username_0 Actually, it's not just about earth. In fact, even for basketball, the idea from the star trails video would apply as is. The direction of rotation is relevant to the _observer_ and not the object that itself is spinning. Imagine Kobe Bryant has held a basketball in his left hand and gives it a _clockwise_ flick to spin the ball atop his finger. From his point of view (north pole of the ball, if you will), it would appear that the ball is spinning clockwise. From the point of view of a kid, looking up at Kobe Bryant, the ball would _appear_ to spin counterclockwise. Does that help? :)  
> **username_0**: @username_3 Ok, got it, thx. The thing bugged me is that the galaxy as a system if it rotates it should go in only one direction, although by observation it appeared in reference to the observer but the direction is only uniquely one on its own. I imagine in the universe there are many galaxies, one goes clockwise, the other goes counterclockwise but they will keep their tendency in constant unless some upheavals caused change of balance.  
> **username_5**: I understand that the direction of objects’ spin is relative to your frame of reference. However, one thing is not relative, and that is whether two objects are spinning in the same direction or not. For example in our solar system, the Sun, Earth, Luna, most planets (except Venus and Uranus), and the planets’ satellites spin all spin in approximately the same direction. Likewise, the orbits of these bodies is in the same direction. Does our galaxy rotate in approximately the same direction? I understand that the rotational plane of our solar system is tilted about 60 degrees relative to the g  
> **username_6**: The majority of the galaxies rotate with trailing arms, but not all.  
> **username_7**: `+1` The OP's question is reasonable, and responsiveness in comments is exemplary. I don't see why it was down voted.  
> **username_0**: @username_7 right, thx. this is bizarre, yeah. my 1st experience with this forum is... uhhhuu  
> **username_0**: @username_5 i think there pull/push force exercised from the centre of the galaxy, the majority of stars in a system should spin in same direction due to the central force influence (a centralized force holding the items to form one system). planet like Venus or Uranus is weirdo, must get some accident to cause it behaved in a different way.  
> **username_0**: @username_6 trailing arms meant the items getting towards the centre of galaxy, that is the galaxy will increase in mass but decrease in size; leading is opposite. not sure this has any explanation or just my hypothetical imagination  
> **username_6**: @username_0 No. It doesn't mean any similar.  

---

## 1 answers

### Answer by username_4 on 2017-07-02. Score: 7

The Milky Way has arms that form due to [density waves](https://astronomy.stackexchange.com/questions/18611/why-do-galaxies-have-arms/18620). Like the majority of spiral galaxies, the arms are trailing. Individual stars orbit in circles (roughly), neither towards or away from the centre.

If you consider a common map of the Milky way (imagined from a point North of the Earth, Celestial North is not the same as Ecliptic North, which are both about 60 degrees off from [Galactic North](https://www.physicsforums.com/attachments/there-planes-angles_no-earth-09oct2016-jpg.107279/))
[![enter image description here](https://i.sstatic.net/UkfQ2.jpg)](https://i.sstatic.net/UkfQ2.jpg)
The stars in the galaxy would be moving in a clockwise fashion.

If you were to view from the other side, it would be as if you had made a mirror image of the galaxy, so the motion would be counterclockwise. If you look at the back of a clock, then from the wrong side, the hand would move counterclockwise. However the rotation of the galaxy is still with its arms trailing.

There are some galaxies that rotate with arms leading: [NGC 4622](http://hubblesite.org/news_release/news/2002-03) is one example.

> **Comments:**
> **username_1**: Good answer but I think if your going to use North as a reference point, you should point out that Celestial North is not the same as Ecliptic North, which are both about 60 degrees off from Galactic North. Maybe a diagram like this would help https://www.physicsforums.com/attachments/there-planes-angles_no-earth-09oct2016-jpg.107279/  
> **username_0**: Thank you so much! This answers all my puzzles. The question on clockwise/counterclockwise is that when I read those articles they seemed suggesting the galaxy could rotate in both directions. The galaxy rotates only in one direction as a system on it's own, constantly; only the observer got the opposing impression due to the reference point of observing, correct?  

---

