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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 depends on whether the Milky Way is observed from above or below. I don't understand how the direction of spin (clockwise & counterclockwise) depends on the point of view of the observer.

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    $\begingroup$ 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. $\endgroup$ – Dean Jun 29 '17 at 10:43
  • $\begingroup$ 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? $\endgroup$ – Mishu 米殊 Jun 29 '17 at 10:49
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    $\begingroup$ 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 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 which are believed to be how spiral arms behave. $\endgroup$ – Dean Jun 29 '17 at 10:55
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    $\begingroup$ @uhoh right, thx. this is bizarre, yeah. my 1st experience with this forum is... uhhhuu $\endgroup$ – Mishu 米殊 Feb 3 '18 at 14:14
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    $\begingroup$ @Mishu米殊 No. It doesn't mean any similar. $\endgroup$ – peterh - Reinstate Monica Feb 3 '18 at 14:45
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The Milky Way has arms that form due to density waves. 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) enter image description here 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 is one example.

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    $\begingroup$ 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 physicsforums.com/attachments/… $\endgroup$ – Dean Jul 2 '17 at 23:11
  • $\begingroup$ 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? $\endgroup$ – Mishu 米殊 Jul 3 '17 at 12:47

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