Galaxies - spiral arms split, why?

  • #1

Main Question or Discussion Point

Why do the spiral arms of galaxies split or bifurcate? Is this due to a previous merger or some kind of current?
 

Answers and Replies

  • #2
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Can you be more specific? What do you mean by "split"?

As it turns out, understanding why galaxies have the spiral structures they do is a complicated question. A simplistic analysis says that all galaxies should have their spiral arms tightly wound up, but this is manifestly not the case. See the Wikipedia http://en.wikipedia.org/wiki/Spiral_galaxy#Origin_of_the_spiral_structure" for further discussion.
 
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  • #3
If you observe some of the better shots of the spiral galaxies, each arm branches in a very peculiar way. It does not make sense to me given only gravity and mass.
 
  • #4
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I saw a documentry that explained it quite well, they talked about slight variations in gravity, causing object to come closer together, to be even more attacked by each other.

They also showed that the objects themselves remain stationary, and the gravity field rotates around the centre.
 
  • #5
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Also spiral arms are something of an illusion. What happens is that spiral arms are places where there are more star formation, and areas with more star formation have lots of blue stars. So when you take a picture with instruments that are particularly sensitive to blue light, you see lots of blue stars in the arms.

In fact it's not true that there is "nothing" between the arms. It turns out that with current models of galactic structure that the areas between the arms have as nearly as much "stuff" as the arms. It's just that the "stuff" between the arms are dark bits of gas and dust that don't show up in picture.
 
  • #6
Ok, I understand that the arms are areas of a type of condensation and that material is more uniform.

Darryl,
You mentioned that the objects (stars?) dont move but that the gravity field rotates instead. I thought I once heard about the orbit time or our star around the galaxy (60 mill years?) so is this true?

Also, the split arms would indicate a kind of offset gravity interference. I also notice strange concentric circles around supernovae.

Anyways, I appreciate the responses and this cool forum. Thanks.
 
  • #7
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i believe the orbit time for our solar system to make one revolution is about 250 million years
 

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