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Why do galaxies have spiral arms?

M

MobiusPizza

Platinum Member
Most galaxies have spiral arms. I just wonder why they would have so regarding the fact they rotate slower compared to the centre of the galaxy. I am not concerned why stars tend to form an distinct arm.

I've came up with some explanations myself but I can't be sure because this is not my expertity.

A. Gravitons travel at speed of light. Strong gravitation fields from centre of galaxy reach the far ends (the arms) slower therefore they have a spiral shape where its relative position is a bit behind.

B. Gravitaional field strength decrease across distance in an inverse square law. Hence the arms are not as strongly attracted at the ends. Though the galaxy spins faster near the centre, it'd spin slower outside due to this weaker connection.


I'd think it's more likely to be B. But I wonder if A also has an effect or not.
 
Im not necessarily sure either theory holds.
If the outer stars were moving that much slower than the inner stars, the arms would eventually break apart.
Therefore I believe all stars in a galaxy must have the same angular velocity with respect to the central black hole, which would mean the outer stars are traveling much (linearly) faster than the inner ones.


You are correct in saying that it takes gravity a long time to reach those stars, but that would only have an effect if the level of gravity changes, it should be reasonably constant.

My guess would be that the arms were formed during the initial stages of the galaxies life, and have remained that way since that time.
 
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Have you ever looked at a hurricane, didn't you notice that it's a spinning mass too and has very similar arms?
 
Exactly. AFAIK spiral arms are just a consequence of ordinary mechanics; the galaxy is rotating and at the same time the stars are attracted to each other.
If you rotate a cloud of particles and introduce some form of pertubation my guess is that arms will start to form.

Edit: Found a link where you can see an animation. The pertubation here is in the form of another galaxy.

http://www.cita.utoronto.ca/~dubinski/nbody/
 
Okay, I'm here 🙂

The formation of arms is actually not well known. It is NOT because the outer parts spin slower than the inner parts though. If you do the modelling and look at the actual radial velocities, the arms would "curl up" within a few million years. By that I mean the galaxy would start to spin, the arms would start to form, the center would keep spinning rapidly and the arms would wind up to the point where the center had undergone 1000 revolutions to the outer edge's 2. You would lose all structure and would be looking at a disk.

The best candidate right now is for that of a density wave. In the early stages of galaxy development a pressure wave (caused mainly by star formation and radiation pressure) causes gas to compress in certain regions of the galaxy. These early forming stars burn out quickly as they are very massive, and then leave behind voids. The compressed gas then triggers new star formation regions which happen to evolve into arms. It's difficult to explain without a picture on hand, but if you look at the types of stars in the arms and where they are, at the leading edges of the arms are hot young blue stars that are newly formed. The rest of the arms are mainly made up of cooler yellow and red stars that have been burning a much longer time. This suggests that star formation happens at the leading edge of the arms as the gas in the voids between the arms is compressed by the density wave. They form new stars which evolve as they fall back through the arms.

Sweet, I found a link that kills both birds with one stone. 🙂
 
Thanks, Silver... I was going to propose the first model, but didn't because I had some uncertainties. I'm glad I didn't now 🙂
 
I can simulate the phylogeny of a galaxy by spinning my morning cappuccino; seriously, the shape the milk forms in relation to the crema on the espresso permutes every galactical form 😀

Thanks for the explanations silverpig.
 
Originally posted by: silverpig
Okay, I'm here 🙂

The formation of arms is actually not well known. It is NOT because the outer parts spin slower than the inner parts though. If you do the modelling and look at the actual radial velocities, the arms would "curl up" within a few million years. By that I mean the galaxy would start to spin, the arms would start to form, the center would keep spinning rapidly and the arms would wind up to the point where the center had undergone 1000 revolutions to the outer edge's 2. You would lose all structure and would be looking at a disk.

The best candidate right now is for that of a density wave. In the early stages of galaxy development a pressure wave (caused mainly by star formation and radiation pressure) causes gas to compress in certain regions of the galaxy. These early forming stars burn out quickly as they are very massive, and then leave behind voids. The compressed gas then triggers new star formation regions which happen to evolve into arms. It's difficult to explain without a picture on hand, but if you look at the types of stars in the arms and where they are, at the leading edges of the arms are hot young blue stars that are newly formed. The rest of the arms are mainly made up of cooler yellow and red stars that have been burning a much longer time. This suggests that star formation happens at the leading edge of the arms as the gas in the voids between the arms is compressed by the density wave. They form new stars which evolve as they fall back through the arms.

Sweet, I found a link that kills both birds with one stone. 🙂
Click to expand...

Wow. Thanks for being an intelligent poster! That's some good stuff you got there 🙂
 
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