Why are some galaxies spiral?

Hi All,

I was just wondering why some galaxies look spiral (e.g. the milky way, andromeda, etc)? Is there a proven explanation for this? Also, it would be interesting to hear your thoughts/opinions..

Thanks! :)
 
Wikipedia sources are a bit insulting...but thanks anyway Drakkith :)
 
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Try not to take it as an insult :wink:. Whenever I link to Wikipedia, I just mean it as a starting point/jumping-off point. Also, there are usually References and External Links sections in Wikipedia articles, which might be useful e.g. :

http://en.wikipedia.org/wiki/Spiral_galaxy#References
http://en.wikipedia.org/wiki/Spiral_galaxy#External_links

http://en.wikipedia.org/wiki/Density_wave_theory#References
http://en.wikipedia.org/wiki/Density_wave_theory#External_sources

(my own knowledge about galaxy formation is very limited, so I've got nothing to add to this discussion :smile:)
 

Chalnoth

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If you are asking instead why most galaxies can be placed into two different classifications: spiral and elliptical, I think the general consensus right now is that it has something to do with dust. Spiral galaxies have a lot of dust in them. This dust experiences friction, which, over time, pushes nearly all of the stars in the galaxy into a single plane. The links that Drakkith mentioned are good at explaining why, once the stars are revolving in the same plane, the galaxy ends up with a spiral shape.

As for why the galaxies switch between these two phases, my understanding is that a lot of work remains to really understand what's going on here, but that it's generally expected to be tightly related to the behavior of the supermassive black hole at the centers of these galaxies: when the supermassive black hole gobbles up too much matter, it gets so bright that it heats up the dust in the galaxy so much that most of it gets pushed out. At least, that's what I recall from memory.
 

Drakkith

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Chronos

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Generally speaking, elliptical galaxies are believed to be the consequence of galactic mergers. On that basis the early universe should be populated mainly by spiral and irregular galaxies. Observation thus far appears to confirm that hypothesis.
 

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