Why Do Some Galaxies Have a Spiral Shape?

In summary: Most spiral galaxies are found in the Local Group, which is made up of the Milky Way, the Andromeda galaxy, the Triangulum galaxy, and several smaller galaxies.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.
  • #1
deepthishan
38
0
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! :)
 
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  • #3
Wikipedia sources are a bit insulting...but thanks anyway Drakkith :)
 
  • #4
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:)
 
  • #5
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.
 
  • #6
deepthishan said:
Wikipedia sources are a bit insulting...but thanks anyway Drakkith :)

What's insulting about them?
 
  • #7
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.
 

1. Why do some galaxies have spiral arms?

There are a few different factors that contribute to the presence of spiral arms in galaxies. One is the rotation of the galaxy, which causes gas and dust to be pulled outward, creating the spiral shape. Another factor is the density waves that form within the galaxy, which can also cause spiral patterns to emerge. Finally, the presence of a central bulge in some galaxies can also influence the formation of spiral arms.

2. What causes the distinctive shape of spiral galaxies?

The shape of spiral galaxies is mainly determined by the balance between the forces of gravity and centrifugal force. As the galaxy rotates, the outer regions are pulled outward, while the inner regions are held in place by gravity. This results in the characteristic spiral shape that we see in these galaxies.

3. Are all spiral galaxies the same shape?

No, not all spiral galaxies have the same shape. There are actually several different subtypes of spiral galaxies, including normal spirals, barred spirals, and flocculent spirals. These different types have varying degrees of central bulges and different patterns in their spiral arms, giving them distinct shapes.

4. Why do some spiral galaxies have bright, defined spiral arms while others do not?

The brightness and definition of spiral arms in galaxies can depend on a variety of factors, including the amount of gas and dust present in the galaxy, the strength of the density waves, and the activity of the galaxy's central supermassive black hole. These factors can all affect the appearance and visibility of spiral arms in a galaxy.

5. Do all galaxies start out as spiral galaxies?

No, not all galaxies start out as spiral galaxies. In fact, it is believed that galaxies go through a process of evolution, with smaller galaxies merging and interacting to form larger, more complex structures. This process can result in the formation of different types of galaxies, including spiral, elliptical, and irregular galaxies.

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