Which forces maintain the shape of the Milky Way?

[KurtLudwig]

TL;DR Summary

In billions of years, stars could just move any which way. Eventually, the shape of our spiral galaxy would be lost.

Gravity, inertia, conservation of momentum and angular momentum are some of the forces and conserved quantities that I could think of which will tend to maintain the shape of the Milky Way.
How is the separation of the spiral arms maintained?

 

[phinds]

How is the separation of the spiral arms maintained?

I think you misunderstand the nature of the spiral arms.

They are not fixed strands as they appear in drawings but rather a result of a "wave" process that is like what happens when cars start bunching up on a highway for no apparent reason and then scatter again only to later bunch up again but with a different set of cars.

That is, the stars in a spiral arm change over time as different stars move at slightly different speeds in their orbit.

 

[mathman]

Gravity (mostly dark matter) holds the galaxy together.

 

[KurtLudwig]

They are not fixed strands as they appear in drawings but rather a result of a "wave" process that is like what happens when cars start bunching up on a highway for no apparent reason and then scatter again only to later bunch up again but with a different set of cars. (phinds)

It is very interesting that the separately appearing spirals are the results of galactic dynamics.

 

[DaveC426913]

Here's a couple of animations for reference. Notice how individual stars move in and out of the spiral arms. The arms are not a solid, fixed object, just a wave crest.

 

[phinds]

It is very interesting that the separately appearing spirals are the results of galactic dynamics.

Yes, just as it is interesting that cars do the same thing. It's one of the reasons, or so I've read, that self-driving cars that talk to one another will be more efficient than human driven cars, because they won't DO that. I've seen the "bunches" on highways get to be miles long and a major annoyance.

 

[DaveC426913]

I've seen the "bunches" on highways get to be miles long and a major annoyance.

And last as much as five hours after the triggering incident is loooong gone.
I read a book on the subject.

 

[phinds]

And last as much as five hours after the triggering incident is loooong gone.
I read a book on the subject.

The point of the wave theory is that there doesn't have to BE a "triggering" incident such as an accident on the other side of the highway, although such things DO tend to cause the worst bunching

 

[Keith_McClary]

 

[KurtLudwig]

Both YouTube videos simulating the evolution of spiral arms were very interesting.

Quote: "Gravity (mostly dark matter) holds the galaxy together."

How is dark matter distributed across the Milky Way galaxy? Is it like a flat pumpkin or a donut? It probably has a more complex shape.

 

[JLowe]

But what causes the spiral arms to appear? I mean gravity dominates of course, but what distribution of gravitational forces causes this? It doesn't appear to be necessary as plenty of galaxies aren't spirals.

 

[phinds]

But what causes the spiral arms to appear? I mean gravity dominates of course, but what distribution of gravitational forces causes this? It doesn't appear to be necessary as plenty of galaxies aren't spirals.

What part of post #2 did you not understand? Not all galaxies are disk-shaped and even for disk-shaped, there is no requirement that spirals form just as there is no requirement that traffic waves form.

 

[JLowe]

What part of post #2 did you not understand? Not all galaxies are disk-shaped and even for disk-shaped, there is no requirement that spirals form just as there is no requirement that traffic waves form.

Ok thanks, I realize my mistake and should have just used google to begin with, which does, of course, give much better answers.

 

[phinds]

Ok thanks, I realize my mistake and should have just used google to begin with, which does, of course, give much better answers.

Hm? Near as I can tell, Google gets you to exactly what post #2 says. Did you find something else?

 

[JLowe]

Hm? Near as I can tell, Google gets you to exactly what post #2 says. Did you find something else?

Yes they are both density waves, but the underlying mechanism is different, which is what I was trying to understand. Density waves on the highway can start by any number things, and since stars don't drop their cigarettes on the floorboard and start swerving all over the road causing other stars to slam on brakes, I only assumed a different reason for such waves appearing in disk shaped galaxies.

 

[Keith_McClary]

It is suggested that spiral galaxies can lose their arms and become lenticular disk galaxies:
https://en.wikipedia.org/wiki/Lenticular_galaxy#Formation_theories
There doesn't seen to be a settled theory.

 

[mathman]

Gravity (mostly dark matter) holds the galaxy together.

https://search.yahoo.com/yhs/search...ter+galaxy+shape&_guc_consent_skip=1614981494

See this article.