Why most stars seem to rotate the same direction in a galaxy?

[fbs7]

After much questioning and soul-searching I got my answers of why galaxies rotate, and for that I greatly thank the gurus in the forum (so I'll not forget: conservation of initial angular momentum, non-isotropic material scattering, slowdown of infalling materials and non-isotropic infalling of materials). Very complicated, yay :^) !

Then a new mystery came in! I have no idea what about paths of stars in an elliptical galaxy, but in a spiral galaxy all of them seem to rotate the same direction. That's awesome, given the distance the stars are from each other!

I understand that in liquid going down the drain - there are collisions between molecules, and the molecules are close together in the liquid, so 100 molecules going one way colliding with 1 molecule going the wrong way will bounce the one in the wrong way around, and eventually you have 101 molecules going a single way - more organization. It's the same thing as people trying to go against flow in a crowd, they are turned around.

But how about stars in a galaxy? These collisions (ie, iteration through gravity, don't mean a head-on collision) are quite different. If there are 100 stars going one way and 1 star going the wrong way, the result after iteration is a big, less organized mess with many stars going in odd directions, not a more organized system going nicely in a beautiful spiral.

So what mysterious and wonderful process works out to get almost all stars in a galaxy to rotate the same direction, instead of ending up in a chaotic mess with stars going all over the place?

 

[Janus]

While the stars as we see them now are small compact objects far apart from each other, this wasn't always the case of the material that they are made up from. They all started as more diffuse and spread out clouds of gas and dust. For instance, the Orion nebula in which large stars are being born right now is 100's of light years across. These spread out nebula are more likely to interact than single compact stars are. Then there is the fact that most of the stars in the galaxy now are second generation stars formed. at least in part by material that were part of earlier stars ( these early stars were likely very massive and lived short lives before going supernova and sowing material into the galaxy). The early galaxy was more of a cloud of gas that collapsed into a more compact cloud. Interactions between the particles of this cloud is what gave the galaxy its present overall rotation. The early stars built from these materials took on this motion, as did the present stars which later formed from material from the first stars.

 

[Vanadium 50]

Do you mean "rotate" or "revolve"? I think you mean "revolve".

 

[fbs7]

Do you mean "rotate" or "revolve"? I think you mean "revolve".

That's right! Revolving around the center of the galaxy, that's what I mean. Thank you :^D

 

[fbs7]

While the stars as we see them now are small compact objects far apart from each other, this wasn't always the case of the material that they are made up from. They all started as more diffuse and spread out clouds of gas and dust. For instance, the Orion nebula in which large stars are being born right now is 100's of light years across. These spread out nebula are more likely to interact than single compact stars are. Then there is the fact that most of the stars in the galaxy now are second generation stars formed. at least in part by material that were part of earlier stars ( these early stars were likely very massive and lived short lives before going supernova and sowing material into the galaxy). The early galaxy was more of a cloud of gas that collapsed into a more compact cloud. Interactions between the particles of this cloud is what gave the galaxy its present overall rotation. The early stars built from these materials took on this motion, as did the present stars which later formed from material from the first stars.

Ohhh.. wow... so stars revolve in the same direction around the galaxy because the gas clouds that formed the galaxy revolved in that direction! I see!

I got it wrong initially! I thought that stars formed first, flying all over the place, and then somehow the galaxy becomes an organized spiral galaxy! But it's the other way around, right?

Does that mean that all (or most) galaxies form initially as spirals?

 

[BillTre]

The makes sense for galaxies evolved directly from gas clouds.
I'll call them primary galaxies.

Secondary galaxies, formed by the collision of of two two primary galaxies, perhaps with stars going on opposing directions?
The resulting secondary galaxies would at least start out with stars going in different directions, revolutionary-wise.
I would not expect a lot of collisions due to mostly empty space. The number and frequency of collisions difference between a liquids molecules and a galaxy's stars is in the number and density of objects and the frequency of their collisions.

I can see how a revolving mass of starts could be gravitationally drawn into a planar shape.
However, I don't see how the direction of revolution of individual stars would changed after combining two separate galaxies.

Are there galaxies, resulting from collisions, with stars revolving in opposite directions?

 

[Vanadium 50]

Are there galaxies, resulting from collisions, with stars revolving in opposite directions?

Yes. Most large ellipticals.