Spiral Galaxy

[Sanduleak]

This is a spiral galaxy arm generated according to the Lin and Chu theory (http://en.wikipedia.org/wiki/Spiral_galaxy).

At the center of the galaxy (made of about 1000 stars) lies the "Core", a supermassive blackhole as big as 8000 suns.  I could not make the whole system coherent without one. And even with the help of the blackhole as a stabilizing factor, I'm not sure the galaxy is in equilibrium on the long term, because the simulation needs to run slowly to avoid unrealistic star collisions.

The first image shows the spiral arm with the projected paths. Subsequent images are taken at start and after 250000 years respectively. I know it doesn't seem to move much, but it does  

Chris

[atomic7732]

Yay! A one armed spiral! 

[Sanduleak]

Yay! A one armed spiral!  

A dual arm version is coming soon  

Maybe I should explain a little bit how these are constructed: each star is following a classical, elliptical orbit around the galaxy center. Each orbit is slightly tiltted from one star to the next, meaning that the argument of the periapsis is slowly increased (or decreased) by one degree or so. The semimajor axis is also increased progressively.

In the beginning of the simulation, the stars are randomly distributed on their orbits: to put it another way, the mean anomaly is chosen at random. Since the orbits are tilted, some stars will appear statistically closer in some regions, forming the so called "arms". But in reality the stars are not staying in the arms, they are running their own elliptical course. The arms are a statistical illusion, so to speak.

[atomic7732]

The way you are saying, wouldn't that make the arm go the wrong direction, but still keep shape?

[Sanduleak]

Yay! A one armed spiral!  

A dual arm version is coming soon  

Here it goes: the 1600 stars dual armed spiral, in its blazing glory  

XML source attached to the post.

The way you are saying, wouldn't that make the arm go the wrong direction, but still keep shape?

Not sure what you mean by the wrong direction? It is true though that we do not seem to be able to control whether the motion will be clockwise or counterclockwise. But the laws of gravity being reversible, you can reverse speeds (I think there is an option somewhere in US).

[atomic7732]

Wrong direction as in, like have the arms pointing as if rotating counter-clockwise, but the galaxy is spinning clockwise.

[Sanduleak]

Wrong direction as in, like have the arms pointing as if rotating counter-clockwise, but the galaxy is spinning clockwise.

I get it. My guess is that the spinning of the galaxy is ultimately commanded by the (averaged) longitude of the ascending nodes of each individual orbits. In my sims I have always left the nodes to their default values (zero), but I think(?) that if you tilt them by 180 degrees, the galaxy will spin the other way with the arms unchanged.

[atomic7732]

Ummm, no I think you need inclination to be 180, retrograde.

[Sanduleak]

Ummm, no I think you need inclination to be 180, retrograde.

You're right. Both solutions are acceptable, they yield the same result.

A third option would be to invert the argument of the periapsis: in this case, the arms would be inverted but the direction of spin of the galaxy would be conserved.

I cannot think of other ways.