Two-arm rotating galaxy with non-crossing spirals
3D particle system
A log spiral r = a e^{bθ}. Two arms offset by π keep a permanent angular gap; scatter is tiny so the bands never merge.
Each star orbits with ω proportional to r^{−0.7}: the core is fast and the rim is slow, instead of the disc spinning as a rigid body. Six satellites ride the outer ring at equal spacing.