The numbers at upper right show elapsed time in units of about 160 million years. As the view above shows, the northern tail appears straight because we view it almost edge-on. All movies follow the same format. The particle is a white point which leaves a green trail as it orbits in the potential. At the same time, the view point circles around to show the orbit in three dimensions.
Boxes are the main orbit family fully consistent with triaxial density distributions.
Theories of Spiral Structure Simulations created for a lecture on spiral structure. Thanks to Joel Welling and Anjana Kar of the Pittsburgh Supercomputing Center for help with the videos. Thanks also to Narayan Raja for advice and comments on this site. Numerical simulations were run at the Pittsburgh Supercomputing Center and the Maui High Performance Computing Center.