Reference: The 3-D structure and evolution of NGC 7009.
Sabbadin et al. (2004) - A&A 416, 955
The fixed image at the start of the movie corresponds to the nebula as seen
from Earth (North is upand East is left).
Each movie is about 80-230 Kby in size (avi) or 110-500 Kby (animated gif).
Rotation around the N-S axis