In the information brought up through the "These waves" link it states:-
"Density wave theory or the Lin-Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies. Their theory introduces the idea of long-lived quasistatic density waves (also called heavy sound), which are sections of the galactic disk that have greater mass density (about 10–20% greater). The theory has also been successfully applied to Saturn's rings."
As I barely understand such as Dark Matter I'll have to ponder for some time on 'heavy sound'
![Exclamation :!:](./images/smilies/icon_exclaim.gif)
. I wonder if short-lived quasistatic density waves are 'light sound'?
PS. While the main APOD site has not been accessible for the last couple of days I've been using the APOD UK mirror site, but oddly (currently at least) that seems to be down!