Density wave theory

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 spiral arm structure of certain galaxies. Their theory introduces the idea of long-lived quasistatic density waves, 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.
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Theory which explains the formation of stars in the arms of spiral galaxies by thinking of the interstellar medium as being compressed in regions of higher density along the leading edges of the spiral arms . These higher density regions cause dense cores to be produced in dark nebulae which collapse to form stars. As the stars are born, OB associations are created which advance the density wave by pushing the interstellar medium onwards via stellar winds . In this way, the density wave advances around the galactic nucleus, triggering a new wave of star formation. Hence the spiral arm appears to move along at the same time as the new stars begin to shine.