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 the spiral arm structure of spiral galaxies.^[1][2] The Lin–Shu theory introduces the idea of long-lived quasistatic spiral structure (QSSS hypothesis).^[1] In this hypothesis, the spiral pattern rotates with a particular angular frequency (pattern speed), whereas the stars in the galactic disk orbit at varying speeds, which depend on their distance to the galaxy center. The presence of spiral density waves in galaxies has implications on star formation, since the gas orbiting around the galaxy may be compressed and cause shock waves periodically.^[3] Theoretically, the formation of a global spiral pattern is treated as an instability of the stellar disk caused by the self-gravity, as opposed to tidal interactions.^[4] The mathematical formulation of the theory has also been extended to other astrophysical disk systems,^[5] such as Saturn's rings.