Аннотация:The geometry and physics of the spiral structure of the giant Hubble type Sc galaxy
NGC 309 is studied. A schematic of two patterns with three arms in each is suggested
for the blue spiral. The red and blue patterns form together a grand design with two-
fold symmetry. A possible gas-dynamics explanation of the phenomenon is suggested
which shows how the two-arm red spiral may induce the formation of the six-arm
coherent blue spiral.
Keywords: Galaxies: individual (NGC 309) { galaxies: spiral}