Grand design spiral galaxy

As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, including M51 (Whirlpool Galaxy), M74 (Phantom Galaxy), M81 (Bode’s Galaxy), M83 (Southern pinwheel galaxy), M101 (Pinwheel Galaxy), NGC 6946 (Fireworks Galaxy) and IC 342 (The Hidden Galaxy).

Origin of structure

Density wave theory is the preferred explanation for the well-defined structure of grand design spirals, first suggested by Chia-Chiao Lin and Frank Shu in 1964. The term "grand design" was not used in this work, but appeared in the 1966 continuation paper; Lin (along with Yuan and Shu) is usually credited with the coining of the term.

According to the density wave theory, the spiral arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxy's disk. Stars and gas are clumped in these dense regions due to gravitational attraction toward the dense material, though their location in the spiral arm may not be permanent. When they come close to the spiral arm, they are pulled toward the dense material by the force of gravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes the gas in particular to clump in the dense regions, which in turn causes gas clouds to collapse, resulting in star formation.