M100 has clearly defined spiral arms and a well organised spiral structure.
Credit: AAO/David Malin

A grand design spiral galaxy is one that has a clearly defined and well organised spiral structure. This is in contrast to galaxies in which the spiral arms are less well defined, often called flocculent spirals. It is thought that the arms in these spirals are actually overdense regions of the disk which trigger star formation as incoming material is compressed in that region. This is the key idea underpinning the density wave model for the origin of spiral arms.