A description of spiral structure is presented that appears to account for both the imperfect
grand-design nature of the patterns and the importance of gas. It is argued that most visible
spirals are short-lived features organized by gravitational forces. The recurrent transient
patterns are self-regulating since they cause the velocity dispersion of the stars to rise
secularly until, in the absence of gas, the disk becomes stable. However, any method of
cooling will ensure that the disk will never completely stabilize and will, consequently …