Notes:
Propagating Star Formation
The idea here is that initially some large and bright stars called OB stars would form (perhaps triggered by a nearby supernova, or a spiral density wave) and the combined resulting ultraviolet radiation from them would generate a shock-wave which travels through the nebula compressing the gas which triggers off more star formation. Though of course this is just a theory until we actually can see evidence of the process, which brings me to my first famous pretty picture.