Transcript: Another idea to explain the existence of spiral arms is called the stochastic star formation theory. In this theory the star formation in one region triggers star formation in neighboring regions of the disk like a chain reaction. For up to a hundred million years a star formation region is lit up by young stars, and during this time differential rotation, the inner part of the region moving faster than the outer part, shears the star formation region into the segment of a spiral. Overall a spiral pattern is seen, but the pattern is transient because different stars are coming and going and being born and dying over the long period of time of the lifetime of the Milky Way galaxy.