Another clump moving down the highway may be seen as cars pile up behind a slow-moving truck. In a galaxy the spiral region of greater gravity concentrates the stars and gas. The spiral regions rotate about as half as fast as the stars move. Stars behind the region of greater gravity are pulled forward into the region and speed up. Stars leaving the region of greater gravity are pulled backward and slow down.
Gas entering spiral wave is compressed.
This is seen in some galaxies with prominent two-armed spiral patterns. But there are some unanswered questions. Another popular theory uses the shock waves from supernova explosions to shape the spiral pattern. When a supernova shock wave reaches a gas cloud, it compresses the cloud to stimulate the formation of stars. Some of them will be massive enough to produce their own supernova explosions to keep the cycle going.