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One popular theory says that the spiral structure is a
wave that moves through the disk causing the stars and gas to
clump up along the wave---a density wave. The spiral arms are
where the stars pile up as they orbit the center. They
are like the entrance ramps onto a busy highway during rush hour:
when a car comes to the point where many other
cars are merging, it slows down and cars jam up. But eventually,
the car moves past the ramp and speeds up. A person
in a helicopter above the traffic giving reports for the local
radio station sees a traffic jam at the entrance ramp even
though individual cars are moving through it. 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. On the downstream side of wave, there should be lots of H II regions (star formation regions). This is seen in some galaxies with prominent two-armed spiral patterns.