Stars closer to the center of a spiral galaxy orbit the galaxy faster than
stars farther away. Over many millions of years, the difference in
orbital rates should wind the spiral tighter and tighter. We do not see
any evidence for this in galaxies of different ages.
Spiral arms are density waves, which, like sound in air, travel through
the galaxy's disk, causing a piling-up of stars and gas at the crests of
the waves. In some galaxies, the central bulge reflects the wave,
giving rise to a giant standing spiral wave with a uniform rotation
rate and a lifetime of about one or two billion years.
The causes of the density waves are still not known, but there are many
possibilities. Tidal effects from a neighboring galaxy probably cause
some of them.
The spiral pattern is energetically favorable. Spiral configurations
develop spontaneously in computer simulations based on gravitational
dynamics (Carlberg et al. 1999).
