Claim CE380:

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.

Source:

Response:

1. 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).

Links:

Carlberg, Ray, Debra M. Elmegreen, Bruce G. Elmegreen, Jerry Sellwood and William Lee Bell. 1999. Ask the experts: Astronomy: What process creates and maintains the beautiful spiral arms around spiral galaxies? http://www.sciam.com/askexpert_question.cfm?articleID=0008A68A-8C7F-1C72-9EB7809EC588F2D7

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