[OAB 32] The claim is made that galaxies would not have coherent spirals if they were not young. However, more recent research has shown that the concentration of mass formed by a galaxy's spiral arm forms a "density wave," whose gravity causes stars to continue to congregate in the wave and thus to perpetuate the spiral arm. Stars move into and out of the density wave, in the same way that water molecules move into and out of an ocean wave at the beach, yet the wave itself maintains its shape. Detailed calculations have shown that density waves can be stable over extremely long periods of time, so the continuing existence of spiral galaxies does not place any limit on the age of the Universe.