MILKY WAY

"Why do so many galaxies exhibit a spiral pattern? Spiral galaxies like our own Milky Way have a relatively thin disk (like a pancake) composed of gas, dust (miniature grains), and stars. The entire galactic disk is rotating about the galactic center. In the vicinity of the sun, for example, the orbital speed around the Milky Way's center is 140 miles per second, and it takes material about 225 million years to complete one revolution. At other distances from the center the speed is different--higher closer to the center, lower at greater distances--that is, galactic discs do not rotate like a solid compact disk but differently." P. 121, THE GOLDEN RATIO, THE STORY OF PHI, THE WORLD'S MOST ASTONISHING NUMBER by Mario Livio (2003)