A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. As the shapes of galaxies go, the spiral disk — with its characteristic pinwheel profile — is by far the most pedestrian. Our own Milky Way, astronomers believe, is a spiral. Our solar system and Earth reside somewhere near one of its swept-back arms. And nearly 70 percent of the galaxies closest to the Milky Way are spirals, suggesting they have taken the most ordinary of galactic forms in a universe with billions of galaxies. Despite their common morphology, how galaxies like ours get into a spiral shape and maintain their characteristic arms has proved to be an enduring puzzle in astrophysics. How do the arms of spiral galaxies arise?