A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae 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?