Spiral arms are regions of higher density found in spiral galaxies, such as the Milky Way. These arms are composed of stars, gas, and dust, and they extend outward from the galaxy's center. The spiral structure is created by the gravitational forces acting on the galaxy, which cause stars and other materials to cluster along these arms. The formation of spiral arms is often linked to density waves, which are areas of increased gravitational attraction. As stars and gas move through these waves, they can become compressed, leading to the formation of new stars. This process contributes to the dynamic and evolving nature of spiral galaxies.