The Milky Way is a spiral, and so is the nearest bright galaxy, in the constellation Andromeda. Spirals are flat disks of stars with bright bulges in their centers. Spiral arms wrap around these bulges. Spiral arms probably form as the result of waves that sweep through the galactic disk. In the case of galaxies, they squeeze clouds of interstellar gas, triggering new star formation.
Some newborn stars are massive, hot, and bright, so they make the spiral arms appear bright.
These massive stars are blue or white, so the spiral arms look blue-white, too. The gaps between the arms contain older stars, which are not as bright. In some spirals, a wave organizes the stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar. The Milky Way may fall into this class of spirals.