The Local Group Galaxies Fade from Sight Types of Galaxies Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, who discovered that the universe is filled with many galaxies beyond our own Milky Way. Hubble's system is based on the shapes of galaxies. The most beautiful galaxies are called spirals. 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.