A spiral galaxy is a coil-shaped collection of stars and planets which are held together by gravity. Our very own Milky Way is a spiral galaxy!
Most spiral galaxies have a concentration of stars at the center, surrounded by spiral "arms" made up of stars, planets, dust, and gas. The majority of galaxies discovered by astronomers are spiral in shape, and the Hubble Space Telescope has captured beautiful images of several. NASA has produced particularly detailed photos of the Andromeda Galaxy's twisting, coiled branches, containing over 100 million stars. The Greek roots of spiral galaxy are speira, "winding," and galaxias, "milky circle."