Galaxy that has a spiral shape, with a central bulge and arms that spiral outwards. These arms are made up of stars, gas, and dust and are where new stars are formed. The Milky Way, our own galaxy, is an example of a spiral galaxy. The spiral shape is created by the rotation of the galaxy and the gravitational pull of the central bulge. These forces cause the gas and dust in the arms to compress, leading to the formation of new stars. The bright central bulge of a spiral galaxy is usually composed of older stars, while the arms contain younger, more luminous stars. The spiral structure of a galaxy is thought to be maintained by the rotation of the galaxy and by the gravitational pull of dark matter, a mysterious substance that makes up most of the mass in the universe.