Stars tend to form cluster known as galaxies. A galaxy is a massive system of stars, dust, and gas that is held together by gravitational forces. There are billions of galaxies in the universe, and they come in different shapes and sizes. Our own solar system is located within the Milky Way galaxy, which is a barred spiral galaxy. Galaxies can contain anywhere from a few million to a trillion stars, as well as various types of interstellar matter such as gas, dust, and dark matter.