A galaxy is a huge group of stars, gas, and dust that are held together by gravity. They are so big that it is hard to even imagine how big they are! There are many different types of galaxies, and they come in all sorts of shapes and sizes. Some galaxies are small and have just a few hundred stars, while others are so big that they have trillions of stars. Our own galaxy, the Milky Way, is a medium-sized galaxy and is shaped like a spiral. It is made up of a central bulge, a flat disk, and a halo. The stars in a galaxy are held together by the force of gravity, and they orbit around the center of the galaxy just like planets orbit around the Sun.