A large cloud of gas and dust is called a nebula. Nebulae are interstellar clouds that are made up of dust, gas, and plasma. They are often the birthplace of stars and planets, and can be found in a variety of shapes and sizes. Some nebulae are so large that they can span hundreds of light-years, while others are only a few light-years across. Nebulae are often classified by their appearance, such as emission nebulae, reflection nebulae, and planetary nebulae.