Nebulas are clouds of dust in space. Nebulas originally included galaxies, before telescopes could resolve that galaxies were actually billions of stars, and not clouds of gas. There are two main types of nebula, a diffuse nebula and planetary nebula. A planetary nebula is usually small disk surrounding what is left of a star in the last phase of its evolution. There are several types of diffuse nebula, an emission nebula is compromised of hot hydrogen gas ionized by nearby hot blue-white stars and are generally red in color. A reflection nebula exists near cooler stars and just reflects starlight from them. Reflection nebula are generally blue in color. A dark nebula is clouds of gas that obscure either stars or a bright nebula that lie behind it. 

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