The term "nebula" simply means "fuzzy thing." As such, there are many types of nebula observable, and shown in these image pages. Emission nebula typically represent star forming regions, where a few very large, hot stars excite the surrounding gas, making it glow, typically in the red color of hydrogen alpha emission. Reflection nebulae are just that - they are the reflected light of bright stars on interstellar gas and dust. Planetary nebula happen near the end of the life of a star similar to the sun when the outer layers are ejected from a hot core. More details about the different nebula types are included on the individual image pages.