A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle.There are several types of nebulae:Emission nebula tend to be red in color when photographed. They are made of gas which has been heated by a nearby star.Reflection nebula are most often blue. They too are gas, but their starlight is reflected.Planetary nebula occur when stars shed their outer layer. They can occur in any color.Supernova remnants are the remains of exploded stars.Absorption nebula are dark clouds of dust which obscure the area behind them. Many nebulae are composed of more than one of the types described above.Learn more about nebulae here: WikipediaBelow are some nebulae I have imaged over the past few years. Click on the image to get more detailed information.