Nebulae and star clusters

A nebular is a cloud of dust and gas inside a galaxy. Nebulae become visible of the gas glows, or if the cloud reflects starlight or obscures light from more distant objects. Emission nebulae shine because their gas emits light when it is simulated by radiation from hot young stars. Reflection nebulae shine because their dust reflects light from stars in or around the nebula.

Dark nebulae appear as silhouettes because they block out light from shining nebulae or stars behind them. Two types of nebula are associated with dying stars planetary nebulae and supernova remnants. Both consist of expanding shells of gas that were once the outer layers of a star.