Emission, Reflection, and Dark Nebulae

Nebulae are clouds of gas and dust that span tens to hundreds of light years. Three common types are called emission nebulae, reflection nebulae, and dark nebulae. Emission nebulae are glowing clouds of dense gas and dust. Emission nebulae contain or are close to hot O and B stars. The ultraviolet light emitted by these stars ionizes the gas in the nebulae. As the electrons recombine with the atoms, light is emitted. The color of the light depends on the elements contained in the gas. Reflection nebulae are clouds of dust that simply reflect or scatter light from nearby stars. These clouds are typically regions where stars are forming. Blue light is scattered more readily than red light, so often, reflection nebulae appear blue. Reflection nebulae often appear with emission nebulae. Dark nebulae are clouds of dust that block light from behind them. The light blocked may be from stars, or it may be from emission or reflection nebulae.