The HII regions (emission nebulae) are so named because they are composed mostly of a plasma of ionized hydrogen (HII) and free electrons. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby star or stars.

Emission Nebulae: The Final Frontier Emission nebulae derive their name from the light that is emitted from gas excited by energetic radiation from nearby hot stars. The gas is always rich in hydrogen, which happens to be the most abundant element. Nebulae therefore radiate with a distinctive red hue. The hydrogen often mixes with oxygen, which provides green light. Green and red combine to ...