In astronomy, emission nebulae are clouds of ionized gas, emitting light of various colors.
The most common source for ionization are high-energy photons emitted from a nearby hot star. If the star is a young massive star, type O or B the nebula is called an HII region. If it is an old white dwarf the nebula is called a planetary nebula.
Usually, a young star will ionize part of the same cloud from which it was born. Only big, hot stars can release the amount of energy required to ionize a significant part of a cloud. Often, an entire cluster of young stars is doing the work.
The nebula's color depends on its chemical composition and amount of ionization. Due to the high prevalence of Hydrogen in interstellar gas, and its relative low energy requirement for ionization, many emission nebulae are red. If more energy is available, other elements can be ionized and green and blue nebulae are possible. By examining the spectra of nebulae, astronomers deduce their chemical content. Most emission nebulae are about 90% hydrogen, with the remainder helium, oxygen, nitrogen, and other elements.
Some of the most beautiful emission nebulae visible from the northern hemisphere are the Lagoon Nebula and the Orion Nebula.
Emission nebulae often have dark spots in them which result from clouds of dust which block the light. The combination of emission nebula and dust cloud make for some interesting looking objects, and many of these nebulae bear the name of objects that they resemble, such as the American Nebula or the Cone Nebula.
Some nebulae are made up of both reflection and emission components such as the Trifid nebula.