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An emission nebula is a cloud of ionized gas (i.e. a plasma) emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star. Among the several different types of emission nebula are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
Usually, a young star will ionize part of the same cloud from which it was born although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud. In many emission nebulae, an entire cluster of young stars is doing the work.
The nebula’s color depends on its chemical composition and degree of ionization. Due to the prevalence of hydrogen in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to the strong emissions of the Balmer series. If more energy is available, other elements will be ionized and green and blue nebulae become 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 prominent emission nebulae visible from the northern hemisphere are the North America Nebula (NGC 7000) and Veil Nebula NGC 6960/6992 in Cygnus, while in the south celestial hemisphere, the Lagoon Nebula M8 / NGC 6523 in Sagittarius and the Orion Nebula M42.1 Further in the southern hemisphere is the bright Carina Nebula NGC 3372.
Emission nebulae often have dark areas in them which result from clouds of dust which block the light. The combination of emission nebula and dust cloud make for some beautiful objects, and many of these nebulae bear the name of objects that they early astronomers thought they resembled, such as the North America Nebula or the Cone Nebula.
Many nebulae are made up of both reflection and emission components such as the Trifid Nebula.
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