Emission nebula
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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 ionising photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
[edit] General information
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 prominent emission nebulae visible from the northern hemisphere are the North America Nebula (NGC 7000) and Veil Nebula NGC 6960/6992 in Cygnus. Good visible are, while in the south celestial hemisphere, the Lagoon Nebula M8 / NGC 6523 in Sagittarius and the Orion Nebula M42. Further in the southern hemisphere is the bright Carina Nebula NGC 3372.
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 North America Nebula or the Cone Nebula.
Some nebulae are made up of both reflection and emission components such as the Trifid Nebula.
[edit] Science fiction
An emission nebula is mentioned in the Star Trek: Deep Space Nine episode "Return to Grace".
