- Emission nebula
An emission nebula is a
cloudof ionized gas("i.e." a plasma) emitting light of various colors. The most common source for ionizationare 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.
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 hemisphereare the North America Nebula(NGC 7000) and Veil NebulaNGC 6960/6992 in Cygnus, while in the south celestial hemisphere, the Lagoon NebulaM8 / NGC 6523 in Sagittarius and the Orion NebulaM42. [McArthur,Frommert and Kronberg. "Messier 42" "Messier Object 42". 12 April 2006. 17 July 2007. [http://seds.org/messier/m/m042.html] ] Further in the southern hemisphere is the bright Carina NebulaNGC 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 Nebulaor the Cone Nebula.
Many nebulae are made up of both reflection and emission components such as the
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