- Emission nebula
An emission nebula is a
cloud ofionized gas ("i.e." a plasma) emitting light of various colors. The most common source forionization are high-energyphoton s emitted from a nearby hotstar . Among the several different types of emission nebula areH II region s, in which star formation is taking place and young, massive stars are the source of the ionising photons; andplanetary nebula e, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.General information
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 theBalmer 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 remainderhelium ,oxygen ,nitrogen , and other elements.Some of the most prominent emission nebulae visible from the
northern hemisphere are theNorth America Nebula (NGC 7000) andVeil Nebula NGC 6960/6992 in Cygnus, while in the south celestial hemisphere, theLagoon Nebula M8 / NGC 6523 in Sagittarius and theOrion Nebula M42. [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 brightCarina 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 theCone Nebula .Many nebulae are made up of both reflection and emission components such as the
Trifid Nebula .References
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