Anemission nebula is anebula formed ofionized gases that emit light of various wavelengths. The most common source ofionization is high-energyultravioletphotons emitted from a nearby hotstar. Among the several different types of emission nebulae areH II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; andplanetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.[1][2]
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 entirecluster of young stars is contributing energy.[1][2]
Stars that are hotter than 25,000 K generally emit enoughionizing ultraviolet radiation (wavelength shorter than 91.2 nm) to cause the emission nebulae around them to be brighter than the reflection nebulae.[3] The radiation emitted by cooler stars is generally not energetic enough to ionize hydrogen, which results in thereflection nebulae around these stars giving off less light than the emission nebulae.
The nebula's color depends on its chemical composition and degree of ionization. Due to the prevalence ofhydrogen in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to strong emissions of theBalmer series. If more energy is available, other elements will be ionized, and green and blue nebulae become possible. By examining thespectra of nebulae, astronomers infer their chemical content. Most emission nebulae are about 90% hydrogen, with the remaininghelium,oxygen,nitrogen, and other elements.
Some of the most prominent emission nebulae visible from thenorthern celestial hemisphere are theNorth America Nebula (NGC 7000) andVeil Nebula NGC 6960/6992 inCygnus, while in the south celestial hemisphere, theLagoon Nebula M8 / NGC 6523 inSagittarius and theOrion Nebula M42.[4] Further in the southern hemisphere is the brightCarina Nebula NGC 3372.
Emission nebulae often have dark areas in them which result fromclouds of dust which block the light.
Many nebulae are made up of bothreflection and emission components such as theTrifid Nebula.
![Emission nebula LHA 120-N 55 in the Large Magellanic Cloud.[5]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aThe_glowing_gas_cloud_LHA_120-N55_in_the_Large_Magellanic_Cloud.jpg)
![Hubble image of emission nebula NGC 2174.[6]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNew_Hubble_image_of_NGC_2174.jpg)
![Hubble image of emission nebula NGC 2313.[7]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCosmic_Silver_Lining.jpg)
| Visible nebula | |
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| Pre-stellar nebulae | |
| Stellar nebula | |
| Post-stellar nebulae | |
| Clouds | |
| Morphology | |
| Intergalactic blobs | |
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