Editing Emission nebula

{{short description|Nebula formed of ionized gases that emit light of various wavelengths}}
[[File:Ring Nebula.jpg|thumb|Planetary nebulae, represented here by the [[Ring Nebula]], are examples of emission nebulae.]]
An '''emission nebula''' is a [[nebula]] formed of [[ionized gas]]es that emit light of various wavelengths. The most common source of [[ionization]] is high-energy [[ultraviolet]] [[photon]]s emitted from a nearby hot [[star]]. Among the several different types of emission nebulae are [[H II region]]s, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and [[planetary nebula]]e, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.<ref name=wphys>"[https://go.gale.com/ps/retrieve.do?tabID=Reference&resultListType=RESULT_LIST&searchResultsType=MultiTab&retrievalId=a9d210ac-60fe-4f08-8065-9cd44227c4d7&hitCount=12&searchType=BasicSearchForm&currentPosition=1&docId=GALE%7CCV2434500333&docType=Topic+overview&sort=Relevance&contentSegment=ZXBE-MOD1&prodId=SCIC&pageNum=1&contentSet=GALE%7CCV2434500333&searchId=R1&userGroupName=mlin_b_umass&inPS=true Nebulae]." (October 2014) ''World of Physics'', Gale In Context: Science, Doc no. CV2434500333, Retrieved May 27, 2023.</ref><ref name=Hubble>{{cite web| url = https://www.nasa.gov/content/discoveries-hubbles-nebulae| title = Emission Nebulae| last = Belleville| first = Michelle| date = Jan 13, 2023| website = Hubble's Nebulae| publisher = NASA| access-date =May 27, 2023}}</ref>

==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 [[star cluster|cluster]] of young stars is contributing energy.<ref name=wphys/><ref name=Hubble/>

Stars that are hotter than 25,000&nbsp;K generally emit enough [[ionizing radiation|ionizing]] ultraviolet radiation (wavelength shorter than 91.2&nbsp;nm) to cause the emission nebulae around them to be brighter than the reflection nebulae.<ref>{{Cite web|title=20.3 Cosmic Dust - Astronomy {{!}} OpenStax|url=https://openstax.org/books/astronomy/pages/20-3-cosmic-dust|access-date=2022-02-15|website=openstax.org|date=13 October 2016 |language=en}}</ref> The radiation emitted by cooler stars is generally not energetic enough to ionize hydrogen, which results in the [[Reflection nebula|reflection nebulae]] around these stars giving off less light than the emission nebulae. 
[[File:Emission_spectrum-H.svg|thumb|The four visible hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line at the right.]]
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 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 [[stellar spectrum|spectra]] of nebulae, astronomers infer their chemical content. Most emission nebulae are about 90% hydrogen, with the remaining [[helium]], [[oxygen]], [[nitrogen]], and other elements.

Some of the most prominent emission nebulae visible from the [[northern celestial hemisphere]] are the [[North America Nebula]] (NGC 7000) and [[Veil Nebula]] NGC 6960/6992 in [[Cygnus (constellation)|Cygnus]], while in the south celestial hemisphere, the [[Lagoon Nebula]] M8 / NGC 6523 in [[Sagittarius (constellation)|Sagittarius]] and the [[Orion Nebula]] M42.<ref>{{cite web |last1=McArthur |first1=Frommert |last2=Kronberg |first2=Christine |date=12 April 2006 |title=Messier 42 |work=Messier Object Index |url=http://messier.seds.org/m/m042.html |access-date=17 July 2007}}</ref> Further in the southern hemisphere is the bright [[Carina Nebula]] NGC 3372.

Emission nebulae often have dark areas in them which result from [[interstellar medium|clouds]] of dust which block the light.

Many nebulae are made up of both [[reflection nebula|reflection]] and emission components such as the [[Trifid Nebula]].

==Image gallery==
<gallery>
The glowing gas cloud LHA 120-N55 in the Large Magellanic Cloud.jpg|Emission nebula [[LHA 120-N 55]] in the [[Large Magellanic Cloud]].<ref>{{cite web|title=A Beautiful Instance of Stellar Ornamentation|url=https://www.eso.org/public/news/eso1616/|date=18 May 2016|access-date=25 January 2023}}</ref>
New Hubble image of NGC 2174.jpg|Hubble image of emission nebula [[NGC 2174]].<ref>{{cite news|title=Hubble revisits the Monkey Head Nebula for 24th birthday snap|url=https://esahubble.org/news/heic1406/|date=17 March 2014|access-date=25 January 2023|newspaper=ESA/Hubble Press Release}}</ref>
Cosmic_Silver_Lining.jpg|Hubble image of emission nebula [[NGC 2313]].<ref>{{cite news|title=Cosmic Silver Lining|url=https://esahubble.org/images/potw2119a/|access-date=25 January 2023}}</ref>
</gallery>

==See also==
*[[Herbig–Haro object]]
*[[N41 (nebula)]]

==Further reading==
*{{Citation
| last = O'Meara 
| first = Stephen James 
| author-link = 
| title = Discover 10 weird emission nebulae
| newspaper = [[Astronomy (magazine)|Astronomy magazine]]
| date = August 14, 2019
| url = https://www.astronomy.com/observing/discover-10-weird-emission-nebulae/
| archive-url = 
| archive-date = 
| access-date =May 27, 2023}}
*{{cite web
| url = https://www.astronomy.com/?s=Meet+the+nebula
| title = Meet the nebula
| date = 2023
| website = 
| publisher = Astronomy magazine
| access-date = May 27, 2023}}

==References==
{{Commons category|Emission nebulae}}
{{Reflist}}

{{Nebulae}}
{{Authority control}}

[[Category:Emission nebulae|*]]
[[Category:Nebulae]]