Editing Emission nebula (section)

==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]].