Editing Nebula (section)

==Formation==
[[File:Nursery of New Stars - GPN-2000-000972.jpg|thumb|[[NGC 604]], a nebula in the [[Triangulum Galaxy]]]]
There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the [[interstellar medium]] while others are produced by stars. Examples of the former case are [[giant molecular clouds]], the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its [[stellar evolution]].

[[Star-forming region]]s are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their [[ultraviolet radiation]] [[ion]]izes the surrounding gas, making it visible at optical [[wavelength]]s. The region of ionized hydrogen surrounding the massive stars is known as an [[H II region]] while the shells of neutral hydrogen surrounding the H II region are known as [[photodissociation region]]. Examples of star-forming regions are the [[Orion Nebula]], the [[Rosette Nebula]] and the [[Omega Nebula]]. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years. 

Other nebulae form as the result of [[supernova]] explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces.  One of the best examples of this is the [[Crab Nebula]], in [[Taurus (constellation)|Taurus]]. The supernova event was recorded in the year 1054 and is labeled [[SN 1054]]. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a [[neutron star]].

Still other nebulae form as [[planetary nebulae]]. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into [[red giant]]s and slowly lose their outer layers during pulsations in their atmospheres.  When a star has lost enough material, its temperature increases and the [[ultraviolet radiation]] it emits can [[ion]]ize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a [[white dwarf]].
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