Editing Planetary nebula (section)

==Lifetime==
[[File:Necklace Nebula.jpg|thumb| The [[Necklace Nebula]] consists of a bright ring, measuring about two light-years across, dotted with dense, bright knots of gas that resemble diamonds in a necklace. The knots glow brightly due to absorption of ultraviolet light from the central stars.<ref>{{cite web|title=Hubble Offers a Dazzling Necklace|url=http://www.spacetelescope.org/images/potw1133a/|work=Picture of the Week|publisher=ESA/Hubble|access-date=18 August 2011}}</ref>]]

After a star passes through the [[asymptotic giant branch]] (AGB) phase, the short planetary nebula phase of stellar evolution begins<ref name=KwokJun2005 /> as gases blow away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB.<ref name=KwokJun2005 /> As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling when the hydrogen shell is exhausted through fusion and mass loss.<ref name=KwokJun2005 /> In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse.<ref name=Kwok1/><ref name=KwokJun2005 /> During the first phase, the central star maintains constant luminosity,<ref name=KwokJun2005 /> while at the same time it grows ever hotter, eventually reaching temperatures around 100,000&nbsp;K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a [[white dwarf]], and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution.<ref name=KwokJun2005 /> For a typical planetary nebula, about 10,000&nbsp;years<ref name=KwokJun2005 /> passes between its formation and recombination of the resulting [[Plasma (physics)|plasma]].<ref name=Kwok1/>