Editing Nebula (section)

===Supernova remnants===

[[Image:Crab Nebula.jpg|thumb|The [[Crab Nebula]], an example of a [[supernova remnant]]]]

A [[supernova]] occurs when a high-mass star reaches the end of its life.  When [[nuclear fusion]] in the core of the star stops, the star collapses.  The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.<ref name="shu1982"/> The expanding shell of gas forms a [[supernova remnant]], a special [[diffuse nebula]].<ref name="shu1982"/> Although much of the optical and [[X-ray]] emission from supernova remnants originates from ionized gas, a great amount of the [[radio wave|radio]] emission is a form of non-thermal emission called [[synchrotron emission]].<ref name="shu1982"/> This emission originates from high-velocity [[electron]]s oscillating within [[magnetic field]]s.