Editing White dwarf (section)

=== Cataclysmic variables ===
{{Main|Cataclysmic variable star}}
Before accretion of material pushes a white dwarf close to the Chandrasekhar limit, accreted hydrogen-rich material on the surface may ignite in a less destructive type of thermonuclear explosion powered by [[Nuclear fusion|hydrogen fusion]]. These surface explosions can be repeated as long as the white dwarf's core remains intact. This weaker kind of repetitive cataclysmic phenomenon is called a (classical) nova. Astronomers have also observed [[dwarf nova]]e, which have smaller, more frequent luminosity peaks than the classical novae. These are thought to be caused by the release of [[gravitational potential energy]] when part of the [[accretion disc]] collapses onto the star, rather than through a release of energy due to fusion. In general, binary systems with a white dwarf accreting matter from a stellar companion are called [[cataclysmic variable]]s. As well as novae and dwarf novae, several other classes of these variables are known, including [[Polar (star)|polars]] and [[intermediate polar]]s, both of which feature highly magnetic white dwarfs.<ref name="osln" /><ref name="sniamodels" /><ref name="nasa1">{{cite web |url=http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html |series=Imagine the Universe! |title=Cataclysmic Variables |archive-url=https://web.archive.org/web/20070709185919/http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html |archive-date=9 July 2007 |department=fact sheet |publisher=NASA Goddard |access-date=4 May 2007}}</ref><ref name="nasa2">{{cite web |url=http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |title=Introduction to Cataclysmic Variables (CVs) |archive-url=https://web.archive.org/web/20120206213752/http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |archive-date=6 February 2012 |url-status=live |department=fact sheet |publisher=NASA Goddard |access-date=4 May 2007}}</ref> Both fusion- and accretion-powered cataclysmic variables have been observed to be X-ray sources.<ref name="nasa2" />