the hydrogen fuel that causes a white dwarf to nova or supernova comes from helium.
where does the hydrogen fuel come from to cause a white dwarf to nova or supernova?
helium
classical nova eruptions are the most common type of nova. they are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. when the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. this atmosphere is mostly hydrogen and is thermally heated by the hot white dwarf, which eventually reaches a critical temperature causing rapid runaway ignition by fusion.
nova
a white dwarf's stellar and planetary system is inherited from its progenitor star and may interact with the white dwarf in various ways. infrared spectroscopic observations made by nasa's spitzer space telescope of the central star of the helix nebula suggest the presence of a dust cloud, which may be caused by cometary collisions. it is possible that infalling material from this may cause x-ray emission from the central star. similarly, observations made in 2004 indicated the presence of a dust cloud around the young (estimated to have formed from its agb progenitor about 500 million years ago) white dwarf g29-38, which may have been created by tidal disruption of a comet passing close to the white dwarf. some estimations based on the metal content of the atmospheres of the white dwarfs consider that at least a 15% of them may be orbited by planets and/or asteroids, or at least their debris. another suggested idea is that white dwarfs could be orbited by the stripped cores of rocky planets, that would have survived the red giant phase of their star but losing their outer layers and, given those planetary remnants would likely be made of metals, to attempt to detect them looking for the signatures of their interaction with the white dwarf's magnetic field. other suggested ideas of how white dwarfs are polluted with dust involve the scattering of asteroids by planets or via planet-planet scattering. liberation of exomoons from their host planet could cause white dwarf pollution with dust. either the liberation could cause asteroids to be scattered towards the white dwarf or the exomoon could be scattered into the roche-radius of the white dwarf. the mechanism behind the pollution of white dwarfs in binaries was also explored as these systems are more likely to lack a major planet, but this idea cannot explain the presence of dust around single white dwarfs. while old white dwarfs show evidence of dust accretion, white dwarfs older than ~1 billion years or 7000 k with dusty infrared excess were not detected until the discovery of lspm j0207+3331 in 2018, which has a cooling age of ~3 billion years. the white dwarf shows two dusty components that are being explained with two rings with different temperatures.
white dwarf