Nova
If the orbital period of the system is a few days or less, the white dwarf is close enough to its companion star to draw accreted matter onto its surface, creating a dense but shallow atmosphere.
This atmosphere, mostly consisting of hydrogen, is heated by the hot white dwarf and eventually reaches a critical temperature, causing ignition of rapid runaway fusion.
The sudden increase in energy expels the atmosphere into interstellar space, creating the envelope seen as visible light during the nova event.
Under certain conditions, mass accretion can eventually trigger runaway fusion that destroys the white dwarf rather than merely expelling its atmosphere.
[1] During the sixteenth century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia.
In this work he argued that a nearby object should be seen to move relative to the fixed stars, and thus the nova had to be very far away.
One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star.
The second star—which may be either a main-sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe.
As the white dwarf consists of degenerate matter, the accreted hydrogen is unable to expand even though its temperature increases.
Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning via the CNO cycle.
This blows the remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light.
[2][6] In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova also can emit gamma rays (>100 MeV).
[7] Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star.
Eventually, the white dwarf can explode as a Type Ia supernova if it approaches the Chandrasekhar limit.
[12] Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.
[2] V Sagittae is unusual in that the time of its next eruption can be predicted fairly accurately; it is expected to recur in approximately 2083, plus or minus about 11 years.
On 20 April 2016, the Sky & Telescope website reported a sustained brightening of T Coronae Borealis from magnitude 10.5 to about 9.2 starting in February 2015.
