What process triggers the sudden brightening observed in a nova event?

A massive thermonuclear reaction on the surface of an accumulating white dwarf

A nova arises specifically within a binary star system that includes a white dwarf. This white dwarf gravitationally captures material, primarily hydrogen, from its orbiting companion star. As this accreted material builds up on the white dwarf's surface, the pressure and temperature eventually become sufficient to ignite a massive thermonuclear reaction. This sudden ignition causes the star system to erupt in a spectacular, temporary brightening. Crucially, this eruption only involves the surface layer of the white dwarf, meaning the star itself survives the event and can potentially repeat the process over vast timescales due to the ongoing accretion cycle.