Nova outbursts are cause by a thermonuclear runaway reaction (TNR) on the surface of a white dwarf star. The star accretes hydrogen rich material from a secondary star.
In Classical Novae hydrogen burning occurs mainly via CNO cycle. The white drarf is cold and accretes matter through Roche lobe overflow of the main sequence binary companion. The accretion timescale is in the order of 10,000 to 100,000 years, and depends upon the mass of the stars, nuclear timescale, and the dynamical timescale. Thermonuclear runaway reaction occurs when degenerate conditions exist, preventing further envelope expansion. During the TNR convection brings new material to the burning shell.
Novae contribute to the chemical evolution of the galaxy due to nucleosynthesis. Overproduction compared to solar abudances occur for CO nova (Li, C, N, O, F, Al), and ONe novae (Li, C, N, O, F, Ne, Na, Mg, Al, Si, P, and S). Isotopes of galactic 17O are almost entirely produced by novae. Novae contribute to radioactivity of the galaxy. Gamma-rays produced by 13N and 18F have a short duration of several hours and occur before visual maximum. Medium-lived nuclei are 7Be and 22Na. Long-lived isotope 26Al has been observed by HEAO3 satellite at 1809 keV.