Giant star mass, when on the main sequence, was below approximately , it will never reach the central temperatures necessary to fuse helium. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf. According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe. In stars above about the core temperature eventually reaches 10 K and helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process. When the core is
Nova 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 a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of