Hydrogen-deficient star radii. Hydrogen deficiency results from stellar evolution. Over the course of a star's evolution, both the consumption of hydrogen in nuclear fusion and the removal of hydrogen layers by explosive processes can lead to a deficiency of hydrogen in its atmosphere. Detailed theoretical models are still in their infancy. Modeling of hydrogen-deficient star evolution involves either a single-star approach or a binary-star approach. For example, there have been two theories put forward to explain the formation of extreme helium stars. The helium final flash scenario is a single-star approach in which a helium flash serves to consume the hydrogen from
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