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
Fleming 1 a torus of expanding hot gas forming the inner bright ellipse. Fleming 1 is probably 5,000 years old. Like any other planetary nebula Fleming 1 was formed when an old asymptotic giant branch (AGB) star lost its outer hydrogen-rich envelope, leaving behind a hot core (young white dwarf)—the central star of the nebula. The star in the center of Fleming 1 has a temperature of and mass of 0.56 . Observations performed by European Southern Observatory showed that the central star is in fact a double degenerate (made of two white dwarfs) binary with a period of days. The companion