Hydrogen-deficient star the outer layer of the star. The double degenerate scenario is a binary-star approach in which a smaller degenerate helium white dwarf and a larger carbon-oxygen white dwarf orbit each other so closely that they eventually inspiral due to gravitational wave losses. At the Roche limit, mass transfer takes place from the helium to the carbon-oxygen star. The latter undergoes helium shell burning to form a supergiant and evolve to a hydrogen-deficient star. The double degenerate scenario provides a better fit to the observational data. Hydrogen-deficient star A hydrogen-deficient star is a type of star that has little or no
Red giant temperature and pressure are adequate to cause fusion to resume in a shell around the core. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the star's life. As the star expands, the energy produced in the burning shell of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red—hence it becomes a "red giant". At this time, the star is said to be ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram. The evolutionary path