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
Hydrogen as dark matter and dark energy.) This element is found in great abundance in stars and gas giant planets. Molecular clouds of H are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction and the CNO cycle of nuclear fusion. Throughout the universe, hydrogen is mostly found in the atomic and plasma states, with properties quite different from those of molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged