Red giant causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. The helium fusion results in the build up of a carbon–oxygen core. A star below about will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass
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