Hydrogen ($\mathrm{H}$)

Metallic hydrogen is a high-pressure conducting state of hydrogen. Its fluid form in the deep interiors of gas giants supplies a conducting medium for a planetary dynamo, unlike the iron-alloy conducting cores of terrestrial planets.

This stable hydrogen isotope has one proton and one neutron. In the Proton–proton chain it is produced by the slow weak proton-pair reaction and is rapidly consumed by proton capture to make helium-3. Its short nuclear lifetime inside a stellar core allows a deuterium quasi-equilibrium in proton-proton burning approximation.

A negative hydrogen ion is a hydrogen nucleus with two bound electrons. Its weakly bound extra electron permits absorption at visible and infrared wavelengths in cool stellar atmospheres.