The hydrogen anion is an important constituent of the atmosphere of stars, such as the Sun.

The ion has two electrons bound by the electromagnetic force to a nucleus containing one proton.

It is measured to be 0.754195(19) eV or 0.0277161(62) hartree (see Electron affinity (data page)).

The hydrogen anion is the dominant bound-free opacity source at visible and near-infrared wavelengths in the atmospheres of stars like the Sun and cooler;[2] its importance was first noted in the 1930s.

[7] H− is unusual because, in its free form, it has no bound excited states, as was finally proven in 1977.