A sequence of emission and absorption lines in the visible part of the spectrum, caused by the singly ionized helium atom, He II. Such an ion is very similar to a hydrogen atom in that both contain only a single electron. However, the helium nucleus is four times heavier and has twice the nuclear electric charge. For these reasons, alternating lines in the Pickering series almost align with lines in the hydrogen Balmer series: Pickering-β‎, for example, is at 656 nm, close to the Ηα‎ line. The series limit is at 364.4 nm (almost coincident with the Balmer limit). Four times as much energy is needed to ionize a helium atom as a hydrogen atom, so He II lines are seen only in very hot stars (spectral type O and Wolf–Rayet stars) and accretion disks. The Pickering series is named after E. C. Pickering.