The Pickering series consists of three lines of singly ionized helium found, usually in absorption, in the spectra of hot stars like Wolf-Rayet stars. The name comes from Edward Charles Pickering. The lines are produced by transitions of an electron with principal quantum number n=4 state from a higher energy level. The lines have wavelengths:
- 4551 angstroms (n=9 to n=4)
- 5411 angstroms (n=7 to n=4)
- 10123 angstroms (n=5 to n=4)
The n=6 and n=8 transitions overlap with hydrogen lines and were not seen.
Pickering assumed that the lines were created by a new form of hydrogen with half-integer transition levels. This turned out to be false.