An Etymological Dictionary of Astronomy and Astrophysics

Any of the → spectral lines arising from → singly ionized helium in the atmosphere of → O-type and → Wolf-Rayet stars. He II lines are chiefly in absorption, but some of them, such as 4686 Å occur in emission in hotter stars. The presence of He II → absorption lines separates O types from → B-type stars.

A number of these He II lines belong to the → Pickering series involving transitions with → principal quantum numbern = 4 and higher.

Although the n = 3-4 (4686 Å) transition also belongs to ionized helium and often occurs in these hot stars, it does not belong to the Pickering series because it has a lower landing level quantum number (n = 3).
The same goes for n = 2-5 (4026 Å).

See also: → helium; → line.