Which spectral classification features a combination of He I and He II lines?

The classification of stars into spectral types is based on their temperature and the presence of various absorption lines in their spectra. In the case of B stars, they are characterized by high temperatures ranging from about 10,000 to 30,000 Kelvin.

What distinguishes B stars is the presence of both neutral helium (He I) and ionized helium (He II) lines in their spectra. The detection of these lines signals that the star's surface temperature is sufficiently high to ionize helium, leading to the formation of He II lines, while still having enough energy to retain neutral helium, resulting in He I lines as well.

O stars, characterized by even higher temperatures, primarily exhibit He II lines and few He I lines. A stars have cooler temperatures and primarily show absorption features from hydrogen, with no significant presence of He II. F stars have temperatures that are even lower still, and their spectra do not include the features found in B stars.

Thus, the spectral classification that encompasses both He I and He II lines is specifically associated with B stars.