The H and K lines are prominent absorption lines in the spectra of stars like the Sun and cooler due to singly ionized calcium (Ca II).

 

The H line is produced by the transition of an electron from the n = 3 to n = 2 energy level, while the K line is produced by the transition of an electron from the n = 4 to n = 2 energy level.

 

Named by Joseph von Fraunhofer, they occur in the near-ultraviolet at wavelengths of 3969 and 3934 Å, respectively (see Fraunhofer lines). Emission in the H and K lines is also common in some kinds of eruptive variable, such as flare stars and RS Canum Venaticorum stars.