H and K lines
The H and K lines of ionized calcium appear to the far left in this spectrum showing the most prominent Fraunhofer lines.
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). 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.