calcium H and K lines
/ KAL-see-um H and K /
Among the dark lines crossing the Sun's spectrum, two of the strongest and broadest sit at the violet edge of visible light. Fraunhofer labelled them H and K, and we now know both come from calcium — specifically calcium that has lost one electron, written Ca II or singly-ionised calcium. They are a pair of deep, dark absorption lines at 396.8 and 393.4 nanometres.
Calcium is far less abundant than hydrogen, yet its H and K lines are among the boldest in many stellar spectra. The reason is a matter of conditions: in the cool outer layers of Sun-like and cooler stars, almost every calcium atom is in just the right state — singly ionised and ready to absorb at exactly these wavelengths — so even modest amounts of calcium leave a strong mark. It is a vivid reminder that line strength reflects the physics of the gas, not simply how much of an element is present.
The H and K lines are workhorses of astronomy. Their great strength makes them easy to measure even in faint, distant galaxies, so they are a favourite tool for finding redshifts and motions. In the Sun and similar stars, bright emission peaking inside the dark H and K cores betrays magnetic activity in the chromosphere, letting astronomers track starspots and stellar cycles much like our own eleven-year solar cycle.
When astronomers measure the redshift of a distant elliptical galaxy, the broad calcium H and K lines are often the easiest features to lock onto, because they remain strong even in old, faint starlight.
Two strong violet absorption lines from ionised calcium — bold out of all proportion to how rare calcium is.
The H line shares its letter with the calcium feature only by coincidence of Fraunhofer's labelling; do not confuse it with hydrogen, which it has nothing to do with. Both H and K arise from calcium, not hydrogen.