The K-line (or iron K-line or iron Kα, or sometimes in context, just iron line) is an iron spectral line that is useful in classifying stars. "K" indicates the smallest electron shell (n=1), the letters K, L, M, etc., being commonly used for the electron shells when indicating X-ray lines. Kα, which is analogous to hydrogen's Lyman alpha, has a photon energy of 6.4 eV, within the soft X-ray portion of the spectrum. The exact energy depends upon the atom's state of ionization, and line broadening this produces is used to analyze the source, providing a clue to its temperature and constituents.
The K-line is useful in studying the X-ray emission from active galactic nuclei. In the latter case, it can be iron fluorescence powered by even higher energy X-ray photons which are part of a very high-temperature black-body radiation due to the energy release of accretion. It is often the primary line showing a Compton reflection.
Note that the term K-line is also used for a calcium Fraunhofer line.