Caption: Grotrian diagram of Ca II (i.e., once-ionized calcium).

Features:

1. The labeled atomic line wavelengths are in angstroms (Å).

2. Ca II K & H lines (with, respectively, λ = 3933.66 Å and λ = 3968.47 Å) are usually the strongest lines of Ca II seen in an absorption line spectrum from near UV/visible/near IR.

3. This is because the Ca II H & K lines have as their lower energy level the ground state of Ca II as seen in the figure.

   The ground state in all ordinary circumstances has overwhelmingly the largest occupation number among all the energy levels of atoms and molecules.

   So absorption lines arising from the ground state are often very strong since there are many atoms or molecules targets to be excited by incident photons.

4. However, forbidden lines and semi-forbidden lines arising from the ground state can still be weak since they have very low cross sections for photon absorption.

   "Forbidden" and "semi-forbidden" in this context means that the interaction is forbidden by the main process, but can still happen by secondary processes.

   Atomic lines that are NOT forbidden lines or semi-forbidden lines are allowed lines.

5. The Ca II 7291 Å and 7323 Å lines are forbidden lines as indicated by their representation as dot-dash lines in the figure (see Moore & Merrill 1968, p. 3).

6. The Ca II H & K lines are seen in AFGK stars of the OBAFGKM spectral types (see Characteristics of the Stellar Spectral Types, Richard Pogge). They are strongest for the G stars and K stars???.

7. EOF