What element's absorption is notably increased in K stars?

In K stars, the absorption of calcium ions (Ca II) is notably increased due to the specific temperature and luminosity characteristics of these stars. K stars are cooler than other types of stars, particularly G and A stars, which leads to a significant enhancement of the Ca II spectral lines, especially the H and K lines at 3968 Å and 3934 Å.

The absorption occurs because the ionization energy for calcium is well-suited to the temperature of K stars, allowing for a substantial population of neutral and ionized calcium atoms. This results in stronger absorption features in the spectrum of these stars, making Ca II lines prominent.

While other elements like iron, hydrogen, and sodium do also have absorption features in different types of stars, the unique conditions found in K stars lead to the increased visibility of calcium absorption compared to the others mentioned. This distinct feature is a key aspect for astronomers when classifying and studying K stars in stellar astrophysics.