Historical sketch of the study of stellar activity

It has long been known that emission components are present in the cores of the Ca II H and K resonance lines in the spectra of many stars of spectral type G and later. The discovery paper by Schwarzschild and Eberhard (1913) was followed by a stream of usually brief papers from which lists of stars with references were collected by Joy and Wilson (1949) and Bidelman (1954). In the early 1950s, a vigorous study of the Ca II H and K resonance lines in stellar spectra was started by Olin C. Wilson at the Mt. Wilson Observatory; see Section 9.3. This research was boosted once more when in 1977 the efficient Ca II HK photometer (Vaughan et al., 1978b) was installed at the Mt. Wilson 60-in. telescope. The productivity of the Ca II HK photometer was enhanced by letting researchers (including Ph.D. students) from other institutes use the instrument. The Catania Observatory has a long tradition of pursuing solar and stellar observations in parallel, also aimed at a better understanding of magnetic activity (Godoli, 1967).

From the early investigations onward, the researchers were aware that the mechanisms causing the stellar Ca II H and K emission are probably similar to those of the solar chromosphere.