Figure 1: Examples of spectra showing different chromospheric emission levels in four solar-like stars.
From: Observational evidence for enhanced magnetic activity of superflare stars
The spectra show the Ca II H and K absorption lines at 396.8 and 393.4 nm, respectively. The four stars shown here are, bottom to top, KIC 8493735, KIC 9025370, KIC 8552540 and KIC 8396230. The spectra for these four stars have been normalized to one in the spectral range shown, but for clarity we have applied an offset of 0.2 between each spectrum. The measured S index for these four stars are: 0.15, 0.23, 0.30 and 0.34, respectively. The difference in S index can be seen as the increase in emission in the core of the absorption lines. The blue shaded regions show the wavelength bands used to measure the S index.
