Evolution of coronal X-ray temperature and luminosity along the cycle for the Sun (magenta stars; data adapted from Orlando et al. 2001) and HD 81809 (red filled circles). The figure also reports the simulated data obtained assuming a mixture of solar-like coronal regions: active regions and cores (namely non-flaring regions; AR+CO, blue crosses), and active regions, cores, and flares (AR+CO+FLs, green diamonds). The crossed square (AR+CO+XFL) indicates the synthetic values considering the contribution of a very intense flare (GOES class X9) to the non-flaring corona with maximum coverage of cores (~ 40%). Labels A–D denote the simulated data points corresponding to the EM(T) distributions in Fig. 2. The data from actual spectra of June 2002 and April 2015 are indicated.