Are coronae of late type stars made of solar-like structures? The Fx-HR   diagram and the pressure-temperature correlation
  We show that stellar coronae can be composed of X-ray emitting structures like those in the solar corona, using a large set of ROSAT/PSPC observations of late-type-stars, and a large set of solar X-ray data collected with Yohkoh/SXT. We have considered data on the solar corona at various phases of the cycle and various kinds of X-ray coronal structures, from flares to the background corona. The surface flux (F_x) vs. spectral hardness ratio (HR) diagram is a fundamental tool for our study. We find that $F_X$ is strongly correlated to HR in stellar coronae, in the solar corona at all phases of the cycle, and in the individual solar coronal structures; all follow the same law. We therefore claim that coronae of late type stars are formed with X-ray structures very similar to the solar ones. In this scenario, the fraction of the stellar surface covered with active regions and with their bright cores increases with activity; the most active stars are brighter and hotter than if they were entirely covered with active regions so they can be explained only with the additional presence of several flares (or flare-like structures) at any time. Using the F_x vs. HR correlation (F_X ~ T^6) we derive new laws relating the temperature, pressure, volumetric heating and characteristic loop length of the coronal plasma, on all the late type stars Individual solar coronal structures and the whole solar corona, follow the same laws. We claim that the strong correlation between surface flux and temperature and the laws mentioned above are just the effect of more fundamental physical mechanisms driving the coronal structures of all the late-type stars from the emergence of new magnetic structures to their dissipation. 
G. Peres, S. Orlando, F. Reale