How different measures of X-ray emission scale with hardness ratio for stars of different masses. The hardness ratios and levels of X-ray emission were measured by ROSAT and collected using the NEXXUS database (Schmitt & Liefke 2004). The blue and red points represent stars with masses below and above 0.65 M_⊙ respectively. The three quantities of interest are X-ray luminosity (left panel), X-ray surface flux (middle panel), and X-ray luminosity divided by the bolometric luminosity (right panel). In order to calculate F_X and R_X we estimate the surface areas and the bolometric luminosities by assuming and .