Evolution of stellar bolometric emission as a fraction of the value at 5 Gyr for different stellar masses from the stellar evolution models of Spada et al. (2013). The values on the y-axis can be interpreted both as the normalised bolometric luminosities and as the normalised bolometric fluxes in the habitable zone. The habitable zone orbital distances that we use are based on the 5 Gyr stellar properties and are shown in the inset as the dashed black line.