The trick is now how to measure the total flux (which is referred to as the bolometric flux). The atmosphere of the Earth only allows the visible and radio (plus some bands in other wavelength regions to hit the Earth's surface) and so, in order to measure the bolometric luminosity of the Sun one must send probes above the Earth's atmosphere to measure the Solar spectrum. The Solar luminosity is 3.9x1026 Watts.
This is the way in which other star's luminosities must also be determined. The bolometric flux is always difficult to measure, but the major difficulty is finding accurate distances to stars.
