Experiments have shown that the nervous system of the human eye effectively takes about 30 "frames" per second (like a movie camera) and that when the eye is fully dark-adapted, it needs to receive only about 500 photons per frame from an object to register it. Our sun radiates a power of about 3.9 x 1026 W at all wavelengths, peaking in the yellow region of the spectrum (only about onehalf of this energy is in the visible range, however). Estimate how far away a star like the sun could be
and still be visible.