The threshold intensity for a human eye to sense the presence of light is 50 photons falling on the retina within a period of 0.1 s. The Sun radiates an output of 2.0x10²⁶ W in the visible wavelengths, with a peak at λ = 550 nm. Estimate how far away a star like the Sun can be and still be visible to the unaided eye on a dark night.