> Essentially, when a single molecule of this protein absorbs a single 
> photon, it undergoes a structural change which causes it to react with 
> another protein, which causes one of the ion pumps on the cell wall to 
> shut off, thereby macroscopically altering the ion concentration inside 
> the cell, generating a minute but measurable net charge for the entire 
> cell, which leads to a nerve impulse being fired... or something like 
> that. It's *complicated*!

With some quick calculations and rather dubious assumptions I estimate that 
from a star like Betelgeuse approximately 10^6 photons are going into your 
eye per second.  This is about the same as a 100 W light bulb 10 km away.

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