Caltech Question of the Month: If a lightbulb were one light-year away, how many watts would it have to be for us the see it with the naked eye?
Submitted by R. Anderson of Pomona, California, and answered by Dr. George Djorgovski, Professor of Astronomy
Star brightness is measured on a magnitude scale. The higher the magnitude, the less bright the object is. For example, Jupiter shines at about -2.5 in the night sky. The dimmest naked-eye object that we can see in the night sky (assuming we are looking someplace where it is dark, i.e., not Los Angeles) is 6th magnitude. Therefore, for the light from a lightbulb one light-year away to be 6th magnitude when it reaches Earth, the bulb would have to emit 10^27 watts of power. That is a billion, billion, billion watts.
Meanwhile, the faintest objects we can see with the Hubble Space Telescope, or the 10-meter Keck Telescopes are a few billion times fainter than what an unaided human eye (with a good vision) can see. While even these telescopes would not allow us to see a regular lightbulb placed one light-year away, they could easily detect a lightbulb on the Moon.