13. [3pt] The star Lelande 21 185 lies at a distance of 8.6 light-years and has
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Question
13. [3pt]
The star Lelande 21 185 lies at a distance of 8.6 light-years and has an apparent brightness in our night sky of 2.6×10-11 W/m2. Use the inverse square law for light to calculate the luminosity of Lelande 21 185.
Answer:
14. [3pt]
Suppose you have a light bulb that emits 200 watts of visible light. (Note: This is not the case for a standard 200-watt light bulb, in which most of the 200 watts goes to heat and only about 20-30 watts is emitted as visible light.) How far away would you have to put the light bulb for it to have the same apparent brightness as Lelande 21 185 in our sky?
Explanation / Answer
13. I = L / (4 pi R^2)
R = 8.6 x 3 x 10^8 x 365 x 24 x 3600
and I = 2.6 x 10^-11
L = 2.163 x 10^24 W
14. 2.6 x 10^-11 = (200) / (4 pi r^2)
r = 782 x 10^3 m or 782 km
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