Question

# Alpha Centauri A lies at a distance of 4.4 light-years and has an apparent brightness in...

Alpha Centauri A lies at a distance of 4.4 light-years and has an apparent brightness in our night sky of 2.7×10−8 2.7 × 10 − 8 watt/m2 w a t t / m 2 . Recall that 1light−year=9.5×1012km=9.5×1015m 1 l i g h t − y e a r = 9.5 × 10 12 k m = 9.5 × 10 15 m. Suppose you have a light bulb that emits 200 watts w a t t s of visible light. (Note: This is not the case for a standard 200-watt light bulb, in which most of the 200 watts w a t t s goes to heat and only about 20-30 watts w a t t s 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 Alpha Centauri A in our sky? (Hint: Use 200 watts w a t t s as L L in the inverse square law for light, and use the apparent brightness given above for Alpha Centauri A. Then solve for the distance.

Therefore light bulb must be placed 5×10¹² m far from it

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