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If the Sun's temperature were to rise from T to 2T and its radius from R to 2R, by what factor would the radiant energy received per unit area at the Earth increase compared with before?

1. 4
2. 16
3. 32
4. 64

Correct answer: 64

Solution

Luminosity is proportional to R²*T⁴; doubling both gives (2²)*(2⁴) = 4*16 = 64, and the Earth-Sun distance is unchanged so the received flux rises by 64.

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