Example 12.1 Assume that the Sun converts 0.8% of its mass into energy. Find an upper limit for the age of the Sun, assuming that its luminosity has remained constant.

The total amount of energy released is E = mc2 = 0.008 MQc2 = 0.008 x 2 x 1030 kg x (3 x 108 ms-1)2 = 1.4 x 1045J .

The time needed to radiate this energy is _ E _ 1.4 x 1045J t = L^ = 3.9 x 1026W = 3.6 x 1018s - 1011 years .

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