The Sun has a mass of and radiates energy at the rate of . (a) We have to find the rate with which the mass of the Sun is decreasing. (b) We have to find the fraction of the original mass of the Sun that it has lost in this way since it began to burn hydrogen ago.

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(a)

The Sun has a mass of and radiates energy at the rate of . We use the Einstein’s mass-energy relation for calculating the rate of decrease in mass of the Sun.

(b)

The mass lost by the Sun since it began to radiate ago will be

The fraction of the original mass of the Sun lost till now will therefore be