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Fundamentals Of Physics
Found in: Page 1332

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Short Answer

The Sun has mass 2.0x1030kgand radiates energy at the rate3.9x1026W. (a) At what rate is its mass changing? (b) What fraction of its original mass has it lost in this way since it began to burn hydrogen, about4.5x109y ago?

  1. The rate at which the mass of the Sun is changing is4.3×109kgs.
  2. The fraction of the original mass lost in this way is3.1×10-4.
See the step by step solution

Step by Step Solution

Step 1: The given data

  1. Mass of the Sun,msun=2.0x1030kg
  2. Rate of energy radiation,P=3.9x1026W
  3. Time of change,dt=4.5x109y

Step 2: Understanding the concept of energy change

In the given problem, the Sun radiates energy in the form of electromagnetic radiation. Thus, this change in energy per given time is given by the change in its mass rate as per the energy relation given by Einstein's energy relation. This results in a change in the mass of the sun which is given by the duration of the change.


The rate of energy in the form of radiationas follows:

P=dEdt…… (i)

The energy as per the Einstein’s mass-energy relation:

E=mc2…… (ii)

The mass change of a body is as follows:

m=dmdtt…… (iii)

Step 3: a) Calculate the rate of mass changing

Letmbe the mass of the Sun at time t andEbe the energy radiated to that time. Then, the power output is given using equation (ii) with the given data in equation (i) as follows:


Substitute the values and solve as:


Substitute the values and solve as:

dmdt=3.9×1026W3×108ms2 =4.3×109kgs

Hence, the rate of mass change is4.3×109kgs.

Step 4: b) Calculate the original mass lost by the Sun

Using the given data and the above calculated data in equation (iii), the mass change of the body in the given timeis calculated as follows:

m=4.3×109kgs4.3×109y3.156x107sy =6.15×1026kg

Now, the required fractional change of this quantity for the given original mass of the Sun is calculated as follows:

mm+m=6.15×1026kg2.0×1030kg+6.15×1026kg =3.1×10-4

Hence, the value of fraction is3.1×10-4.

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