Let's repeat the calculation from last time about the length of time you can power the Sun, only this time use nuclear reactions.
We can now repeat the calculation done earlier as to whether this fuel source can keep the Sun shining for a sufficiently long period of time. In the case of chemical reactions, we found that the Sun would burn out after less than two hundred years.
Remember that the luminosity of the sun is
Watts. The requirement is that E= P T, where E is the
total amount of energy in its fuel tank and T is the amount of time it can keep
this up.
The total mass of the Sun is 
kilograms.
Of that, only about the inner 10 percent is at high enough temperatures for
these reactions to be occurring. So we have 
kilograms to
work with.
Of this, 0.7 % or 
kilograms is converted into energy.
Now, we have
Joules.
The time it can keep the Sun shining is
seconds.
Question for audience: How many years is this?
Much more detailed calculations essentially bear out this result.