1. The problem statement, all variables and given/known data Kelvin could only see the photosphere (the glowing outer surface) of the sun, so didn't know how the mass was distributed inside it. How would the potential energy of the sun change if it was a hollow shell? (No numbers needed; just indicate whether it would be more or less than in the uniform density case). Similarly, what if the outer part is just fluff, so that most of the mass is in the center? Which of these two cases makes a bigger difference to our calculation for the lifetime of the sun? 2. Relevant equations Potential energy = GM^2 / r There is a factor in this equation that depends on the density distribution within the object (e.g. 0.6 for a uniform sphere) - but for rough calculations we can ignore that. 3. The attempt at a solution I don't understand how this factor of density distribution fits into the equation. But I'm assuming that if the sun was hollow it would significantly lower it's solar lifetime since the photosphere would have to radiate inside the shell as well thus depleting its potential energy quicker.