  Some entities, such as humans, survive longest if their environment is
neither too hot nor too cold, and the sun is no exception. It is rather obvious
that if the sun were enclosed inside a thermally conducting sphere surrounded
by a heat bath kept much hotter than the present central temperature of the
sun, its nuclear burning would occur faster, so that the sun would last for a
shorter time. It is less obvious that if the sun were surrounded by a perfectly
reflecting sphere to prevent its radiation from escaping to cold empty space,
it could actually last longer. Here I shall show that this is the case for such
a sphere at least somewhat larger than the present solar radius. This naively
paradoxical result is a consequence of the negative specific heat of many
gravitating systems, so as the energy emitted by the sun is reflected back to
increase the thermal energy, the sun expands and its central temperature goes
down rather than up and reduces the nuclear burning rate, so that the sun can
last much longer than five billion years, for a lifetime growing roughly
exponentially with the cube root of the radius of the perfectly reflecting
sphere.
