I agree that the Sun isn't externally powered. But I think that there is more to it that simple cooling. Not that you really put it that way, but I think that in order to get the actual nature of energy release that we see, you need a charge separation, and for there to be recombination. And then, it's not just that the whole thing cools, and you're left with a huge piece of condensation where there used to be a white-hot plasma. Rather, the lighter elements are blown off, and you're left with the heavier-element core. Why does Venus still have an atmosphere, while Mercury and Mars do not? The solar wind should have swept all of the inner planets clean of their atmospheres a long time ago. And Venus doesn't have a very powerful magnetic field, so it isn't protected the way the Earth's geomagnetic field (supposedly) protects us, by (supposedly) deflecting the solar wind. So what binds Venus' atmosphere in such a way that the onslaught of solar wind doesn't blow it away? That can only be electric charge.