As an addendum to some of the (semi drunken) discussions we had after breakfast, The Sky At Night episode on iPlayer currently, briefly talks about the sun, and the rate at which mass is converted into energy. I thought that the sun lost around half a million tons a second, but that programme states the rate of loss as being around four million tons a second, so I underestimated it by around an order of magnitude!
Simon also queried what the pressure inside the Sun was, and I didn't know a number of the top of my head. A quick check suggests that near the centre of the sun, where fusion occurs, and hence that's also where the mass loss occurs, the pressure is probably just over 100,000,000,000 atmospheres, or over 7,000,000,000 times the pressure in a road bike tyre. It needs to be quite high for fusion to occur, but the force produced by that then stops the Sun compressing any further. If you want really high pressures, you can look at the interior of a neutron star, where the internal pressure is probably around 1,500,000,000,000,000,000,000,000,000,000 atmospheres, and the only thing stopping it collapsing infinitely (which is in effect what happens with a black hole) is quantum degeneracy pressure.
