One further question related to this. Would I be correct in saying that this means that at any moment around 10^-7 of the Sun's mass is actually made up of radiant energy?

I guess that the choice of word "mass" there might be contentious (equally so if "energy" is used). So I think I need to add that a large bunch of photons bouncing about for a long time more or less in place do have mass in the sense that the ensemble is not in motion. If someone can suggest better words to describe this then that would also be appreciated.