Assuming mass-energy equivalence, what would be the diameter of the Sun be given its invariant (resting) mass?

Question

If the Sun could slow down (or cool) instantaneously, how large would the Sun be? The size of the Earth? Pluto? Smaller?

Answer 1

I assume you mean cool to zero temperature or there abouts. Hard to say. It would be classified as a black dwarf. They don't exist in the present universe. To figure it out from scratch, though, even if you assumed it was pure hydrogen and zero tempurature, you would need to know its density as a function of pressure at zero T to figure it out. It's far from an idea gas at the densities entailed. There's probably a theoretical model for this data somewhere, but It would take some looking. I would hesitate to speculate that it would be bigger than earth.

Answer 2

the mass will be slightly smaller, almost negligibly