Why does the sun take so long to burn out?

Steve - The sun's doing fusion very slowly. It doesn't have to do it any faster to keep itself held up. If the sun did fusion faster it would expand and then the fusion would go slower and it would contract back down again. If the sun was doing fusion too slowly it would contract until the fusion got fast enough to hold it up. The sun is just self-regulating and that's why it's going to last 5 billion years.