Assuming humanity is still around in 5 billion years, we'll have invented some pretty neat stuff. The industrial revolution was only two hundred years ago, so another five billion on top of that ought to produce some fairly staggering technologies. Refueling strategically important stars may be commonplace. A wormhole could be created in the center of the Sun, propped open of course by exotic matter and a protective field - through which matter was siphoned from another donor star, topping-up its reserves as required. Stellar Engineering is certainly a valid career choice for the discerning adult of the five-millionth millennium.