About 5 billion years from now, the Sun will begin to swell into a red giant. This will cause some problems, because life on a planet orbiting a red giant is hard. Even if Earth isn't engulfed by the expanding Sun, it's going to be scorched. We're looking at surface temperatures anywhere from 500$^\circ$F to about 3200$^\circ$ F, which is highly unpleasant. 3 billion years later, the Sun will have settled down to become a white dwarf, surrounded by a protoplanetary nebula, and things will become relatively quiet and boring for what remains of the Solar System.
I'm working on a story set in the far future. Specifically, it involves a couple of intrepid time travelers intent on seeing the wonders of the galaxy - pretty standard time travel stuff. Most of their journey is irrelevant for now, but the key event is that they decide to travel 8 billion years into the future to see what the protoplanetary nebula around the Solar System looks like, out of sheer morbid curiosity. I'd like them to find, to their surprise, that the Sun is no longer a white dwarf, but a normal (-ish?) star, insofar as it's undergoing significant nuclear fusion again. Life might even be possible again, at some part of the Solar System.
Needless to say, our explorers are more than a little shocked. The thing is, this whole premise rests on the idea that after the Sun becomes a white dwarf, it could somehow revert to a normal star, and when I started writing, I wasn't sure if this would be possible. Can the Sun somehow, 8 billion years in the future, leave the white dwarf track and start fusing hydrogen, helium, or heavy elements again? Here are my criteria, which are pretty strict:
- Fusion must be stable on a timescale of at least a few thousand years. That's admittedly small on stellar timescales, but I'm not pushing my luck. A few million years would be nice, but my expectations are low.
- The Sun must have a luminosity at least half as large as its current luminosity - preferably a couple times greater.
- I'd rather there not have been any catastrophic astronomical events, like a collision (or interaction) with another star, because then there's the possibility that the Solar System could be thrown into disarray.
- The event should be natural, not prompted by an advanced civilization or anything far-out like that. For instance, I would disallow star lifting, or the creation of some sort of megastructure.