Could the Sun be born again?
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.
1 answer
I actually figured this out shortly after beginning the story, and I'm going to therefore post a self-answer, because it might be useful to others. Obviously, I'm still open to more ideas if this one is flawed or incomplete.
TL;DR: Yes, it could work, on short enough timescales.
So, the Sun will transition from a red giant to a white dwarf as it traverses the asymptotic giant branch of stellar evolution, becoming an AGB star. It will remain in this phase for a few million years at the most, losing dramatic amounts of mass via stellar pulsations. After this extreme mass loss - on the order of half a solar mass - it will transition into a post-AGB star, and head towards the white dwarf track.
Now, the pulsations in the star continue throughout the AGB phase, and include shell helium flashes, where helium fusion suddenly begins in a shell of hydrogen in the star. This happens several times for an AGB star, and may continue as the Sun transitions through the post-AGB phase and forms a planetary nebula.
Under the right conditions, something called a very late thermal pulse may occur as the star enters the white dwarf branch. This rapidly depletes the hydrogen and can initiate the fusion of heavy elements inside the star. The now former white dwarf travels back to the AGB phase extremely quickly - within decades or a century - and begins life as an AGB star again, staying on the branch for hundreds of years, depending on its mass loss. At the end of this era, the star will be forced to become a white dwarf for good, as it will have lost almost all of its hydrogen.
So, does this happen? Very likely! There are several notable candidate cases:
- Sakurai's Object, which began to display this behavior in 1996. The very late thermal pulse model was applied by Duerbeck & Benetti (1996) later that year.
- V605 Aquilae, which brightened in 1918. It may also have undergone a nova instead (or merged with another star), but the born-again hypothesis remains plausible.
- FG Sagittae, which may have begun to transition back to an AGB star about a century ago. It has continued cooling, which makes sense, as white dwarfs are usually hotter than AGB stars or supergiants.
FG Sagittae may have stopped cooling; the first two stars will likely stop cooling in a few decades (Lawlor & MacDonald (2003)). After that, they will enter the AGB phase again. Now, Sakurai's Object formed a planetary nebula $\sim$17,000 years ago, and it's still visible. Protoplanetary nebulae don't last long, on astronomical timescales, but this one isn't going anywhere any time soon.
Now, let's say the Sun also undergoes a very late thermal pulse. What sort of conditions are we looking at? The three objects above give us a pretty good idea:
- It will have a mass about half that of its current mass, as per models of AGB evolution.
- Its luminosity will be $\sim$10,000 times its current luminosity. This means that the effective temperature of a planet $\sim$100 AU from the star will have the same effective temperature as Earth does right now.
- The cooling phase should last about a century, and the second AGB phase will last at least several hundred years (models predict different timescales; see Herwig (2001)). $\sim$1,000 years is likely the upper limit.
Earth likely won't be habitable, although it's possible that Planet Nine (or any moons it might have), at its closest approach to the Sun, would be (its orbital elements aren't terribly well-constrained). The Kuiper Belt would also be warmer, which would be nice. Perhaps even the outer ice giants could provide refuge for any life-forms daring enough. Of course, 1,000 years (or several hundred years!) isn't much time at all for life to arise, but perhaps life could arrive via panspermia. It's still possible.
Some less well-developed ideas
I also had some other, less promising ideas that could still work, so I'll outline them here. I like them a bit less because of a lack of observational support for their evolutionary paths.
So, if fusion begins again, the Sun is going to be a hydrogen-deficient star, because it will have lost all of its hydrogen during the AGB and post-AGB phases. Now, we know of some classes of stars that are hydrogen-deficient, besides AGB stars:
- Wolf-Rayet stars, massive, luminous and unstable supergiants that fuse helium.
- O-type and B-type subdwarfs, which have a large helium layer surrounding a carbon and oxygen core.
The Sun will never become a Wolf-Rayet star; it's not massive enough. But these particular subdwarfs show promise. Subdwarf B stars may form when red giants prematurely lose their outer layers of hydrogen (possibly on the horizontal branch), and may evolve in subdwarf O stars (which could also form from the merger of white dwarfs). The only problem, of course, is that this early hydrogen loss is not yet well-understood; a binary system may be necessary.
0 comment threads
0 comment threads