Since they were first discovered in 1952, blue stragglers have baffled astronomers. Appearing strangely youthful amid their ageing brethren, many now believe blue stragglers are formed when two smaller stars merge. There is, however, one theory which is rather more… outlandish. In a fascinating paper dating back to 1990, Martin Beech suggests an alternative explanation. What if an advanced civilization, having invested so much time and effort into their home star system, decided to artificially prolong the lifetime of their host star?
There are billions of stars in the Milky Way. Assuming that we’re not alone in the Universe and some of those stars also have civilisations around them, statistically at least a handful of civilisations must be facing mortal peril. A fair amount of discussion has been devoted over the years to how an advanced civilisation may survive the demise of their parent star. Stars are, after all, inconstant. After leaving the main sequence and swelling into a red giant the slow inexorable increase in stellar radius and luminosity would steadily fry the inner planets. Most discussion about how to survive this is centred around a civilisation moving their homeworld to a higher orbit (keeping the stellar radiation they receive constant) or simply fleeing to the expanses of space in the hope of finding a new homeworld capable of weathering a few billion years more. The possibility explored by Beech is, at least in concept, simple. Stop your host star leaving the main sequence, and your homeworld could simply stay where it is.
Simply put, it’s possible to drastically alter the course of stellar evolution, and if a civilisation were to do so, it’s host star would bear a striking resemblance to one of the mysterious blue stragglers. Stars leave the main sequence when their cores run out of hydrogen to fuse, so they start to fuse helium instead. The thing is, the bulk of the star will still be rich in hydrogen. If you could mix it all together, chemically homogenising the star, it could burn for longer. This is astroengineering in its truest sense — the engineering of a whole star. But that isn’t all that’s required. A chemically homogenised star would still evolve to have a hugely greater luminosity, and while it’s lifespan would be longer, it wouldn’t be significantly longer. The key to this problem lies in mass loss. Less mass means less pressure in the core, and less pressure in the core means less helium burning, resulting in a lower output. The end result is that the star becomes much hotter (and therefore appears bluer), while maintaining roughly the same luminosity output and with a minimal variation in its radius.
So by chemically homogenising the star and causing it to lose a sufficient amount of mass (around 60-80% of it’s original mass over the course of its life is about ideal), a star’s lifespan can be increased to almost 10 times it’s natural length. This means that for a star like the Sun, instead of lasting for only 8 billion years on its main sequence, it could last closer to 80 billion years! Even if only one civilisation in the whole universe has achieved this, they wouldn’t need to move anywhere for a long, long time to come.
As for how to achieve all of this, there are a few ideas. Mixing the star’s interior could be achieved by creating hotspots inside the star, near its core, to cause convection. This could be done with hydrogen bombs, for instance, or with huge targeted laser blasts. Interestingly, this means the human race already has the technology (albeit not the ability) to do this! Alternatively, an increased rotation speed or magnetic field could cause mixing within the star (and many blue stragglers have been seen to have higher rotation speeds than average).
For the mass loss, an increased rotation rate might cause more material to be centrifuged off the star’s surface. Another idea, actually considered by Hawking to explain blue stragglers, involves a relatively small black hole centred in the star’s core, which would cause the effective mass loss while devouring the star from the inside.
Personally though, I’d like to add the sci fi option. Virtually unimaginable for a race with our current level of technology, “Star lifting” is a concept which might allow an advanced civilisation to mine a star. Constrained to science fiction for us, the Star Forge from the Star Wars universe (pictured to the left) is quite a good illustration. Mining a star this way would give two benefits to such a civilisation, both helping them prolong their star’s lifespan, and providing a virtually limitless supply of resources for the duration of that lifespan.
Of course, Beech doesn’t intend to make the claim that all blue stragglers are artificially produced (indeed, some may be stealing fuel from their neighbours), admitting that his discussion is, by definition, quite speculative. Simply the possibility that some of these curious stars might be artificially created is a fascinating one. Perhaps he has a good point, and perhaps blue stragglers might be a good place to look for evidence of advanced civilisations and astroengineering activities — particularly any which lie outside the globular clusters where most of them usually reside. While admittedly, such civilisations would still eventually have to venture out into space to fend for themselves, they may not need to do so for significantly longer than we’d previously thought.
Delaying the demise of your host star… You have to admit, as a survival strategy, it kinda works.
Martin Beech (1990). Blue stragglers as indicators of extraterrestrial civilisations? Earth, Moon and Planets, 49 (2), 177-186 DOI: 10.1007/BF00053979